In 1967 the American biologist Lyn Margulis developed an idea that had been considered earlier in the 20th century. It proposed that the complex architecture of eukaryote cells had arisen by several simpler prokaryote cells becoming incorporated inside a membrane to form other bodies or organelles that co-existed and interacted. That is, a complex of mutual dependence within a cell wall, called endosymbiosis. For instance, the mitochondria of modern animal cells resemble a class of aerobic bacteria or Proteobacteria (gram-negative bacteria), some of which are responsible for several modern diseases, such as tick-bite fevers. Another is the resemblance of the photosynthesising chloroplasts of plants to cyanobacteria. A similar origin might apply to eukaryote nuclei and other organelles inside eukaryote cells; some have their own DNA molecules. I summarised Margulis’s late-20th century concept of endosymbiosis in my 1999 book Stepping Stones.
AI generated cartoon of symbiogenesis. Credit PhysOrg
Since then the rapid development of genome analysis has seen major advances in the field now known as symbiogenesis. In the most general sense, that is now regarded as a sequence of mergers between early members of the two prokaryote domains of Archaea and Bacteria: not a simple topic! In 2017 a group of archaeons called Promethearchaeati – ‘Asgard’ for short – were found to contain proteins – and thus the genes that produce them – akin to those in eukaryotes. So the Asgards are prime candidates for a role in symbiogenesis. Their symbiotic merger with a Proteobacteria may have begun the evolution of all eukaryotes. The entry of cyanobacteria – a candidate for chloroplasts – into one of the evolving groups divided plants from animals. A new AI analysis of thousands of genomes in living microbial organisms by Catalan scientists in Barcelona has enabled them to flesh-out and critique this hypothesis to a remarkable extent (Bernabeu, M. et al. 2026. Gene ancestries reveal diverse microbial associations during eukaryogenesis. Nature, v. 654; DOI: 10.1038/s41586-026-10639-9). Their work possibly revolutionises the study of biological evolution
Moisès Bernabeu and his three colleagues drastically ‘pruned’ the eukaryotic tree of life, which over-represents animals and species found in common ecosystems. They also stripped the limited number of eukaryote genomes of genes that do simple jobs or are closely related – i.e. those that seem to duplicate large sections from the oldest, ancestral genes. Two further ‘edits’ enabled the team to judge from their analysis what sort of roles may have been played by the genetics of the last eukaryote common ancestor (LECA). At this level of simplification it appeared that our ancestors inhabited oxygenated environments and got their energy by eating other organisms or their dead remains.
About 30% of the genes in eukaryotes seem to be unique to them and evolved after LECA had emerged. Many of the rest came from diverse prokaryote organisms. Alphaproteobacteria (previously termed ‘purple’ bacteria) and the Asgard archaea figure strongly, together with a range of other bacteria. As suggested previously, a vital process could have been transfer of genes from one prokaryote to another. Bernabeu et al.’s study highlights waves of such gene transfers prior to LECA’s acquisition of mitochondria, widely deemed to have been incorporation of an early proteobacterium. They also provide evidence for a central role played by giant viruses in enabling such gene transfers, also hypothesised previously.
Rather than being a simple case of a ‘one-off’ symbiosis between two separate prokaryotes, an archaeon and a bacterium, with the other organelles and genes added during a later evolutionary stage, the genesis of LECA was probably a long and complex interaction that involved diverse participants. It also seems certain that all the prokaryotes must have interacted in a stable, long-lived ecosystem for such a complex process to reach a tangible and enduring outcome after innumerable fits and starts. That oxygen became such an essential inorganic ‘player’ clearly suggests a microbial-mat ecosystem of organisms that involved oxygenic photosynthesis. The whole ecosystem and its members, pro- and eukaryotic, seem likely to have been evolving together, like modern ecosystems but on a microscopic scale. All this may have taken millions of years during the Palaeoproterozoic Era (2.5 to 1.6 Ga)
Lately, North American ground squirrels have been observed hunting, dismembering and eating voles. European tree squirrels also have a side that negates their nut-nibbling popular personae. They regularly take fledglings from bird nests. No more Mr Cute Squirrel then! In fact they’ll eat just about anything, including roadkill and even washed-up dead whales. A team of forensic ecologists from Canada, Sweden, Denmark and the US has harnessed this trait into a possibly ground-breaking study of how the Yukon Territory ecosystem evolved during the Pleistocene since 700 ka ago (Murchie, T.J. and 15 others 2026. Ground squirrel coprolites preserve complex archives of ancient environmental DNA over 700,000 years. Nature Communications, v. 17, article 4868; DOI: 10.1038/s41467-026-72977-6). Between 2007 and 2021 Tyler Murchie and colleagues collected ground squirrels’ faecal pellets from 14 latrine chambers or middens in their ancient burrows in a sequence of permafrost layers at the famous Klondike goldfields. The uppermost layers were dated using the 14C method, and for samples from deeper levels – older than 50 ka – using volcanic ash layers in the frozen sediments. Fourteen of the samples spanning 17 to 700 ka ago yielded fragmentary DNA from the squirrels’ diet.
Ground-squirrel midden in tunnelled permafrost. Credit Scott Cocker, University of Alberta)
Obviously this was dominated by their own DNA and gut bacteria, but contained fragments from an astonishing range of organisms that they had eaten. There were signs of at least 200 plant species: trees, shrubs grasses and flowering herbs known from the Pleistocene ‘mammoth steppe’ and tundra. Animal DNA included that from spiders, ants, moths, beetles, and grasshoppers, together with parasitic worms. But the most astonishing range of their appetites covers a great many mammals. As well as small mammals, such as mice, there are also signs of bison, mammoths, horses, sheep, wolves, and big cats having been eaten. It hardly needs to be emphasised that the Pleistocene ground squirrels did not hunt and overwhelm such prey, but they certainly did not reject a free meal of carrion lying on the tundra.
The wealth of species unwittingly archived by ground squirrels’ tendency to hide their droppings within their burrow systems offers a novel means of tracking the evolution of the ecosystem of which they were a part. It seems to outweigh the use of DNA extraction from soil horizons or even fossil bones. But to take matters further would require many more samples spread more evenly through the history of the mammoth steppe and tundra – most of the samples are from the last 90 ka. The Klondike goldfields are not representative of the whole of Arctic North America, being in a rugged terrain. Moreover, the Yukon Territory was repeatedly glaciated, as was the Canadian Shield itself. So, intact permafrost sequences spanning even the last glacial period are rare.
The notion of large-scale use of hydrogen as an energy source has a surprisingly long history. It was first proposed by J.B.S. Haldane in 1923, who envisaged electrolysis of water – releasing hydrogen and oxygen – using power from wind turbines to address this renewable source’s highly variable output effectively by storing it in the form of hydrogen. Since the only other output is oxygen, a hydrogen economy might seem to avoid global warming from the current release of greenhouse gases. However, as a 2023 post on Earth-logs concluded, of all the means for mass production and use of hydrogen only one source is a truly ‘green’ energy source: that emitted from rock by natural processes: so-called ‘white’ hydrogen. It is known to be generated by the breakdown of the mineral olivine [(Fe,Mg)2SiO4] by water in the absence of oxygen:
3Fe2SiO4 + 2H2O → 2 Fe3O4 + 3SiO2 +3H2
A more complex reaction is the hydration of olivine to the mineral serpentine [Mg3Si2O5(OH)4], which also yields hydrogen. Olivine is the most important mineral in the Earth’s mantle and abundant in crustal basalts and ultramafic rocks too. Oceanic lithosphere (ophiolites) added by tectonics to the continental crust form obvious targets for seeking natural hydrogen seepage. Yet such surface gas escapes have been documented only from a few sites, including an irrigation well in rural Mali that emitted gas containing 98% hydrogen, and a few natural springs from the Oman ophiolite.
The latest study may have taken the hydrogen economy to a literally deeper level (Sherwood Lollar, B. & Warr, O. 2026. Decadal record of continental H2 reservoirs reveals potential for subsurface microbial life and natural H2 exploration. Proceedings of the National Academy of Sciences, v. 123, article e2603895123; DOI: 10.1073/pnas.2603895123. PDF requests to owarr@uOttawa.ca and/or barbara.sherwoodlollar@utoronto.ca). Over fifteen years Barbara Sherwood Lollar and Oliver Warr of the Universities of Toronto and Ottawa, Canada monitored gas released by 35 boreholes originally drilled to assess and plan mining of an orebody in Precambrian basement rocks at Kidd Creek near Timmins, Ontario. On average, each of the boreholes released 8 kg of hydrogen per year. Scaled up to the mine’s 15 thousand exploratory boreholes, the mine itself is estimated to be yielding 140 metric tons of the gas annually. That could provide 4.7 gigawatts of energy per annum, sufficient for the needs of more than 400 Canadian homes.
Schematic cross section through the Kidd Creek Mine, Ontario, Canada. Source American Museum of Natural History
The Timmins mining district is typical of Archaean greenstone belts in the Canadian Shield and in cratons across the world: supracrustal rocks including ultramafic and mafic volcanics and a variety of metasedimentary rocks. The Timmins district is historically Canada’s largest gold producer, but also hosts ores of many other metals. The Kidd Creek Cu-Ag-Zn mine is one of the deepest in North America, which penetrates interlayered felsic, mafic, ultramafic, and metasedimentary rocks to a depth of 2.9 km below the surface. The ores formed by submarine hydrothermal processes around 2.7 Ga ago. The sampled boreholes were drilled horizontally at mine levels between 2.04 to 2.9 km below the surface to penetrate the ore zone and its mafic-ultramafic host rocks. Rather than yielding gas, the holes release briny fluids in which hydrogen, helium and various hydrocarbon gases are dissolved. They are similar to fluids issuing from other deep mines, but differ in showing their formation mainly to be through inorganic reactions with the bed rock rather than as a result of microbial metabolism that exploits a variety of chemical interactions in the ore, such as reduction of sulfate ions to sulfide. The authors have studied hydrogen yields from a number of other mines in mafic-ultramafic rocks, which are comparable with Kidd Creek. So it may be that hydrogen in vast volumes is being emitted by existing and abandoned metal mines in such igneous terrains.
Sherwood Lollar and Warr authoritatively outline the economic potential of hydrogen production for remote communities and mines in greenstone-belt terrains. They also assess active serpentinisation of ophiolites and kimberlites by near-surface groundwater and associated microbial ecosystems as hydrogen sources, the few that have been studied seeming to produce even larger amounts of hydrogen. But they also note that their closer proximity to the surface means that these geological features are generally ‘open-systems’ prone to rapid loss of gases. However, in the manner of hydrocarbon gas fields, some ophiolites may host large amounts of hydrogen if they are capped by younger clay-rich sedimentary strata. Whatever, the global warming of what might be called the ‘Hydrocarbon Age’ is set to become a disaster. Breaking its death grip should be the principal economic agenda, which requires the most rapid turn to long-term energy alternatives. Natural hydrogen could be a part of that, and hopefully the work of Sherwood Lollar and Warr, and others like them, should lead to determined exploration and assessment of this novel physical resource. In Scandinavia a Nordic Hydrogen Route is being proposed. This Swedish-Finnish initiative is based on the Scandinavian Shield and its greenstone terrains and numerous mines driven into them. One would hope that its entrepreneurs are considering naturally emitted hydrogen rather than or as well as sources given other coloured labels.
That Earth has always been such an active planet is largely due to water having continually being shifted into the mantle by subduction of oceanic lithosphere. Emplaced at temperatures around 1200°C the basaltic crust and ultramafic rocks of the lithospheric mantle become hydrothermally altered by interaction with the ocean, so that they contain a range of hydrous minerals. The mantle is estimated to contain between a quarter and four times the present volume of all ocean water. The vast bulk of the mantle is not undergoing partial melting at any one time. Most magmatic activity is linked to plate tectonics through linear belts such as those along oceanic rift systems and above subduction zones, with a small proportion at ocean islands above isolated mantle plumes and similarly, though more sparsely above hot spots below continents. Such plume-related magmatism has at intervals in the past been vastly bigger than now, forming flood-basalt provinces and ocean-floor plateaus, such as the Deccan Traps and the Ontong Java Plateau; some linked to extinction events. The particular role of water in mantle melting is its reduction in the temperature at which melting begins at depth: ‘dry’ mantle does not melt but remains solid, albeit ductile. Two recent studies have provided important insights into previously unsuspected roles that water can play deep in the mantle.
Four different modes of subduction. Credit: Li et al. Fig 6
Jianfeng Yang of the Chinese Academy of Science and colleagues from China and University of Padua, Italy provide evidence that ancient subducted slabs that gather at the mantle transition zone (MTZ) may trigger ocean-island and ocean-plateau volcanism (Yang, J. et al. 2026. Subduction legacies in the mantle transition zone modulate intraplate oceanic volcanism. Nature Communications, in press; DOI: 10.1038/s41467-026-73403-7). In fact there is a multiplicity of modes wherein subducted slabs interact with the MTZ, some are retained within it while, in one way or another, others eventually pass through it to the deeper mantle. Long-dead relics of subduction zones trapped there form ‘reservoirs’ of water 410 to 660 km below the surface at concentrations far higher (1 to 3 %) than does pristine mantle (less than 0.1%). It is stored as OH ions in dense mafic minerals, such as ringwoodite a high-pressure form of olivine (Mg2SiO4) containing up to 2.6 % of OH ions, and bridgmanite (MgSiO3), which forms once subducted slabs pass into the mantle transition zone. If that transformed lithosphere rises above about 410 km, such minerals transform back into anhydrous olivine, thereby liberating their water. At such depths, where temperature in the surrounding dry mantle is about 1800°C the emergence of water triggers a decrease in the temperature at which the ancient slab and also the surrounding mantle can melt. The authors cite evidence that such a process has contributed to the Azores oceanic plateau where the crust is 10 to 20 km thick. It is conceivable that a similar process of deep water ‘recycling’ may have been associated with continental flood basalts. Yang et al.’s new insight may also help unravel hitherto puzzling geochemical anomalies in other kinds of basaltic igneous rocks, such as those which well-up at mid ocean ridges to form modern oceanic crust.
Slabs that descend deeper into the mantle retain their dense mafic minerals and thus the water trapped within them. That water may eventually be involved in transformations at much higher pressures and temperatures, as deep as the core-mantle boundary. One possibility is their retention in mantle plumes that rise from the CMB to facilitate partial melting once they pass through the MTZ
Hominin fossils referred to as Homo erectus have been found in Africa, Central Europe and China. Those from Africa have also been attributed to H. ergaster and by some to ‘African H. erectus’– a point of lingering dispute and confusion. The African lineage spanned a long time period after appearing around 1.7 Ma ago, possibly to as late as 0.6 Ma. The confusion deepened with the discovery of similar, well-preserved remains at Dmanisi in Georgia that are actually older (1.77–1.85 Ma) than the African specimens. But they are so anatomically diverse that the five skulls might easily be assigned to five different hominin species had they been found at separate locations. Asia is again very odd in an H. erectus context. The species was first proposed in 1891 by Eugene Dubois from remains in sediments oof the Solo River near Trinil in Java – he originally suggested the name Pithecanthropus erectus. Remarkably, the Solo sediments were dated at 53 to 27 ka in 2019, so did Homo erectus co-exist with anatomically modern humans (AMH) on Java? Similarly heavy-browed crania emerged from several sites in China. Curiously, the first complete Denisovan cranium, found near Harbin in China matched H. erectus in the eyebrow department. However, mtDNA from its dental plaque and bone proteomics tie in with those found in fragments from the eponymous Denisova Cave in Siberia and with a fragmentary mandible from Tibet. Without such evidence, and were itnot so young (146 ka), most palaeoanthropologists would probably have called the Harbin individual H. erectus, There are quite a few records of older Chinese hominin crania, dubbed H. erectus on anatomical grounds, including one dated at 1.7 Ma. There are a great many oddities and contradictions that need resolving.
Skulls of Homo erectus from Dmanisi , Georgia (credits; M.S. Ponce de Leon & P.E. Zollkofer, University of Zurich)
The cranium found near Harbin, China belonged to a Denisovan. Credit: Hebei Geo University
On 13 May 2026 a team led by palaeogeneticist Qiaomei Fu of the Chinese Academy of Sciences published data on proteins and amino acids yielded by enamel from six teeth of these ancient Chinese fossils, which emerged from three localities of Middle Pleistocene age (580 to 400 ka) (Fu, Q. and 11 others 2026. Enamel proteins from six Homo erectus specimens across China. Nature, v. 10.1038/s41586-026-10478-8). One is from the Zhoukoudian Cave near Beijing, famous for ‘Peking Man’, the primary reference for the anatomy of Asian H. erectus. Older fossils are unlikely to yield meaningful data of this kind because of chemical degradation; the reason why DNA has so far proved elusive from these specimens. Tooth enamel is extremely durable and can protect proteins and amino acids. Since both are produced by genes on DNA they are proxies for variants of those long-vanished genes
The key protein in the supposed H. erectus teeth isameloblastin which is involved in the formation of tooth enamel. The ameloblastin of all six teeth shared two amino acid variants; one previously unknown in other hominin lineages and perhaps unique to H. erectus, the other has been identified in Denisovans. Fu and colleagues suggest that the original bearers of the teeth – presumed to be H. erectus – had interbred with Denisovans and passed on the second variant gene. In turn that had been passed on to Asian AMH with some of whom Denisovans had interbred; remarkably 21% of living people on the Philippine archipelago carry that gene. The authors go on further to suggest that their findings support the notion that H. erectus was the source by gene-flow for ‘super-archaic’ sections of DNA found in actual Denisovan DNA from one member of that group. That is certainly a possibility, but is not the only one.
Neither the proteomics nor the morphology of the teeth, nor the anatomy of the fossils that accompany them in any way prove that they are from actual 400 ka old Homo erectus individuals. That would require at the very least protein analyses from specimens that definitely pre-date the divergence of Denisovans from Neanderthals about 600 ka ago. Remarkably, proteins have been extracted from a ~1.8 Ma old tooth yielded by the Dmanisi H. erectus site in Georgia, but that failed to reveal anything useful in this context. Maybe future work on older Chinese hominin teeth could resolve the issue. Another hypothesis is that the bearers of the analysed teeth were a population of Denisovans who themselves developed genetic variations rather than inheriting them. Proteomics is at about the same stage in its development as human genomics was before 2010 in the run-up to discovering Neanderthal and Denisovan genomes. But in the case of H. erectus the problem began with biologists’ long record of trying to simplify the natural world, especially fossils, through ‘lumping’ rather than ‘splitting’.
Humans that science has designated as different species were capable of interbreeding over tens and hundreds of thousand years, probably repeatedly and maybe habitually. That fact makes it hard to defend the concept of their speciation. There were few environments where they could not thrive, yet their migrations spread small numbers over vast areas. Continually shifting, isolated populations would diverge genetically and physically, the more so the fewer individuals were banded together. Occasionally populations would meet: an opportunity for celebration, and more, for conscious beings facing the rigours of exploration with neither territory nor resources to defend.
During the past 539 Ma (the Phanerozoic Eon) Earth’s geological history saw the explosion of rapidly evolving life in the oceans and on the land. The pace of that evolution swung up and down through a complex sequence of extinctions and adaptive radiations. They resulted from many intertwined inorganic changes: tectonics; impacts; igneous events; global climate change; atmosphere and sea-water composition. Although palaeoclimatic knowledge has become ever more detailed over the last few decades, its most important record, the varying temperature of the land surface and oceans, is lacking in precision. The timing of climatic events is not the issue, but the magnitude of changes in global mean surface temperature. The latter is largely down to the main tool in assessing past temperatures: the isotopic composition of oxygen (δ18O) in marine fossils. In particular, the record for the Lower Palaeozoic has remained stubbornly odd. In the Cambrian and Ordovician Periods it implies that low-latitude seawater temperatures reached levels of 40 to 50 °C, that seem literally life threatening: phytoplankton at the base of modern marine ecosystems die at water temperatures above 35°C. Yet the fossil record is teeming throughout the Lower Palaeozoic at all latitudes. Some manner of imprecision in the oxygen-isotope method gives the impression of wild fluctuations and a dramatic overall cooling of the planet through the Phanerozoic: the temperature record as it stands seems implausible.
The carbonate-silicate cycle within the longer-term carbon cycle. Source: Wikimedia Commons
A group of palaeoclimatologists from China, the UK, Australia and the US have combined a variety of geochemical proxies, sedimentary records and climate modelling to correct the marine-carbonate δ18O record (Zheng, D. and 12 others 2026. Tight regulation of Earth’s long-term temperature over Phanerozoic time. Nature Communications, in press 4 May 2026; DOI: 10.1038/s41467-026-72672-6). Their approach is based on a chemical index of alteration (CIA), i.e. a measure of the degree of chemical weathering of the source for sedimentary rocks. The CIA compares their content of immobile aluminium oxide (Al2O3) with calcium, sodium and potassium oxides that are more easily moved in solution. Analyses of recent river sediments show a positive correlation between CIA and local temperature, so CIA in ancient sedimentary rocks is a potential proxy for the ambient temperature of the region from which those sediments were derived. The CIA also depends on other factors, such as the intensity of physical erosion and transport. However, allowing for these factors in modern environments does not affect the correlation with ambient temperature: the method remains robust. The geochemical data from sedimentary rocks required to use CIA as an independent check on O-isotope derived temperature are available in abundance from all continents for most of the Phanerozoic.
The study by Zheng et al. suggests that throughout the Phanerozoic global mean temperature remained consistently within the 10 to 30°C range. Thus Palaeozoic ocean temperatures were comparable with those of the succeeding Mesozoic and Cenozoic Eras. The team concludes that various negative feedback processes inherent in the Earth System have been able to regulate its surface temperature through the Phanerozoic. The most important of these is climate-dependent silicate weathering in which acidic rain – produced by CO2 dissolved from the atmosphere – breaks down silicates to yield dissolved bicarbonate ions that combine with calcium and magnesium ions to precipitate carbonates. Such a process draws down the main greenhouse gas from the atmosphere. There are other aspects of the carbon cycle that also draw down atmospheric CO2 and reduce the greenhouse effect, such as burial of organic debris. Tectonics also shapes climate by modulating both silicate weathering and CO2 emissions from volcanic activity.
It should be emphasised that anthropogenic global warming is proceeding at a far higher rate than natural negative feedback processes. We simply cannot rely on silicate weathering to reverse whatever climatic outcome results from what the current global economy does so very quickly. Yet the findings by Zheng et al. do seem likely to force a change in thinking about climate change on a geological timescale.
Octopuses defy common sense. They are invertebrate molluscs, so we don’t expect them to show well-developed intelligence, which they do. As well as tiptoeing around on their eight tentacles, they can move at high speed using a kind of backwards jet propulsion and some are even able to cross dry land.. Each of their tentacles has a sort of brain, as well their central one: distributed, semi-autonomous cognition in which their tentacles taste, touch, and move independently. Three hearts circulate their blue blood. Masters of swift camouflage using specialised skin cells that contain different coloured pigments, which behave like pixels in a TV screen. Octopuses can also rapidly manipulate their body texture and shape. They also seem to use such bizarre displays to communicate mood, at the very least.
Shape-shifting octopuses can squeeze through gaps far smaller than their own size to hide from both predators and their prey, which also makes them escapologists far outranking Harry Houdini. Their eyes look like those of goats, with horizontally linear pupils, although they evolved separately from the eyes of other animals. Satan is said to have goat-eyes, hence the colloquial name for an octopus: devil fish. Mariners of old (and maybe some of the present day) reputedly feared giant octopuses to be capable of crushing ships and devouring the crew: the Kraken! Even small octopuses possess greater intelligence than a dog: some seem to enjoy playing, building dens, negotiating mazes and watching the antics of humans …
Since they are almost entirely soft flesh, the fossil record of octopuses is unsurprisingly meagre, apart from their jaws that use chitinous ‘beaks’ to munch their victims. The evolution of other cephalopods, for instance ammonites and squids, is better known from their external and internal skeletal remains and extends back to the Cambrian Period. So collecting and analysing fossil octopus jaws is the only option for palaeontologists. Shin Ikegami of Hokkaido University, together with Jörg Mutterlose of Ruhr University in Bochum Germany and eight other Japanese scientists, developed a new approach to supplement data on octopus jaws previously excavated from Cretaceous strata on Hokkaido and Vancouver Islands. (Ikegami, S. and 9 others 2026. Earliest octopuses were giant top predators in Cretaceous oceans. Science, v. 392, p. 406-410; DOI: 10.1126/science.aea6285).
Cretaceous marine predators (at maximum estimated size) with a scuba diver for scale. Credit: After Ikegami et al. Fig. 4, and Jacobs 2026.
Ikegami et al. ground layer by layer through Japanese and Canadian sedimentary rocks to produce 3-D tomographic models of fossil beaks within them – quicker CT scanning proved inefficient in showing details. All the ‘beaks’ showed signs of wear from cracking the harder bones of their prey. Assuming that the same variation of beak size and body mass as in modern octopuses is relevant to those of Cretaceous age, the researchers came up with an astonishing result. Cretaceous octopuses reached huge sizes. They estimated one Nanaimoteuthis jeletzkyito have been 19 metres long, roughly the size of an articulated truck. During the Cretaceous Period the top predator of the oceans had long been supposed to have been the formidable marine reptile Mosasaurus at around 15 m long.
We know what mosasaurs ate from fossilised stomach contents of two specimens: more or less anything, including other substantial marine reptiles, sharks, cephalopods and even other mosasaurs, some whole, some dismembered. As for Nanaimoteuthis and other giant Cretaceous octopuses, reconstructed from their fossilised beaks, there is little obvious evidence for what they ate, other than it would have had to have been in large amounts. Judging from the wear exhibited by their beaks at least a proportion of their diet was crunched up shells and bones of ammonites and fish. Modern octopus species, both small and moderately large, have other sorts of feeding strategies. Some eat planktonic animals, others drill holes in shells and suck out their innards rendered to the texture of a ‘smoothie’ by corrosive saliva.
It is not surprising that the media have made quite a fuss of these Cretaceous ‘krakens’, some suggesting that they preyed on formidable marine reptiles such as mosasaurs. That would definitely have made them ‘top’ marine predators. Yet such massive, moving mounds of nourishing flesh would have made them a worthwhile catch for a whole school of toothy reptiles and sharks. The modern sperm whale is known to devour giant squid at the great depths to which they can dive, as witnessed by numerous cephalopod beaks in their stomachs. So it is equally possible that the octopus beaks found in Cretaceous sediments of Hokkaido were excreted by marine reptiles
Our hominin ancestors in Africa first fashioned tools about 3.5 Ma ago. Since then regular intake of animal protein through hunting, followed by the later discovery of fire and cooking, may progressively have encouraged the evolution of larger hominin brains. Both behavioural leaps would have reduced the length of the ‘working day’ needed to sustain hominin groups. That would have lengthened opportunities for cognitive reflection. social life and culture, and thus further evolution. They also expanded the opportunities for migration, beginning with Homo ergaster venturing beyond Africa at least 1.8 Ma ago. Hominins evolved to such an extent that several separate species occupied our home world at any one time until about 45 thousand years ago. After that only H. sapiens occupied Africa, Eurasia and Australasia.
Such protracted and meandering evolution and dispersal clearly involved episodic physiological and cultural changes, but all we have to go on are fragmentary fossil remains and artifacts of various kinds. DNA has yet to be extracted from hominin bones older than 400 ka (an early Neanderthal from northern Spain). Though H. sapiens first appeared in Morocco about 300 ka ago, DNA from our species dates back to only 45 ka (western and central Europe). What is today termed ‘ancient’ human DNA, is actually very young and restricted to climate zones where its decomposition has been slow. At present there is little point in analysing fossil material from tropical and subtropical latitudes; the DNA is degraded beyond recovery by even the most up-to-date techniques. Fascinating as discussion of human evolution is, in reality most is merely inferred from comparative anatomy and anthropological interpretation.
By 45 ka the heavy evolutionary lifting had been done, resulting in anatomically modern humans, but we have little, if any, chance of explaining in genetic terms how it was achieved. There has been much speculation about the conditions, particularly climatic ones, which may have driven the changes. During the last 2.6 Ma – the Quaternary Period – global climate has been the most changeable in the last 300 Ma. Ice ages have come and gone, first in 40 ka cycles and during the last million years every 100 ka. Much more rapid changes, such as millennial Dansgaard-Oeschger cycles, appeared during each glacial episode, the last being the Younger Dryas between 12.9 and 11.7 ka. For a long while ideas on the drivers of human evolution have been dominated by those concerning environmental stress. Unsurprisingly, genetic change has also been ascribed to such a Darwinian-ecological cause: adaptability to adversity. To test such a hypothesis requires genetic data, of course. But, except for the climatically more stable Holocene Epoch since 11.7 ka, ancient human genomes are in very short supply.
Renowned researcher into ancient human genetics David Reich of the Harvard Medical School in Boston, USA has collated more than 15 thousand ancient human genomes extracted from the remains of individuals who lived and died in Europe and parts of the Middle East during the last ten thousand years. These have been analysed statistically in the context of ‘directional selection’. This is a type of natural selection that occurs when one version of a gene – an allele – confers an extreme form of a trait. If it proves advantageous it rapidly gets passed on to more descendants than do less advantageous alleles, and thus rises in frequency across a population. This differs from other causes of gene frequency changes, such as human migration, population mixing, and random genetic fluctuations that occur in small populations. Well-known examples of directional selection are rapid changes among European Peppered moths, African cichlid fish, Alaskan Sockeye salmon and Big Cats which change over time in response to variations in their habitats. A human example is a genetic variant that maintains the ability to digest the sugar lactose in milk beyond infancy, which enables many modern Europeans to digest milk throughout their lives. The algorithm needed to separate signs of directional selection from other types of genetic change was developed by Ali Akbari, a computational geneticist also at Harvard Medical School. A recent paper by Akbari, Reich and colleagues in the US, Iran, Germany, and Austria (Akbari, A. and 15 others 2026. Ancient DNA reveals pervasive directional selection across West Eurasia. Nature, advance online publication; DOI: 10.1038/s41586-026-10358-1) seems set dramatically to change the research into recent human evolutionary genetics.
Akbari et al. discovered that directional selection has driven the spread or decline of hundreds of gene variants in human populations throughout Western Europe in the last ten millennia. In particular, selection accelerated with the adoption of farming rather than a hunter-gatherer lifestyle. Among the gene variants are those connected with light skin, red hair, risk of celiac disease – linked to gluten in cereals – susceptibility to gout, resistance to leprosy, baldness, rheumatoid arthritis and alcoholism. There are many more (see Figure 3 in the paper): the team identified 479 gene variants affected by directional selection, some that can be explained by changes in lifestyle, others less explicable and yet more that underlie complex traits such as mental illness and cognition. Some of the variants sprang up and were sustained in the population, others rose and then dwindled. The Neolithic began a period of fundamental life style changes in Europe, summed up as a shift from hunting and foraging to farming of cereals and livestock, as early as about 10 ka ago in what is now Türkiye. The pace of genetic changes of this kind reached a peak around the Bronze Age, perhaps because human activities in Europe became more complex then with the mass migration westwards of Yamnaya horse- and wagon-using people from the steppes to dominate Europe
The shift from small wandering bands to living in settlements was a drastic change from a lifestyle that had continued throughout all previous human history. So, it is hardly surprising that there was a major shift in humans’ genetic makeup. But such a change in human labour was not unique to Europe and is known to have occurred on all inhabited continents, with the exception of Australia, at different times during the Holocene. Other regional genetic databases can be analysed in much the same way, once sufficient ancient DNA is collected in Asia, Africa and the Americas. Yet not much is available. The authors comment: ‘A variant that now correlates to household income or years of schooling [remarkably, there are some!] had to have meant something different in the Stone Age. So these results do not mean that Europeans evolved to be smarter or healthier.’ Moreover, the research results in the paper seem likely to be amplified as the data set is so large and complex.
Around 20 thousand years ago, the Earth began to emerge from the grip of the Last Glacial Maximum (LGM). Huge ice sheets had locked up so much water that sea level was then about 125 m lower than it is today. At 12,870 years ago the warming and sea-level rise were reversed for 1,170 years in the Northern Hemisphere: an episode of near-full glacial conditions known as the Younger Dryas (YD). The adjectives ‘sudden’ or ‘abrupt’ grossly understate the pace of initial cooling – 3°, 6° and 15° C in North America, Europe and Greenland, respectively. Isotopic evidence from Greenland ice cores suggest that the cooling took place over three years or less. Such a degree of precision stems from the continuous annual layering in the Greenland ice cap. As far as humans were concerned, this would have been catastrophic for hunter gatherers following game northwards in Eurasia and North America as conditions ameliorated during the seven thousand years since the LGM. The archaeological record, or rather the lack of one, for what are now temperate zones suggests humans either retreated south or were blotted out.
There is no counterpart for the YD in the end stages of early glacial episodes. Some authors have suggested that it was the outcome of an appropriately catastrophic geological event, such as a large meteorite strike, as proposed in 2007 (See: Whizz-bang view of Younger Dryas; July 2007). This hypothesis gained traction in 2013, at least for its authors, with the discovery of anomalously high concentrations of the noble metal platinum (Pt) and other platinum Group metals, such as iridium (Ir) at or around the start of the YD in the GISP2 ice core. New research on this anomaly (Green, C.E. et al 2025. A possible volcanic origin for the Greenland ice core Pt anomaly near the Bølling-Allerød/Younger Dryas boundary. PLOS One, v. 20, article e0331811; DOI: 10.1371/journal.pone.0331811) offers a different scenario. Charlotte Green of Royal Holloway, University of London and colleagues from universities in the UK, Germany and Austria examine the timing of this Pt spike and its detailed geochemistry.
The ‘killer’ observation is that the anomaly occurs in ice that formed 45 years after the onset of the Younger Dryas and has a spread of about 14 years. Whatever kind of event released the platinum, it definitely did not somehow trigger the onset of the YD. Moreover, the anomaly was significantly deficient in iridium compared with a wide range of meteorites and terrestrial igneous rocks. It also differed markedly in other elements, such as lutetium and hafnium, and in all three elements in melt rocks and ejecta sediments associated with five proven impact structures. The closest match is to volcanic gas condensates from a recent eruption of a submarine volcano near Tonga
Both the GISP2 and NGRIP cores through the Greenland ice also record a large, 12-year long spike in sulfate of volcanic origin spread across the very start of the YD. That roughly matches the age of an explosive eruption, which formed the circular Laacher See in the Eifel volcanic field in Germany. That eruption is thought to have blasted 6.3 km3 of highly alkaline magma into the atmosphere: about the magnitude of the 1991 Pinatubo eruption, but insufficient to yield the size and duration of the sulfate spike that coincides with the start of the YD. The sulfate anomaly suggests a far larger, currently unknown eruption at 12,870 years ago. The Pt and Ir data from the Laacher See event rule it out as a source for the younger Pt anomaly in the GISP2 ice core. One possibility is a nearby Icelandic subglacial fissure eruption at that time.
So, as regards what started the Younger Dryas, there is support for a very large, but so-far unknown volcanic event, and an as yet unresolved, perturbation in the Atlantic Meridional Overturning Circulation (AMOC) resulting from drainage of a huge glacial lake in northern North America (see: The Younger Dryas and the Flood; June 2006), but no support whatever for an impact event. Climatology of the distant past is always likely to be difficult to pin down. That is because, as now, it involves linkages between a large number of variables: not only physical ones, but issues of biogeochemistry, the inner Earth, the rest of the solar system and even cosmology. That is, it is as complex as human affairs and their history. Common sense, linear thinking and the like, simply will not do.
Seismic tomography provides no evidence that slabs of oceanic lithosphere descend intact through the whole mantle to the core-mantle boundary. It might once have happened when they were capped by abundant high-density rocks, such as Precambrian banded-iron formations. A great many actively descending slabs have been shown to cease sinking, slide sideways and accumulate at depths around 660 and 1000 km. Until recently these discontinuities were been generally ascribed to transitions in the structure of the dominant mafic mineral olivine (Mg2SiO4) in mantle peridotite induced by increasing pressure and temperature. The resulting increases in mantle density supposedly form barriers to further slab descent. Pressure-induced mineral transitions in the slabs themselves that increase their density, such as pyroxene to garnet, may somehow be inhibited thereby leading to stagnation in slab descent. That may be true for the 660 km discontinuity, but for stagnation at 1000 km deep no such density-changing mineral transitions have shown up in high-P high-T mineralogical experiments. Some other process must therefore be responsible for slab descent to that depth. Recent work by geoscientists at several universities in China gives insights into what may be going on (Li, J., Li, K., Li, J. et al 2026. Dual slab stagnation depths controlled by grain-size-induced sporadic low-viscosity zones at around 1000 km depth. Nature Communications DOI: 10.1038/s41467-026-69987-9).
Four different modes of subduction at island arcs. Credit: Li et al. Fig 6
Jing Li and colleagues have focussed on the possibility that changes in the bulk viscosity of the mantle may play an important role. Their approach is twofold: experimental mineral physics and geodynamic modelling. Results suggest that recrystallization in the mantle when deeply penetrating slabs pass through it may patchily reduce the mantle’s grain size and thus its viscosity; the more so with larger volumes of subducted slab material. In turn, the resulting physical heterogeneity probably disrupts the steady downward passage of the slabs; fine-grained, less viscous zones ‘lubricating’ slab penetration, unchanged zones hindering it. The authors link such hypothetical micro-structural processes to modes of subduction that are currently active. They consider four modes of active subduction beneath island arcs with either a slow or a fast rate of trench retreat (see Figure). A slowly retreating trench system combined with low-viscosity patches at depth (Mode 1) results in penetration below 660 km and slab stagnation at 1000 km. Slow trench retreat with a homogenous lower mantle (Mode 2) gives rise to penetration and buckling of the descending slab between 660 and 1000 km. Fast trench retreat with a deeper low-viscosity zone (Mode 3), or with a homogeneous lower mantle (Mode 4) both result in slab stagnation at 660 km.
The models developed by Jing Li et al convincingly simulate various results of seismic tomography beneath island arcs. Interestingly, they suggest that the eventual assimilation of older slab materials into the deeper mantle (‘fossil’ slabs) may play a major role in mineral comminution and reduced mantle strength. That may leave behind low viscosity zones that later subduction may exploit. In fact, there are signs of possible fossil slabs in seismic tomograms more than 1000 km below the present Pacific Ocean floor in the form of zones of high P-wave velocity.
This work shows that plate tectonics is far from ‘done-and-dusted’, the mantle being far from uniform in its properties. Li et al’s results potentially open up new insights into whole-mantle convection, in which older tectonic events influence plate motions that are currently operating and the triggering of plumes rising from the deepest mantle. It also hints that such complex physical mixing of subducted material into the mantle may have resulted in the geochemical heterogeneities that increasingly emerge from analysis of magmas with ultimate origins in the mantle.
Followers can now download newly posted annual logs for Human Evolution and Migrations covering the years 2022 to 2025. By downloading them you can get a clear idea of how palaeoanthropology has moved forward since the Covid pandemic.
Enjoy the experience if you have the time and inclination!
Over the last few decades improved analytical techniques have made it possible to analyse tiny mineral grains for a variety of trace elements and several isotopes. Zircons obtained directly from crushed granitic igneous rocks vary in chemistry according to the magmatic processes that generated them and their tectonic context. Elevated ratios between uranium and niobium (U/Nb) and scandium and ytterbium (Sc/Yb) are characteristic of zircons in intermediate granites. These contain 52 to 63 % SiO2 – between mafic and felsic magmas – which formed by melting of hydrated mafic crust in settings akin to modern continental arcs; i.e. in subduction zones. But such partial melting can also take place where the base of continental crust delaminates and ‘drips’ into the mantle. That process is part of what is known as stagnant lid tectonics, believed by many to have been important in the Palaeoarchaean and Hadean. Such a process would have involved nearly anhydrous conditions and thus different geochemical partitioning of elements in the magmas and minerals that crystallised from them. Exposures of crystalline continental crust become increasingly rare further back in geological time, and there are none older than 4.0 Ga – i.e. of Hadean age – with a granitic component. Consequently studying the generation of continental crust in the Hadean and the early Archaean is almost entirely dependent on ancient zircons that found their way into much younger sedimentary rocks. The most famous of these occur as detrital grains in the 3.6 Ga Jack Hills conglomerate of Western Australia. Others have been extracted from similar ~3.3 Ga sedimentary rocks in the Barberton Greenstone Belt of South Africa and Eswatini.
Cartoon of possible Hadean stagnant lid tectonics, dominated by mantle plumes. (Credit: Bédard, J.H. 2018, Fig 3B, DOI: 10.1016/j.gsf.2017.01.005)
John Valley of the University of Wisconsin-Madison, USA, and co-workers from the US, Germany, Australia and France have worked on a large number of zircons newly extracted from Jack Hills. They have radiometrically dated them, and analysed Nb, Sc, U and Yb trace elements and hafnium (Hf) and oxygen isotopes Together with data from earlier studies, including Barberton zircons, they have teased out some remarkable insights into ‘continent-forming’ magmatism as far back in time as 4.4 billion years ago (Valley, J.W. and 11 others 2026. Contemporaneous mobile- and stagnant-lid tectonics on the Hadean Earth. Nature, Open access; DOI: 10.1038/s41586-025-10066-2). More than 70% of the >4.0 Ga Jack Hills zircons have elevated U/Nb and Sc/Yb ratios, which suggest that they formed in a setting akin to continental-arc subduction (CAS) zones, to produce now-vanished Hadean continental crust. The remainder seem to represent processes at mid-ocean ridge (MOR) and oceanic island (OI) settings. In contrast, the bulk of Barberton zircons of Hadean age show OI affinities, with only around 22% showing Nb–Sc–U–Yb signatures of probable CAS origins. From about 4.4 to 3.8 Ga two distinct forms of continental crust generation seem to have operated on Earth. In the erosional source region for the Barberton zircons their host granites seem to have formed during the Hadean and Eoarchaean by remelting of foundered lower crust, i.e. probably in a stagnant-lid-like tectonic setting. But at around 3.6 Ga they ‘flip’ to a subduction-like setting. The zircons yielded by Jack Hills conglomerates suggest substantially different conditions: alternating CAS and OI settings during the Hadean and a fall-off in crust generation during the Eoarchaean (4.0 to 3.8 Ga).
Plots of Sc/Yb and U/Nb against ages of zircons (vertical scale logarithmic). Black points are from Jack Hills, red from Barberton. The yellow field represents zircons formed in subduction zones; green suggests stagnant lid tectonics; grey the overlap between the two settings. Credit: Valley et al. Fig 3 a and b.
The mixed Hadean zircon signatures from Jack Hills possibly indicate that they were derived by erosion and transport from several distinct terranes that had been generated by two different processes: some kind of upper crustal recycling and stagnant lid tectonics. Meanwhile, that part of the Hadean Earth represented by the Barberton zircons may have been a long-lived regime of stagnant lid tectonics, replaced by dominant subduction at the end of the Eoarchaean. Yet the data suggest that into the Palaeoarchaean (3.6 to 3.2 Ga) and perhaps later, lid tectonics continued to operate somewhere, but at no time after 4.4 Ga was the Earth entirely subject to lid tectonics. Likewise, the authors insist that subduction was not of the plate-tectonic style, referring to some form of recycling of hydrated upper crustal mafic and ultramafic rocks into the mantle to undergo partial melting. Plate tectonics as we know it probably developed later in the Archaean. The early Earth had much higher heat flow than in later times, and thus the lithosphere was more ductile rather than brittle. The essence of modern tectonics is a series of rigid plates that extend down to the asthenosphere. When they deform it is largely through brittle failure of the entire lithosphere.
The great megalithic structure is the centrepiece of a vast ritual landscape on a 780 km2 plateau known as Salisbury Plain, underpinned by Cretaceous limestone: the largest remaining area of calcareous grassland in northwest Europe. The earliest sign that the Plain was used for ritual purposes dates to ten thousand years ago (8,000 BCE), when Mesolithic hunter gatherers erected large wooden posts to define by an E-W line the Sun’s rise and setting at the equinoxes. The area seems to have been continuously populated until 4,000 BCE when the first Neolithic farmers settled the Plain and began building burial mounds (barrows) to celebrate notable individuals and families.
The Stonehenge monument began as a circular cemetery around 3,100 BCE. Its development to the astonishing structure that remains largely intact today occupied the Neolithic populace and succeeding Bronze Age immigrants for the next 1,600 years. This involved setting up and then repeatedly shuffling around several kinds of boulders or megaliths. The first, around 2,600 BCE, were 2 to 3 tonne blocks mainly of igneous rock (the ‘bluestones’), now known to have originated from outcrops of Ordovician volcanics in Pembrokeshire about 230 km to the west. Next to arrive was a 6 tonne grey-green sandstone slab, now lying flat (hence its being named the ‘Altar’ Stone) beneath a fallen, far bigger megalith,. Once thought to be of Welsh provenance – in the Brecon Beacons 150 km to the west – the Altar Stone is now beyond a shadow of doubt to have come from Devonian strata in northern Scotland, possibly Orkney. The final erection of 30 truly enormous ‘sarsens’ to create Stonehenge’s signature circle and inner ‘horseshoe’ of vertical slabs capped by lintels took place between 2,600 to2 400 BCE. Weighing up to 50 tonnes, the sarsens are locally derived from remnants of Lower Eocene (~55 Ma) sands cemented by chemically precipitated silica (SiO2) that once covered much of southern England.
After 1,600 BCE, serious fiddling with the various stones, the bluestones in particular, ceased. The monument may have remained in some form of use during the Iron Age: it could hardly have been ignored. The first record of antiquarian interest is from the late 17th century and continued sporadically until systematic excavation of archaeological features on the Plain got underway during the 19th century and continues to the present.
Much recent literature has concentrated on what Stonehenge was for and how it was built, leading to a rich eclecticism and a little experimentation. But given the size of its stones and the obviously exotic nature of some of them, there have been disputes between those who consider them to have been brought by natural means and those who suggest collective human endeavour. The latter would have involved vast amounts of labour, shifting the bluestones over 250 km, entire community muscle power to drag the locally occurring sarsens about 25 km from their probable source, and a journey of at least 700 km to get the Altar Stone in place. Since none of the stones could conceivably have been moved by river flow, the only natural alternative for their transport is by glacial action.
Such an ice-transport theory rests on at least one of the several known advances of Pleistocene ice sheets having reached as far south as Salisbury Plain and deposited upon it glacial till that contains material from NE Scotland and South Wales. The most obvious indicators of glacial transport are large erratic boulders strewn far from their source down a previous ice stream that their distribution helps to reconstruct. In Northern Britain a great many megaliths that people erected long ago are glacial erratics of one kind or another. Of course, glacial tills contain grains of all sizes ripped and ground from the course of glacial flow. No so obvious, but equally capable of revealing transportation paths. After ice sheets melt, the till that they dump is eroded so that exotic rock and mineral grains enter drainage systems, some to remain in stream sediments. Two geologists based at Curtin University in Perth, Western Australia collected river sands from four active drainage systems on Salisbury Plain to test the glacial-transport hypothesis for the Stonehenge megaliths (Clarke, A.J.I. & Kirkland, C.L. 2026. Detrital zircon–apatite fingerprinting challenges glacial transport of Stonehenge’s megaliths. Communications Earth & Environment, v. 7, article 54; DOI: 10.1038/s43247-025-03105-3).
Using standard mineral-separation techniques – removal of low-density minerals (mainly quartz and feldspar) and those that are magnetic – Anthony Clarke and Christopher Kirkland mounted and polished samples of the remaining high-density grains embedded in resin. Using automated X-ray spectroscopy they identified grains of two minerals, zircon and apatite, that can be dated using uranium and lead isotopes. Zircons are virtually absent from the underlying Chalk although phosphorus-rich horizons in that rock sometimes contain apatite, a complex calcium phosphate. Both minerals are commonly found in igneous and metamorphic rocks and, being chemically resistant and hard, are often present in sediments derived by erosion of such parent rocks. The authors analysed U-Pb isotopes using laser ablation plasma mass spectrometry of suitable grains of each mineral. The U-Pb data from 250 apatite grains revealed a dominant age peak at 60 Ma, roughly the base of the once overlying Palaeogene sediments. Far fewer grains hint at older ages (175, 215, 300 and 625 Ma) in the Mesozoic, Palaeozoic and Neoproterozoic. The 550 analysed zircons span an age range from the Silurian to Palaeoproterozoic (432 to 1870 Ma), with a few outliers as young as 285 Ma and as old as 3396 Ma.
These data seem to suggest that they can support virtually any glacial transport hypothesis, including that of the Altar Stone, let alone the Stonehenge bluestones. However, that would be to misunderstand the complexity of sediment transport in relation to their original provenance. Erosion from a bedrock source leads to transport and deposition in sedimentary rock. Later uplift and erosion of that secondary host rock is followed by later sediment transport to another rock repository and so on and so forth through the entire geological history of Britain, across its jumble of many tectonic terranes and the effects of numerous orogenic episodes! The Salisbury Plain chalk lands were covered by Palaeogene sedimentary rocks of the London Basin. And, lo and behold, one of those younger sediments, the Thanet Formation sandstones, tell much the same U-Pb story as do the modern river sediments of Salisbury! Those Palaeocene sands elsewhere directly overlie the Chalk and, in some localities on Salisbury Plain, still do today in the form of the chemically cemented sarsens. About 50 Ma ago (early Eocene) the Palaeocene rocks and those beneath were broadly buckled by the outermost ripples of the Alpine orogeny. Once eroded from above the Plain they would certainly have delivered that signature to the mercy of subsequent back and forth river transport. And indeed the sarsens, hard to miss in that landscape, perhaps still do so. Yet no one has thought to examine their content of heavy-mineral grains.
It does seem to me that the authors, perhaps inadvertently, walked into a sedimentological minefield in a vain attempt to put the lid on the fractious debate about human- versus glacial-transport of the Stonehenge megaliths. It is not their data that fling down a ‘challenge’ to the latter hypothesis (see their Conclusions), but the widely accepted absence of even the tiniest nugget of bluestone or Devonian sandstone on the vast and heavily excavated ritual landscape of Salisbury Plain, or indeed in the gravels of the streams that currently drain the Plain. But this where the plot thickens. A recent paper by one of the proponents of the glacial hypothesis (John, B.S. 2024. A bluestone boulder at Stonehenge: implications for the glacial transport theory. E&G Quaternary Science Journal v. 73, p. 117-134;DOI: 10.5194/egqsj-73-117-2024) describes a small piece of bluestone (22 × 15 × 10 cm) that was found during excavations at Stonehenge in 1924 and mysteriously ‘rescued’ by a Robert Newall and stored in his attic for almost 50 years, eventually examined by geologists and then returned to the attic. In 1976, two years before his death Newall passed it to the curator of Salisbury Museum ‘for safe keeping’. Brian John claims that its shape and surface texture suggests glacial transport. It also has several percussion scars suggesting that it had been worked, perhaps by someone hoping to make a stone tool. Unsurprisingly, Johns succeeded in provoking a storm of criticism from archaeologists largely of the human-transport wing of the controversy. And then there is the Mumbles Erratic, found at the eponymous Mumbles headland to the west of Swansea Bay. It too looks like a ‘bluestone’, but is it an erratic or from a Neolithic ship wreck carrying bluestones from Pembrokeshire?
Maximum extent of glaciation in SW Britain during the Anglian Stage 478 to 424 ka ago (Credit: Wikipedia Commons)
A great deal of work by British glaciologists has established the flow patterns and extent of major ice sheets, but only for four onshore, even though there is offshore evidence for repeated glaciation back as far as 2.5 Ma ago. The most extensive of these was the Anglian Stage between 478 and 424 ka ago. The figure above shows that the Irish Sea Glacier did not reach the Stonehenge area, but it did cross Pembrokeshire to reach Somerset on the eastern side of the Bristol Channel. Bluestone erratics may have been much more easily available than blocks hewn at their source in SW Wales, an hypothesis that is currently in vogue. Nope, the quest is not over …
These days only a fool or a scoundrel would deny anthropogenic global warming and its primary outcome of inevitable sea-level rise. Yet no agency, either national or international, has set out to attempt a detailed global assessment of coastal vulnerability. There is no shortage of relevant data to do that – from remote sensing, digital elevation models, simulations of tides and wave height from meteorological data and much else. Thankfully, a team of geomorphologists, climate scientists, sociologists and economists from The Netherlands and France, led by Vindhya Basnayake of the University of Twente, The Netherlands, have taken up the challenge (Basnayake, V. et al. 2026. A global assessment of coastal vulnerability and dominant contributors. Nature Communications, in-press manuscript; DOI: 10.1038/s41467-025-67275-6).
About 10% of the world’s population – a bit less than a billion – live in coastal zones at less than 10 m elevation above mean sea level, and two-fifths that may bear the brunt of future rise. Coastal flooding and erosion threaten landforms, ecosystems and built infrastructure. Both physical effects of sea-level rise potentially may disrupt population centres, livelihoods and marine and coastal industries. More frequent and severe storms driven by global warming are also expected to increase the frequency and intensity of coastal hazards over time. Basnayake et al. have developed a Coastal Vulnerability Index (CVI) to express the hazard presented by future flooding and erosion to all coastal areas. The CVI is based on geomorphology, geology, coastal slope, coastal relief, wave height, and relative sea level change. It also integrates the local adaptive capacity and community resilience from socioeconomic and geopolitical data. Importantly CVI values are calculated at 1 km intervals along the global coastline at over 350 thousand locations. The approach used by the team incorporates from previous analyses time series for wave and tide heights and for changing sediment supply. The fine spatial resolution of data allows for identification of critical micro-regions – even within generally less vulnerable countries. Such a nuanced approach shows up the complexity of coastal risk that one-size-fits-all approaches are destined to miss.
Steep coastal slopes are less vulnerable than gentle ones, which allow greater penetration by marine hazards. The more rugged coastal terrain, the less vulnerable the coast is by acting like a large scale breakwater. Mean wave height controls the energy impinging on a coast, and is affected by wave ‘fetch’, so that ocean-facing coasts are more vulnerable than more enclosed locations. Offshore seismicity, as in island arcs, increases vulnerability to tsunamis. Tidal range has a counterintuitive effect, large ranges reducing the time a coast is in direct contact with the sea, whereas low ranges place the sea next to land for much longer. Although global sea level is destined to rise uniformly, some coasts are rising through tectonic or glacio-eustatic uplift, while others are actively subsiding; so relative sea-level change is used to address vulnerability. Other considerations assessed by Basnayake et al. are subsidence due to coastal groundwater extraction, the presence of protective coastal vegetation such as mangroves, and the influence of deltas and estuaries.
Coastal vulnerability by country: dark blue – very low; green – low; yellow – moderate; orange – high; dark red – very high. (Credit: Basnayake et al. Fig 2a)
The figure above summarises the results of the CVI study on a country-by-country basis. Eurasia is surprisingly the least vulnerable continent in this respect, especially Britain and Norway that are so exposed to the fierce North Atlantic. That is partly due to those countries high adaptive capacity and communal resilience, but mainly to their rugged and deeply indented western coasts; a legacy of glaciation. It’s important to note that coloration on the figure can be misleading. For instance, the higher resolution data pinpoint extremely high vulnerability of stretches of coast dominated by low-lying deltas, such as those of Pakistan, India, Myanmar and SE Asia. Equally surprising is the high vulnerability of North America at similar latitudes; somewhat ironic for the heartland of climate-change denial. High resolution also points to counterintuitive hazards; for instance coastal defences sometimes exacerbate vulnerability by increasing erosion on nearby undefended stretches and by hindering sediment movement. Increased onshore infrastructure boosts runoff and erosion in the coastal realm and displaces natural buffers, such as coastal forest, to storm surges: perhaps partly responsible for the high vulnerability of coasts around the hurricane belt of the Gulf of Mexico and Caribbean. Of the nineteen countries with greatest vulnerability 12 are in West Africa and NE South America and 2 in the Caribbean area. The paper is well worth reading, to get a flavour of the complexity involved and the vast magnitude of the task of ameliorating risk of coastal devastation that lies ahead in the next decades.
For decades, most of the news concerning our deep ancestry emerged from discoveries in sub-Saharan Africa at sites in Zambia, Tanzania, Kenya, South Africa, Ethiopia. The first week of 2026 decisively shifted that focus northwards to Chad and Morocco in two separate publications.
In 2002 ago the world of palaeoanthropology was in turmoil following the first discovery of fragments of what was then thought to be a hominid, or great-ape, cranium in Chad dated at around 7 Ma ago (Brunet, M. and 37 others 2002. A new hominid from the Upper Miocene of Chad, central Africa. Nature, v. 4418, p. 145-151;DOI:10.1038/nature00879). When pieced together the cranium looked like a cross between that of a chimpanzee and an australopithecine. Some suggested that the creature may have been a ‘missing link’ between the hominids and hominins; perhaps the ultimate ancestor of humans. Sahelanthropus tchadensis (nicknamedToumaï or ‘hope of life’ in the local Goran language) was undoubtedly enigmatic. The ‘molecular-clock’ age estimate for the branching of hominins from a common ancestor with chimpanzees was, in 2002, judged to be two million years later the dating of Sahelanthropus, so controversy was inevitable. Another point of contention was the size of Sahelanthropus’s canine teeth: too large for australopithecines and humans, but more appropriate for a gorilla or chimp. Moreover, Toumaï showed no indisputable evidence for having been bipedal. The Chadian site subsequently yielded three lower jaw bones and a collection of teeth, a partial femur (leg bone) and three fragmentary ulnae (forearm bones). The finds suggested that as many as five individuals had been fossilised. The femur gave an unresolved hint of an upright gait, yet the ulnas suggested Toumaï might equally have been arboreal; as could also be said for the australopithecines.
Reconstructed skull of Sahelanthropus tchadensis. (Credit: Didier Descouens, University of Toulouse)
All the limb bones of Toumaïhave now been anatomically compared with those of hominins and apes (Williams S.A. et al. 2026. Earliest evidence of hominin bipedalism in Sahelanthropus tchadensis.Science Advances, v. 12, article eadv0130; DOI: 10.1126/sciadv.adv0130). Scott Williams of New York University and co-workers from other US institutions show that although the leg bones are much the same size as those of chimpanzees, their proportions were more like those of hominins. They also showed features around the knees and hips needed for bipedalism and an insertion point for a tendon for the gluteus maximus muscle (buttock) vital for sustained upright locomotion, similar to the femurs of Orrorin tugenensis (see: Orrorin walked the walk; May 2008) and Ardipithecus ramidus. Unfortunately, an intact Sahelanthropus cranium showing a foramen magnum – where the skull attaches to the spine – continues to elude field workers. Its position distinguishes upright gait definitively.
The second new advance concerns the joint ancestry of Neanderthals, Denisovans and anatomically modern humans (AMH), whose ancient genetics crudely suggest a last common ancestor living between 765 to 550 ka. This had previously been attributed to Homo antecessor found in the Gran Dolina cave at Atapuerca in northern Spain, roughly dated between 950 ka and 770 ka. (Incidentally, Gran Dolina has yielded plausible evidence of cannibalism). A novel possibility stems from hominin fossils excavated from a cave in raised-beach sediments near Casablanca in Morocco (Hublin, JJ. and 28 others, 2026 Early hominins from Morocco basal to the Homo sapiens lineage. Nature, v. 649 ; DOI: 10.1038/s41586-025-09914-y). The fossil-bearing sediments contain evidence for a shift in the Earth’s magnetic field (the Brunhes–Matuyama reversal) dated at 773 ka, much more precisely than the Atapuerca age span for H. antecessor. Jean-Jacques Hublin of CNRS in Paris and his multinational colleagues report that the fossils are similar in age to H. antecessor, yet are morphologically distinct, displaying a combination of primitive traits and of ‘derived features reminiscent of’ later Neanderthal, Denisovan and AMH fossils. The differences and shared features suggest that there may have been genetic exchanges between the Moroccan and Iberian population over a considerable period. The most obvious route would have been across the Straits of Gibraltar, but would have required some kind of water craft. An important question is ‘which population gave rise to the other?’
Artistic reconstruction of a juvenile Homo antecessor, Based on skeletal remains from Gran Dolina Cave
Larger and more robust hominin remains in Algeria dated at 1,000 ka – H. heidelbergensis? – resemble those found near Casablanca. They may have evolved to the latter. Similar possible progenitors to Iberian Homo antecessor have yet to be found in Western Europe. Homo erectus appeared in Georgia and Romania between 2.0 and 1.9 Ma, but the intervening million years or more have yielded no credible European forebears of H. antecessor. For the moment, incursion of a North African population into Europe followed by sustained contact is Hublin et al’s favoured hypothesis, rather than a European origin for Homo antecessor. For Neanderthals and Denisovans to have originated from such an African group, as has been suggested, requires finds of African fossils with plausible resemblance to what are predominantly Eurasian groups. The Iberian population migrated far and wide in Western Europe, as witnessed by stone tools and footprints dating to between 950 to 850 ka in eastern England. So it is equally possible that the Iberian group were progenitors of Neanderthals and Denisovans in Eurasia itself. At least for the moment, ancient genomes of the two H. antecessor groups are unlikely to be found in either Iberian or African fossils of the same antiquity. But, as usual, that will not stifle debate: a resort to the adage ‘absence of evidence is not evidence of absence’ seems appropriate to several research teams!
The oldest anatomically modern human fossils dated at ~300 ka, were also discovered in Morocco (see: Origin of anatomically modern humans, June 2017). Their isolation in the NW corner of the African continent poses a similar conundrum, as since then such beings went on to occupy wide areas of sub-Saharan Africa and then the world.
I must have been about ten years old when I last saw a ‘lucky dip’ or ‘bran tub’ at a Christmas fair. You paid two shillings (now £0.1) to rootle around in the bran for 30 seconds and grab the first sizeable wrapped object that came to hand:. In my case that would be a cheap toy or trinket, but you never knew your luck as regards the top prize. There is a small asteroid called 101955 Bennu, about half a kilometre across, whose orbit around the Sun crosses that of the Earth. So it’s a bit scary, being predicted to pass within 750,000 km of Earth in September 2060 and has a 1 in 1,880 chance of colliding with us between 2178 and 2290 CE. Because Earth-crossing asteroids are a cheaper target than those in the Asteroid Belt, in 2016 NASA launched a mission named OSIRIS-REx to intercept Bennu, image it in great detail, snaffle a sample and ultimately return the sample to Earth for analysis. This wasn’t a shot in the dark, as a lot of effort and funds were expended to target and then visit Bennu. But unlike me at the fair ground, NASA will be very happy with the outcome.
The asteroid Bennu, showing its oblate spheroidal shape, due to rotation, and its rubbly structure. Source: NASA/Goddard/University of Arizona via Wikimedia Commons
Bennu is a product of what might be regarded as ‘space sedimentation’, indeed a kind of conglomerate, being made up of boulders up to 58 m across set in gravelly and finer debris or ‘regolith’. High-resolution images revealed veins of carbonate minerals in the boulders. They suggest hydrothermal activity in a much larger parent body – one of many proto-planets accreted from interstellar gas and dust as the Solar System first began to form over 4.5 billion years ago. Its collision with another sizeable body knocked off debris to send a particulate cloud towards the Sun, subsequently to clump together as Bennu by mutual gravitational attraction. The carbonate veins can only have formed by circulation of water inside Benno’s parent.
The ‘REx’ in the mission’s name is an acronym for ‘Regolith Explorer’. Sampling was accomplished on 20 October 2020 by a soft landing that drove a sample into a capsule, and then OSIRIS-REx ‘pogo-sticked’ off with the booty. The capsule was dropped off by parachute after the mission’s return on 24 September 2023, in the manner of an Amazon delivery by drone to a happy customer. So, you can understand my ‘lucky dip’ metaphor. And NASA certainly was ‘lucky’ as the contents turned out to be astonishing, as related two years later by the analytical team in the US, led by NASA’s Angel Mojarro (Mojarro, A. et al. 2025.Prebiotic organic compounds in samples of asteroid Bennu indicate heterogeneous aqueous alteration. Proceedings of the National Academy of Science, v. 122, article e2512461122; DOI: 10.1073/pnas.2512461122).
The rock itself is made from bits of carbonaceous chondrite, the most primitive matter orbiting the Sun. It contains fifteen amino acids, including all five nucleobases that make up RNA and DNA – adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U) – as in AUGC and AGCT. Benno’s complement of amino acids included 14 of the 20 used by life on Earth to synthesise proteins. The fifteenth, tryptophan, has never confidently been seen in extraterrestrial material before. Alkylated polycyclic aromatic hydrocarbons, also found in Bennu, are seen in abundance in interstellar gas clouds and comets by detecting their characteristic fluorescence when illuminated by mid-infrared radiation from hot stars using data from the Spitzer and James Webb Space Telescopes. These prebiotic organic compounds have been suggested to have played a role in the origin of life, but exposure to many produced by human activities are implicated in many cancers and cardiovascular issues. A second paper by Japanese biochemists and colleagues from the US was also published in early December 2025 (Furukawa, Y. and 13 others 2025. Bio-essential sugars in samples from asteroid Bennu. Nature Geoscience, v. 12, online article; DOI: 10.1038/s41561-025-01838). The authors identified several kinds of sugars in a sample from Bennu, including ribose – essential for building RNA – and glucose. Bennu also contains formaldehyde – a precursor of sugars – perhaps originally in the same brines in which the amino acids formed.
Yet another publication coinciding with the aforementioned two focuses on products of the oldest event in the formation of Bennu: its content of pre-solar grains (Nguyen, A.N. et al. 2025. Abundant supernova dust and heterogeneous aqueous alteration revealed by stardust in two lithologies of asteroid Bennu. Nature Astronomy, v. 9, p. 1812-1820; DOI: 10.1038/s41550-025-02688-3). In 1969 a 2 tonne carbonaceous chondrite fell near Allende in Mexico. The largest of this class ever found, it contained tiny, pale inclusions that eight years of research revealed to represent materials completely alien to the Solar System. They are characterised by proportions of isotopes of many elements that are very different from those in terrestrial materials. The anomalies could only have formed by decay of extremely short-lived isotopes that highly energetic cosmic rays produce in a manner analogous to neutron bombardment: they are products of nuclear transmutation. It is possible to estimate when the parent isotopes produced the anomalous ‘daughter’ products. One study found ages ranging from 4.6 to 7.5 Ga: up to three billion years before the Solar System began to form. It is likely that the grains are literally ‘star dust’ formed during supernovae in nearby parts of the Milky Way galaxy. Bennu samples contain six-times more presolar grains than any other chondritic meteorites. Nguyen et al. geochemically teased out grains with different nucleosynthetic origins. These ancient relics point to Bennu’s formation in a region of the presolar cloud that preceded the protoplanetary disk and was a mix of products from several stellar settings.
The results from asteroid Bennu support the key idea that that amino acid building blocks for all proteins and the nucleobases of the genetic code, together with other biologically vital compounds arose together in a primitive asteroid. Its evolution provided the physical conditions, especially the trapping of water, for the interaction of simpler components manufactured in interstellar clouds. Such ‘fertile’ planetesimals and debris from them almost certainly accreted to form planets and endowed them with the potential for life. What astonishes me is that Bennu contains the five nucleobases used in terrestrial genetics and 70% of the amino acids from which all known proteins are assembled by terrestrial life. But, as I try to explain in my book Stepping Stones: The Making of Our Home World, life as we know it arose, survived and evolved through a hugely complex concatenation of physical and chemical events lasting more than 4.5 billion years. The major events and the sequences in which they manifested themselves may indeed have been unique. Earth is a product of luck and so are we!
As early as 3.4 Ma bones with cut marks first appear in Ethiopia suggesting that meat had by then entered the hominin diet. Access to such a rich source of protein has been suggested to have encouraged the evolution of larger hominin brains. By around2.4 Ma ago it may have led to the first known human species (Homo habilis) with a brain larger than those of australopithecines. Homo ergaster, with a significantly larger brain size, first appeared at about 1.8 Ma. As the probable inventor of bifacial stone tools and being the first hominin to leave Africa, H. ergaster needed greater cognitive abilities.It is quite likely that discovery of means to cook food then provided a further boost to human evolution. Cooking unfolds the proteins in meat and also breaks down the constituents of raw vegetables making both more palatable and easier to digest. Also, many potentially nutritious tubers are toxic if not processed and cooked. Another evolutionary advantage is that such an increased uptake of nutrients without needing an increase in successful hunting and foraging reduces the length of the ‘working day’. Fire itself provides warmth, protection from large predators and light, which further increases the time available for social and mental activities.
Being able to cook demands the controlled use of fire. But when was fire first harnessed? After 3 Ma the climate in East Africa cooled to open up dry savannah, prone to wildfires. Finding naturally roasted carcases may have been an incentive for hominins to use smouldering patches to cook meat. The next breakthrough would have been carrying embers to light fires elsewhere. The earliest tentative evidence for such a fire was discovered at Swartkrans in South Africa. Crudely dated between 2 to 1 Ma, it was a reddened patch of soil containing charred, cut marked bones and burnt biface tools. Definite evidence only appears with the burnt teeth of large carp-like fish from a 780 ka site found at Gesher Benot Ya’aqob in Israel. So, archaeological evidence for cooking is very rare. Imagine, then, the excitement of a group of archaeologists from the British Museum and the Natural History Museum in London and several universities in the UK and Netherlands at finding a small, 400 ka-old Neanderthal hearth at Barham in Kent, England containing direct evidence of how the fire was lit (Davis, R. and 14 others 2025.Earliest evidence of making fire. Nature, online advance publication. DOI: 10.1038/s41586-025-09855-6). Apparently, there are signs that fires had been lit at the hearth on twelve or more occasions. So, clearly, the spot was used regularly by Neanderthals.
Striking sparks with flint and pyrite. Credit: Craig Williams, The Trustees of the British Museum
The burnt-earth site yielded fire-cracked flint hand axes and fragments of the mineral pyrite (FeS2). Pyrite, named from the Greek word for ‘fire’ – i.e. ‘fire stone’ – creates showers of sparks when scraped with a hard, sharp tool. Struck into dry grass tinder the sparks cause it to smoulder and then burst into flame when blown on. This approach has been used throughout historic times. Interestingly, pyrite is not found in local rocks and had to have been brought from outcrops of Cretaceous Chalk 15 km away, which is also a major source of flint for stone tools. Not only had the Neanderthal ‘campers’ mastered this fire lighting method, they knew where to get the minerals required. With these skills, they could have lit fires on demand wherever they were; to cook, keep warm, light the night and keep predators at bay. The find is a lucky one, for pyrite eventually oxidises in damp air. The skill may have been acquired long before 400 ka. Yet, as most school children used to know, you can also produce fire, or at least embers and smoke(!), by abrading softwood (a schoolroom desk top) with hardwood (a wooden ruler) … Other abrasive methods are available, but none so handy as a flint-pyrite tinderbox.
See also: Smith, K.N. 2025. Getting lit: This is the oldest evidence of people starting fires. Ars Technica; 10 December 2025
The emergence of the eukaryotes – of which we are a late-entry member – has been debated for quite a while. In 2023 Earth-logs reportedthat a study of ‘biomarker’ organic chemicals in Proterozoic sediments suggests that eukaryotes cannot be traced back further than about 900 Ma ago using such an approach. At about the same time another biomarker study showed signs of a eukaryote presence at around 1050 Ma. Both outcomes seriously contradicted a ‘molecular-clock’ approach based on the DNA of modern members of the Eukarya and estimates of the rate of genetic mutation. That method sought to deduce the time in the past when the last eukaryotic common ancestor (LECA) appeared. It pointed to about 2 Ga ago, i.e. a few hundred million years after the Great Oxygenation Event got underway. Since eukaryote metabolism depends on oxygen, the molecular-clock result seems reasonable. The biomarker evidence does not. But were the Palaeo- and Mesoproterozoic Eras truly ‘boring’? A recent paper by Dietmar Müller and colleagues from the Universities of Sydney and Adelaide, Australia definitely shows that geologically they were far from that (Müller, R.D. et al. 2025. Mid-Proterozoic expansion of passive margins and reduction in volcanic outgassing supported marine oxygenation and eukaryogenesis. Earth and Planetary Science Letters, v. 672; DOI: 10.1016/j.epsl.2025.119683).
Carbon influx (million tons per year) into tectonic plates and into the ocean-atmosphere system from 1800 Ma to present. The colour bands represent: total carbon influx into the atmosphere (mauve); sequestered in tectonic plates (green); net atmospheric influx i.e. total minus carbon sequestered into plates (orange). The widths of the bands show the uncertainties of the calculated masses shown as darker coloured lines.
From 1800 to 800 Ma two supercontinents– Nuna-Columbia and Rodinia – aggregated nearly all existing continental masses, and then broke apart. Continents had collided and then split asunder to drift. So plate tectonics was very active and encompassed the entire planet, as Müller et al’s palaeogeographic animation reveals dramatically. Tectonics behaved in much the same fashion through the succeeding Neoproterozoic and Phanerozoic to build-up then fragment the more familiar supercontinent of Pangaea. Such dynamic events emit magma to form new oceanic lithosphere at oceanic rift systems and arc volcanoes above subduction zones, interspersed with plume-related large igneous provinces and they wax and wane. Inevitably, such partial melting delivered carbon dioxide to the atmosphere. Reaction on land and in the rubbly flanks of spreading ridges between new lithosphere and dissolved CO2 drew down and sequestered some of that gas in the form of solid carbonate minerals. Continental collisions raised the land surface and the pace of weathering, which also acted as a carbon sink. But they also involved metamorphism that released carbon dioxide from limestones involved in the crustal transformation. This protracted and changing tectonic evolution is completely bound up through the rock cycle with geochemical change in the carbon cycle.
From the latest knowledge of the tectonic and other factors behind the accretion and break-up of Nuna and Rodinia, Müller et al. were able to model the changes in the carbon cycle during the ‘boring billion’ and their effects on climate and the chemistry of the oceans. For instance, about 1.46 Ga ago, the total length of continental margins doubled while Nuna broke apart. That would have hugely increased the area of shallow shelf seas where living processes would have been concentrated, including the photosynthetic emission of oxygen. In an evolutionary sense this increased, diversified and separated the ecological niches in which evolution could prosper. It also increased the sequestration of greenhouse gas through reactions on the flanks of a multiplicity of oceanic rift systems, thereby cooling the planet. Translating this into a geochemical model of the changing carbon cycle (see figure) suggests that the rate of carbon addition to the atmosphere (outgassing) halved during the Mesoproterozoic. The carbon cycle and probable global cooling bound up with Nuna’s breakup ended with the start of Rodinia’s aggregation about 1000 Ma ago and the time that biomarkers first indicate the presence of eukaryotes.
Simplified structures of (a) a prokaryote cell; (b) a simple eukaryote animal cell. Plants also contain organelles called chloroplasts
So, did tectonics play a major role in the rise of the Eukarya? Well, of course it did, as much as it was subsequently the changing background to the appearance of the Ediacaran animals and the evolutionary carnival of the Phanerozoic. But did it affect the billion-year delay of ‘eukaryogenesis’ during prolonged availability of the oxygen that such a biological revolution demanded? Possibly not. Lyn Margulis’s hypothesis of the origin of the basic eukaryote cell by a process of ‘endosymbiosis’ is still the best candidate 50 years on. She suggested that such cells were built from various forms of bacteria and archaea successively being engulfed within a cell wall to function together through symbiosis. Compared with prokaryote cells those of the eukaryotes are enormously complex. At each stage the symbionts had to be or become compatible to survive. It is highly unlikely that all components entered the relationship together. Each possible kind of cell assembly was also subject to evolutionary pressures. This clearly was a slow evolutionary process, probably only surviving from stage to stage because of the global presence of a little oxygen. But the eukaryote cell may also have been forced to restart again and again until a stable form emerged.
One of the longest-lived hominin species that we know of was Paranthropus boisei, remains of which occur in East African sediments between 2.6 and 1.3 Ma. Others, including our own species, lasted nowhere near as long, except perhaps for Homo erectus who emerged around 1.9 Ma ago and is believed by some to have lingered on in Java until about 112 ka ago. However, when the unresolved muddle in the Middle Pleistocene of similar-looking hominin fossils is eventually unravelled – as now seems to be on the cards – that may limit the range of H. erectus to 1.9 -1.0 Ma. Paranthropoid remains are easily distinguished from those of their contemporary hominins – australopithecines and early species of Homo – being extremely robust compared with the ‘gracile’ members of the human line. They were also bipedal, but their fossil skulls are distinctive: massive teeth and jaws, and a bone crest on top of the cranium to which very powerful chewing muscles were attached. Once regarded as a sort of upright gorilla with vegetarian habits, evidence has accumulated since their first discovery that they may have been far more remarkable.
Reconstruction of a Paranthropus head (Credit: Jerry Humphrey, Pinterest)
The earliest paranthropoid was P. aethiopicus from Ethiopia, dated at around 2.7 to 2.3 Ma, and believed to be the common ancestor of P. boisei and P. robustus found in Tanzania and South Africa respectively. Stone and bone tools associated with paranthropoid remains have been found in South and East Africa, some of which show signs of having been burnt. The connection between paranthropoids and both tool- and fire-making is clearly impossible to verify with certainty, and so too for their known association with australopithecine remains – or even the earliest known humans (Homo habilis) for that matter. Palaeoanthropologists are not likely to find a near-complete skeleton of any of these candidates with a tool grasped in the remains of a hand! The issue can be partly resolved if it can be shown that a fossil hand was capable of making and using tools. The fabled ‘opposable thumb’ that is capable of touching the tips of all four fingers is essential for the necessary ‘precision grip’. Isolated, 2 Ma-old thumb bones probably able to do that were found in the famous Swartkrans Cave in South Africa, but with no clue as to which hominin species had yielded them. In fact had that matter been resolved there and then, it would be not take the hominin story very far, simply because evidence for tool use – tools and cut marks on bone – goes back as far as 3.3 Ma, again with more than one candidate for the usefully endowed hominin species.
The left hand of Paranthropus boisei reconstructed from individual bones, palm-up on the left, palm down on the right. Credit: Mongle et al, Fig 3.
Remarkably, a group of scientists from the US, Canada, Australia, South Africa and Kenya have indeed unearthed from 1.5 Ma sediments on the shore of Lake Turkana in Kenya a near-complete left hand associated with cranial bones and teeth from Paranthropus boisei (Mongle,C.S. and 29 others 2025. New fossils reveal the hand of Paranthropus boisei. Nature v. 647, p. 944–951; DOI: 10.1038/s41586-025-09594-8). It is clear that the P. boisei hand shared some of the manipulative capacity of modern human hands, though it bears some resemblance to gorilla hands. That hand was probably nimble enough to make and use simple stone tools. It would also have had a powerful grip, sufficient for climbing and wielding a large stick. Yet again, it does not indicate which species first adopted tool making and use.
There are several interesting possibilities. It may be that a form of convergent evolution enabled two separate genera to become capable of such skills and the intellect to put them to use: tools, however simple, confer enormous evolutionary advantages. Had the antecedents of humans – presumably a species of Australopithecus – been the first, paranthropoids may have observed and adopted tools or vice versa. Just as possible, the – as yet unknown – common ancestor of both may have made this fundamental leap, which would have benefitted both vegetarian and omnivorous descendants. In that case the physiology of each group may have diverged with their lifestyles. Eating roots and leaves requires considerably more physical effort than getting sufficient protein and fats partly by devouring flesh.
Evidence for the earliest life on Earth has largely relied on finding signs of structures that may have been created during the Archaean Eon by micro-organisms. Actual fossils don’t turn up until the Proterozoic. The most distinctive and diverse of these are members of the Ediacaran fauna dated at around 635 Ma . The oldest widely accepted multi-celled eukaryote fossil was found in 2.1 billion-year old sediments from Gabon (see:The earliest multicelled life; July 2010). There have been a few claims for biogenic material, such as microscopic tubular structures in 3.5 billion-year (Ga) old pillow lavas and 3.2 Ga cherts from South Africa (see: Early biomarkers in South African pillow lavas; April 2004 and Believable Archaean fossils; March 2010) which some researchers dispute. Then there are Archaean stromatolites, which may be evidence for bacterial mats. The oldest of them have been claimed to occur in the famous, 3.77 Ga Isua metasediments of West Greenland. But such early fossils are chance finds, so geochemists have entered the arena with attempts to find irrefutable chemical signatures for life in ancient rocks.
One approach is isotope geochemistry. Carbon isotope data have been widely used, because life processes, such as photosynthesis, result in a deficiency of 13C relative to 12C. This was tried on graphite crystals trapped in sedimentary phosphate minerals from Isua. The results were at first acclaimed as a sign of life at around 3.8 Ga, but then refuted. In 2015 a similar approach was applied to graphite trapped in a 4.1 Ga detrital zircon, seemingly pushing back evidence for life into the Hadean. But zircon is a mineral produced by crystallisation of magma, so the fractionation of carbon isotopes in trapped graphite seem unlikely to shed light on the earliest life. The main drawback to using carbon isotopes is because metamorphism, Fischer-Tropsch mechanisms in hydrothermal environments, and volcanic processes may be responsible for enrichment of lighter carbon isotopes relative to 13C. The relative abundance of the different isotopes of iron in Archaean sediment may give clues to the transient availability of oxygen generated by bacterial photosynthesis that would oxidise soluble Fe2+ to insoluble Fe3+. Promising results were obtained in 2013 from 3.8 Ga banded ironstones at Isua. But doubt was again raised, so the only generally accepted evidence is that of the microfossils found in hydrothermal cherts in Palaeoarchaean pillow lavas from South Africa and Western Australia and the earliest stromatolites, all around 3.4 to 3.5 Ga old. However, recent research may have opened up a more convincing route to tracking down ancient life forms –actual organic molecules that make up or are produced by organisms.
Michael Wong and co-workers at the Carnegie Institution for Science in Washington, DC, USA together with other colleagues from the US, Austria, Canada, China, Belgium, Norway, Australia, the UK and France used artificial intelligence to wade through the results of geochemical analysis of over 400 ancient and modern carbon-bearing samples. (Wong, M.I. and 28 others 2025. Organic geochemical evidence for life in Archean rocks identified by pyrolysis–GC–MS and supervised machine learning. Proceedings of the National Academy of Sciences, v. 122, article e2514534122: DOI: 10.1073/pnas.2514534122). Their objective was to track the presence of organically derived molecules as far back as possible. Their approach bears a passing resemblance to that used to build genomes of ancient fossils from broken bits of DNA that reside in them. Like DNA, bio-molecules degrade over time, but leave fragments in rocks that can be detected using pyrolysis gas chromatography and mass spectrometry. In itself PGC-MS is not especially new, but using artificial intelligence (machine learning) on a massive date set certainly is: perhaps the first major trial of AI in geology.
Percentages of samples designated as biogenic by Wong et al’s AI analysis. Credit: Wong et al, Fig 4
Their samples were not just ancient rocks going back into the Archaean as far back as 3.5 Ga, but included modern biological material, meteorites presumed to have been devoid of life since their origin in pre-solar system times and synthetic samples. Wong et al divided 272 samples with known biological affinities into 9 groups to train the AI algorithm. The analytical method breaks down organic and inorganic carbonaceous materials into fragments of molecules: the opposite of DNA sequencing. When subjected to PGC-MS each type of living organism, from bacteria to animals produces a distinct pattern of molecular fragments. The AI analysis is based on a sophisticated statistical algorithm being trained to recognise ‘debris’ from organic and inorganic carbonaceous compounds according to each sample’s geochemical ‘fingerprint’. Part of the ‘training’ was based on sediments that contain irrefutable fossil samples from as far back in time as the Mesoproterozoic (1000 Ma). Another part was based on definitely inorganic materials, such as carbonaceous meteorites. AI proved able to distinguish biological from inorganic material with a probability up to 0.9 (90%). These results suggested that older, more biologically uncertain material could be assessed.
The AI was able to distinguish general biogenic affinities from inorganic ones in samples with decreasing success going back in time: as high as 0.93 in the Phanerozoic to 0.47 in the Archaean. The oldest samples that reached the probability threshold for this distinction (0.6) were 3.3 Ga cherts from the Barberton Greenstone Belt in South Africa. Another distinction between photosynthetic and and non-photosynthetic affinities among the samples that ‘passed’ as probably biotic reached the 0.6 probability threshold at 2.5 Ga for a sample from South Africa. Non-photosynthetic, but still probably biotic samples extend as far back as 3.5 Ga in South African and Western Australian Greenstone Belts.
Although Wong et al’s preliminary exploration with their novel approach doesn’t take us beyond the current 3.4 to 3.5 Ga age for the earliest tangible suggestions of life. However, they note ‘…our sample inventory is notably lacking in ancient abiogenic samples’. This is a good indication of the promise for further progress that the approach offers. Previous research has sought intact biogenic molecules, with not a great deal of luck, over several decades. Their final conclusion is ‘…information-rich attributes of ancient organic matter, even though highly degraded and with few if any surviving biomolecules, have much to reveal about the nature and evolution of life.’ They have opened a very important avenue in palaeobiological research , as their methodology seems capable of fine tuning to all manner of pro- and eukaryote biochemical distinctions. It could even be used with extraterrestrial material, should we ever get any …
Bioturbated ‘pipe rock’ of the basal Cambrian sandstones of NW Scotland. Credit: British Geological Survey photograph P531881
About 530 Ma ago most of the basic body plans of today’s living organisms can be detected as fossils, i.e. preserved hard parts. Yet studies of trace fossils (ichnofossils) – marks left in sediments by active soft bodied creatures suggest that many modern phyla arose before the start of the Cambrian (~539 Ma), as early as 545 Ma. So the term ‘Cambrian explosion’ seems to be a bit of a misnomer on two counts: it lasted around 15 Ma and began before the Cambrian. Preceding it was the Ediacaran Period that began around 100 Ma earlier in the Neoproterozoic Era. Traces of its eponymous fauna of large soft-bodied organisms are found on all continents, but apparently none of them made it into the Phanerozoic fossil record. Another characteristic of the Ediacaran is that its sedimentary rocks – and those of earlier times – show no signs of burrowing: they are not bioturbated. That may be why the Ediacaran pancake-, bun-, bag- and pen-like lifeforms are so remarkably well preserved. But a lack of burrowing did not extend to the beginning of Cambrian times. The most likely reason why it was absent during the early Ediacaran Period is that sea-floor sediments then were devoid of oxygen so eukaryote animals could not live in them. But the presence of these large organisms showed that seawater must have been oxygenated. Now clear signs of burrowing have emerged from study of Ediacaran rocks exposed in the Yangtze Gorge of Hubei,southern China ( Zhe Chen & Yarong Liu 2025. Advent of three-dimensional sediment exploration reveals Ediacaran-Cambrian ecosystem transition. Science Advances, v. 11, article eadx9449; DOI: 10.1126/sciadv.adx9449).
Tadpole-like trace fossils from the Ediacaran Dengying Formation in the Yangtze Gorge: 5 cm scale bars. The ‘heads’ show tiny depressions suggesting that there maker probed into the sediments as well as foraging horizontally. Credit: Zhe Chen & Yarong Liu; Figs 3B and 3D
Zhe Chen and Yarong Liu of the Nanjing Institute of Geology and Palaeontology and Chinese Academy of Sciences in China examined carbonates of the upper Ediacaran Dengying Formation. This overlies the Doushantuo Formation (550 to 635 Ma), known for tiny fossils of possibly the oldest deuterostome Saccorhytus coronaries; a potential candidate for the ancestor of modern bilaterian phyla. In the Yangtze Gorge locality sediments at this level show only traces of browsing of bacterial mats on the sediment surface; i.e. 2-D feeders. The basal Dengying sediments host clear signs that organisms could then penetrate into the sediments. These 3-D feeders , would have had access to buried organic remains, hitherto unexploited by living organisms. Such animal-sediment interactions would have disturbed and diminished the living microbial mats that held the sediment surface in place, and thus began to dismantle the substrate for the typical Edicaran fauna. Similar 3-D feeders occur throughout the 11 Ma represented by the Dengying Formation to the start of the Cambrian. This beginning of bioturbation heralded a period during which the Ediacaran fauna steadily waned. It also released nutrients into deep water, and opened up new ecological niches for more advanced animals on the seabed. Dissolved oxygen could only slowly enter the sediments since atmospheric and oceanic O2 levels were low. But by the earliest Cambrian it had risen to about 5 to 10% by volume to support many other kinds of burrowing animals that could penetrate more deeply, as witnessed by the abundant sandstones that occur at the base of the Cambrian in Britain.
Artist’s impression of the impact of a roughly Mars-size planet with the proto-Earth to form an incandescent cloud, from part of which the Moon formed.
Geochemists have gradually built a model of the proportions of the 92 naturally occurring elements that characterise the Solar System. It is based on systematic chemical analysis of meteorites, especially the ‘stony’ ones. One hypothesis for Earth formation is that the bulk of it chemically resembles a class of meteorites known as C1 carbonaceous chondrites. But there are important deviations between that and reality. For instance the relative proportions of the isotopes of several elements in meteorites have been found to differ. Because nuclei of all the elements and their individual isotopes have been shown to form in supernovae through nucleosynthesis, such instances are known as ‘nucleosynthetic anomalies’. An example is that of the isotopes of potassium (K), which was investigated by a team of geochemists from the Carnegie Institution for Science in Washington DC, USA and the Chengdu University of Technology, China led by Nicole Nie (Nie, N.X. et al. 2023. Meteorites have inherited nucleosynthetic anomalies of potassium-40 produced in supernovae. Science, v.379, p, 372-376; DOI: 10.1126/science.abn1783).
A measure for the magnitude of this nucleosynthetic anomaly is the ratio between the abundance in a sample of potassium’s rarest (40K) and its most common isotope (39K), divided by the ratio in an accepted standard of terrestrial rock. Since isotopically identical samples would yield a value of 1, the result has 1.0 subtracted from it to emphasise anomalies. Samples that are relatively depleted in 40K give negative values, whereas enriched samples give positive values. This measure is signified by ε40K, ε being the Greek letter epsilon. The authors found significant and variable positive anomalies of ε40K in carbonaceous chondrite (CC) meteorites, compared with non-carbonaceous (NC) meteorites. They also found that ε40K data in terrestrial rocks are quite different from those of CC meteorites. Indeed, they suggested that Earth was more likely to have formed from NC meteoritic material. Clearly, there seems to be something seriously amiss with the hypothesis that Earth largely accreted from C1 carbonaceous chondrites.
The correlation between ε40K and ε100Ru in meteorites (EC – enstatite chondrites, OC – ordinary chondrites; CC – carbonaceous chondrites), Earth and a geochemically modelled proto-Earth. Credit: Da Wang et al., Fig 2
Three of the authors of Nie et al. and other researchers from MIT in Cambridge MA and Scripps Institution of Oceanography in San Diego CA, USA and ETH in Zurich, Switzerland have produced more extensive potassium isotope data to examine Earth’s possible discrepancy with the chondritic Earth hypothesis (Da Wang et al. 2025. Potassium-40 isotopic evidence for an extant pre-giant-impact component of Earth’s mantle. Nature Geoscience, v. 18, online article; DOI: 10.1038/s41561-025-01811-3). To better approximate the bulk Earth’s potassium isotopes they analysed a large number of terrestrial rock samples of all kinds and ages to compare with meteorites of different classes. Meteorites also have variable nucleosynthetic anomalies for ruthenium-100 (ε100Ru). So, ε40K and ε100Ru may be useful tracers with regards to Earth’s history. But, for some reason, the research group did not analyse ruthenium isotopes in the terrestrial samples.
Most samples of igneous rocks from different kinds of Phanerozoic volcanic provinces (continental flood basalts, island arcs, and ocean ridge basalts) showed no evidence of anomalous potassium isotopes. However, some young ocean-island basalts from Réunion and Hawaii showed considerable depletion in 40K. A quarter of early Archaean (>3.5 Ga) metamorphosed basaltic rocks from greenstone belts also showed clear 40K depletion. Yet no samples of granitic crust of similar antiquity showed any anomaly and nor did marine sediments derived from younger continental crust. Even the oldest known minerals – zircon grains from Jack Hills Western Australia – showed no anomalies. The authors suggest that both the anomalous groups of young and very ancient terrestrial basalts show signs that their parent magmas may have formed by partial mantle melting of substantial bodies of the relics of proto-Earth. To account for this anomalous mantle Da Wang et al. suggest from modelling that proto-Earths 40K deficit may have arisen from early accretion of meteorites with that property. Later addition of material more enriched with that isotope, perhaps as meteorites or through the impact with a smaller planet that triggered Moon-formation. That cataclysm was so huge that it left the Earth depleted in ‘volatile’ elements and in a semi-molten state. It reset Earth geochemistry as a result of several processes including the mixing induced by very large-scale melting. No radiometric dating has penetrated that far back in Earth history. However, in February 2004, Alex Halliday used evidence from several isotopic systems (Pb, Xe, Sr, W) to show that about two thirds of Earth’s final mass may have accreted in the first 11 to 40 Ma of its history.
Curiously, none of the hundreds of meteorites that have been geochemically analysed show the level of 40K depletion in the terrestrial samples. Nicole Nie has comments, “… our study shows that the current meteorite inventory is not complete, and there is much more to learn about where our planet came from.”
I’m persuaded to write this by ‘Piso Mojado’. And today – 23rd October – is the anniversary of the Creation of Earth, Life and the Universe in 4004 BCE, according to Archbishop James Ussher (1581-1656) by biblical reckoning, which always tickles me!
A symposium hosted by the Royal Society in 1965 aimed at resurrecting Alfred Wegener’s hypothesis of continental drift. During the half century since Wegener made his proposal in 1915, it had been studiously ignored by most geologists. The majority had bumbled along with the fixist ideology of their Victorian predecessors. The symposium launched what can only be regarded as a revolution in the Earth Sciences. In the three years following the symposium, the basic elements of plate tectonics had emerged from a flurry of papers, mainly centred on geophysical evidence. Geology itself became part of this cause célèbre through young scientists eager to make a name for themselves. The geological history of Britain, together with that of the eastern North America, became beneficiaries only four years after the Royal Society meeting (Dewey, J. 1969. Evolution of the Appalachian/Caledonian Orogen. Nature222, 124–129; DOI: 10.1038/222124a0).
In Britain John Dewey, like a few other geologists, saw plate theory as key to understanding the many peculiarities revealed by geological structure, igneous activity and stratigraphy of the early Palaeozoic. These included very different Cambrian and Ordovician fossil assemblages in Scotland and Wales, now only a few hundred kilometres apart. The Cambro-Ordovician of NW Scotland was bounded to the SE by a belt of highly deformed and metamorphosed Proterozoic to Ordovician sediments and volcanics forming the Scottish Highlands. That was terminated to the SE by a gigantic fault zone containing slivers of possible oceanic lithosphere. The contorted and ‘shuffled’ Ordovician and Silurian sediments of the Southern Uplands of Scotland. The oldest strata seemed to have ocean-floor affinities, being deposited on another sliver of ophiolites. A few tens of km south of that there was a very different Lower Palaeozoic stratigraphy in the Lake District of northern England. It included volcanic rocks with affinities to those of modern island arcs. A gap covered by only mildly deformed later Palaeozoic shelf and terrestrial sediments, dotted by inliers of Proterozoic sediments and volcanics separated the Lake District from yet another Lower Palaeozoic assembly of arc volcanics and marine sediments in Wales. Intervening in Anglesey was another Proterozoic block of deformed sediments that also included ophiolites.
Dewey’s tectonic assessment from this geological hodge-podge, which had made Britain irresistible to geologists through the 19th and early 20th centuries, was that it had resulted from blocks of crust (terranes), once separated by thousands of kilometres, being driven into each other. Britain was thus formed by the evolution and eventual destruction of an early Palaeozoic ocean, Iapetus: a product of plate tectonics. Scotland had a fundamentally different history from England and Wales; the unification of several terranes having taken over 150 Ma of diverse tectonic processes. Dewey concluded that the line of final convergence lay at a now dead, major subduction zone – the Iapetus Suture – roughly beneath the Solway Firth. During the 56 years since Dewey’s seminal paper on the Caledonian-Appalachian Orogeny details and modifications have been added at a rate of around one to two publications per year. The latest seeks to date when and where the accretion of 6 or 7 terranes was finally completed (Waldron, J.W.F. et al. 2025. Is Britain divided by an Acadian suture?Geology, v. 53, p. 847–852; DOI: 10.1130/G53431.1).
Kernel density plots – smoothed versions of histograms – of detrital zircon ages in Silurian and Devonian sandstones from Wales. The bracketed words are stratigraphic epochs. Credit: Waldron et al. 2025, Fig 3A
John Waldron and colleagues from the University of Alberta and Acadia University in Canada and the British Geological Survey addressed this issue by extracting zircons from four late Silurian and early Devonian sandstones in North and South Wales. These sediments had been deposited between 433 and 393 Ma ago at the southernmost edge of the British Caledonide terrane assemblage towards the end of terrane assembly. The team dated roughly 250 zircons from each sandstone using the 207Pb/206Pb and 206Pb/238U methods. Each produced a range of ages, presumed to be those of igneous rocks from whose magma the zircon grains had crystallised. These data are expressed as plots of probable frequency against age. Each pattern of ages is assumed to be a ‘fingerprint’ for the continental crust from which the zircons were eroded and transported to their resting place in their host sediment. In this case, the researchers were hoping to see signs of continental crust from the other side of the Caledonian orogen; i.e. from the Precambrian basement of the Laurentia continent.
The three late-Silurian sediments showed distinct zircon-age peaks around 600 Ma and a spread of smaller peaks extending to 2.2 Ga. This tallied with a sediment source in Africa, from which the southernmost Caledonian terrane was said to have split and moved northwards. The Devonian sediment lacked signs of such an African ‘heritage’ but had a prominent age peak at about 1.0 Ga, absent from the Welsh Silurian sediments. Not only is this a sign of different sediment provenance but closely follows the known age of a widespread magmatic pulse in the Laurentian continent. So, sediment transport from the opposite side of the Iapetus Ocean across the entire Caledonian orogenic belt was only possible after the end of the Silurian Period at around 410 Ma. There must have been an intervening barrier to sediment movement from Laurentia before that, such as deep ocean water further north. Previous studies from more northern Caledonian terranes show that Laurentian zircons arrived in the Southern Uplands of Scotland and the English Lake District around 432 Ma in the mid-Silurian. Waldron et al. suggest, on these grounds that the suture marking the final closure of the Iapetus Ocean lies between the English Lake District and Anglesey, rather than beneath the Solway. They hint that the late-Silurian to early Devonian granite magmatism that permeated the northern parts of the Caledonian-Appalachian orogen formed above northward subduction of the last relics of Iapetus, which presaged widespread crustal thickening known as the Acadian orogeny in North America.
Readers interested in this episode of Earth history should download Waldron et al.’s paper for its excellent graphics, which cannot be reproduced adequately here.
Much of the Archaean Eon is represented by cratons, which occur at the core of continental parts of tectonic plates. Having low geothermal heat flow they are the most rigid parts of the continental crust. The Superior Craton is an area that makes up much of the eastern part of the Canadian Shield, and formed during the Late Archaean from ~4.3 to 2.6 billion years (Ga) ago. Covering an area in excess of 1.5 million km2, it is the world’s largest craton. One of its most intensely studied components is the Abitibi Terrane, which hosts many mines. A granite-greenstone terrain, it consists of volcano-sedimentary supracrustal rocks in several typically linear greenstone belts separated by areas of mainly intrusive granitic bodies. Many Archaean terrains show much the same ‘stripey’ aspect on the grand scale. Greenstone belts are dominated by metamorphosed basaltic volcanic rock, together with lesser proportions of ultramafic lavas and intrusions, and overlying metasedimentary rocks, also of Archaean age. Various hypotheses have been suggested for the formation of granite-greenstone terrains, the latest turning to a process of ‘sagduction’. However the relative flat nature of cratonic areas tells geologists little about their deeper parts. They tend to have resisted large-scale later deformation by their very nature, so none have been tilted or wholly obducted onto other such stable crustal masses during later collisional tectonic processes. Geophysics does offer insights however, using seismic profiling, geomagnetic and gravity surveys.
The Geological Survey of Canada has produced masses of geophysical data as a means of coping with the vast size and logistical challenges of the Canadian Shield. Recently five Canadian geoscientists have used gravity data from the Canadian Geodetic Survey to model the deep crust beneath the huge Abitibi granite-greenstone terrain, specifically addressing variations in its density in three dimensions. They also used cross sections produced by seismic reflection and refraction data along 2-D survey lines (Galley, C. et al. 2025. Archean rifts and triple-junctions revealed by gravity modeling of the southern Superior Craton. Nature Communications, v. 16, article 8872; DOI: 10.1038/s41467-025-63931-z). The group found that entirely new insights emerge from the variation in crustal density down to its base at the Moho (Mohorovičić discontinuity). These data show large linear bulges in the Moho separated by broad zones of thicker crust.
Geology of the Abitibi Terrane (upper),; Depth to the Moho beneath the Abitibi Terrane with rifts and VMS deposits superimposed (lower). Credit: After Galley et al. Figs 1 and 5.
Galley et al. suggest that the zones are former sites of lithospheric extensional tectonics and crustal thinning: rifts from which ultramafic to mafic magmas emerged. They consider them to be akin to modern mid-ocean and continental rifts. Most of the rifts roughly parallel the trend of the greenstone belts and the large, long-lived faults that run west to east across the Abitibi Terrain. This suggests that rifts formed under the more ductile lithospheric condition of the Neoarchaean set the gross fabric of the granites and greenstones. Moreover, there are signs of two triple junctions where three rifts converge: fundamental features of modern plate tectonics. However, both rifts and junctions are on a smaller scale than those active at present. The rift patterns suggest plate tectonics in miniature, perhaps indicative of more vigorous mantle convection during the Archaean Eon.
There is an interesting spin-off. The Abitibi Terrane is rich in a variety of mineral resources, especially volcanic massive-sulfide deposits (VMS). Most of them are associated with the suggested rift zones. Such deposits form through sea-floor hydrothermal processes, which Archaean rifting and triple junctions would have focused to generate clusters of ‘black smokers’ precipitating large amounts of metal sulfides. Galley et al’s work is set to be applied to other large cratons, including those that formed earlier in the Archaean: the Pilbara and Kaapvaal cratons of Australia and South Africa. That could yield better insights into earlier tectonic processes and test some of the hypotheses proposed for them
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