More dinosaur trackways from the Jurassic of the Isle of Skye, Scotland

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The Isle of Skye off the northwestern coast of Scotland is one of several areas in Britain that are world-class geological gems. Except for the Cuillin Hills that require advanced mountaineering skills it is easy to explore and has become a major destination for both beginners and expert geoscientists of all kinds. Together with the adjacent Isle of Raasay the area is covered by a superb, free geological guidebook (Bell, B. 2024. The Geology of the Isles of Skye and Raasay. Geological Society of Glasgow) together with 60 standalone excursion guides, and even an introduction to Gaelic place names and pronunciation. It is freely available from https://www.skyegeology.com/

Fig Dinosaur trackways at Prince Charles’s Point on the Isle of Skye: Left carnivorous theropods; Right herbivorous sauropods. The black scales are 1 m long. The images are enhanced fine-scale elevation models of the exposed surfaces that were derived from vertical photographs. Credit: Blakesley et al., Figs 9 and 27.

Since 2018 Skye has also become a must-visit area for vertebrate palaeontologists. Beneath Palaeocene flood basalts is a sequence of Jurassic strata, both shallow marine and terrestrial. One formation, the Great Estuarine Group of Middle Jurassic (Bathonian, 174–164 Ma) age covers the time when meat-eating theropod- and herbivorous sauropod dinosaurs began to grow to colossal sizes from diminutive forebears. While other Jurassic sequences on Skye have notable marine faunas, its Bathonian strata have yielded a major surprise: some exposed bedding surfaces are liberally  dotted with trackways of the two best known groups of dinosaur. The first to be discovered were at Rubha Nam Brathairean (Brothers’ Point) suggesting a rich diversity of species that had wandered across a wide coastal plain, also including the somewhat bizarre Stegosaurus. The latest finds are from a rocky beach at Prince Charles’s Point where the Young Pretender to the British throne, Charles Edward Stuart, landed and hid during his flight from the disastrous Battle of Culloden (16 April 1746). It was only in the last year or so that palaeontologists from the universities of Edinburgh and Liverpool, and the Staffin Museum came across yet more footprints (131 tracks) left there by numerous dinosaurs in the rippled sands of a Bathonian lagoon (Blakesley, T. et al. 2025. A new Middle Jurassic lagoon margin assemblage of theropod and sauropod dinosaur trackways from the Isle of Skye, Scotland. PLOS One, v. 20, article e0319862; DOI: 10.1371/journal.pone.0319862.

The Prince Charles’s Point site is partly covered by large basalt boulders, which perhaps account for the excellent preservation of the bedding surfaces from wave action. Two kinds of footprint are preserved (see image): those made by three-toed feet and by elephant-like feet that ‘squidged-up’ sediment surrounding than. Respectively these are suggested to represent the hind limbs of bipedal carnivorous theropods and quadrupedal herbivorous sauropods. They show that individual dinosaurs moved in multiple directions, but there is no evidence for gregarious behaviour, such as parallel trackways of several animals. They occur on two adjacent bedding surfaces so represent a very short period of time, perhaps a few days. The authors suggest that several individual animals were milling around, with more sauropods than theropods. What such behaviour represents is unclear. The water in an estuarine lagoon would likely have been fresh or brackish. They may have been drinking or perhaps there was some plants or carcases worth eating ? That might explain both kinds of dinosaurs’ milling around. The sizes of both sauropod and theropod prints average about 0.5 m. The stride lengths of the theropods suggest that they were between 5 to 7 metres long with a hip height of around 1.85 m. Their footprints resemble those reconstructed from skeletal remains of Middle Jurassic Megalosaurus, the first dinosaur to be named (by William Buckland in 1827). The sauropods had estimated hip heights of around 2 m so they may have been similar in size (around 16 m) to the Middle Jurassic Cetiosaurus, the first sauropod to be named (by Richard Owen in 1842).

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

What drove the Cambrian Explosion?

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The origin of animals occurred sometime during the Proterozoic Eon, perhaps as early as 2.1 Ga (billion years ago) after the Great Oxygenation Event. Available oxygen is a prerequisite for animal life, and that is about as far back as palaeobiologists can push it. More familiar are the trace fossils known as the Ediacaran fauna which emerged after the environmentally highly stressful Cryogenian Period that was marked by two Snowball Earth events. Traces of these animals may have been big enough to be easily found, but they were not particularly diverse and are difficult to place in any particular modern group. Most modern animals have front- and rear ends, tops and bottoms, and input and output orifices. The earliest of these bilaterian beasts may have emerged during the Ediacaran as well, but were not very prepossessing. It was during the Cambrian Period (541 to 485 Ma) that most modern animal phyla became recognisable to palaeobiologists. That carnival of diversification is widely known as the Cambrian Explosion. Yet it was later in geological time that the full panoply of Phanerozoic diversity among taxa below the level of the phylum truly exploded, punctuated by mass extinctions and the diversification that followed each of them. So, what lay behind the initial emergence of the characteristics that form the basic templates of the phyla themselves?

Cartoon of the Cambrian Explosion in benthic faunas. Credit: Gabriela Mangano and Luis A. Buatois, 2016 The Cambrian Explosion, Fig 3.15

A multinational team of modellers and geoscientists have moved the focus from long-term shifts in climate and atmospheric chemistry to what might change from day to night in an ecosystem during the diel cycle (Hammarlund, E.U. and 13 others 2025. Benthic diel oxygen variability and stress as potential drivers for animal diversification in the Neoproterozoic-Palaeozoic Nature Communications, v. 16, article 2223; DOI:10.1038/s41467-025-57345-0). During the Neoproterozoic oxygen levels in Earth atmosphere rose to about half the amount present today. But animals arose and evolved in sea water. The most prolific source of food for them would have been in shallow water (the benthic zone), simply because sunlight in the photic zone encourages photosynthesis. As well as a thriving base for animal life’s food chain shallow water is where oxygen is produced; but only during daylight hours. At night decay of organic matter on the seabed draws down dissolved oxygen. Emma Hammarlund and colleagues wondered if day-night changes in oxygen levels might have exerted sufficient stress to force early animals to adapt and thus diversify. Their model shows that in warm, shallow water the lower oxygen levels at the start of the Phanerozoic could change dramatically in the diel cycle. Algae at the base of the food chain would swiftly oxygenate the water in daylight, but at night would consume it to produce much lower levels. Animals that were better adapted to the stress of this daily ‘feast-and-famine’ cycle in oxygen availability would outcompete others that were less resilient for the available nutrients. Environmental stress had flipped from an obstacle to evolution to a catalyst for it. The earliest appearances of organisms in the 10 modern phyla seem to coincide with global warming at low latitudes to an air temperature of about 25° C at the start of the Cambrian, perhaps when this shift began.

Another empirical coincidence lies in the sedimentary rock record. On modern continents the base of Phanerozoic sediments is widely marked by shallow-water sandstones often at an unconformity. Often white and containing abundant burrows, the sandstones are signs of abundant life, though rarely contain body fossils. They represent global sea-level rise that flooded the existing continents, so the highly productive benthic environment became about four times more widespread at the end of the Cambrian than it was during the previous Ediacaran Period. Abundant life forms were under stress more or less everywhere. Thereafter these ‘shelf seas’ halved in total area, but the basic ‘templates’ for animal life were well-established and the numbers of classes, orders, families etcetera steadily burgeoned. By the end of the Cambrian oxygen production rose so that atmospheric concentration of the gas reached 25%, higher then it is at present.

See also: Hammarlund, E. 2025. How dramatic daily swings in oxygen shaped early animal life. The Conversation, 21 March 2025.

The earliest known impact structure

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Earth has been through a great many catastrophes, but the vast majority of those of which we know were slow-burning in a geological sense. They resulted in unusually high numbers of extinctions at the species- to family levels over a few million years and the true mass extinctions seem to have been dominated by build ups of greenhouse gases emitted by large igneous provinces. Even the most famous at the end of the Cretaceous Period, which did for the dinosaurs and considerably more organisms that the media hasn’t puffed, was partly connected to the eruption of the Deccan flood basalts of western India. Yet the event that did the real damage was a catastrophe that appeared in a matter of seconds: the time taken for the asteroid that gouged the Chicxulub crater to pass through the atmosphere. Its energy was huge and because it was delivered in such a short time its sheer power was unimaginable. Gradually geologists have recognised signs of an increasing number of tangible structures produced by Earth’s colliding with extraterrestrial objects, which now stands at 190 that have been confirmed.

Landsat image mosaic of the Palaeoarchaean granite-greenstone terrain of the Pilbara Craton, Western Australia. Granite bodies show as pale blobs, the volcanic and sedimentary greenstone belts in shades of grey. The site of Kirkland et al.’s study site is at the tip of the red arrow

The frequency of impact craters falls off with age, most having formed in the last ~550 million years (Ma) during the Phanerozoic Eon, only 25 being known from the Precambrian, which spanned around 88 percent of geological time. That is largely a consequence of the dynamic processes of tectonics, erosion and sedimentation that may have obliterated or hidden a larger number. Earth is unique in that respect, the surfaces of other rocky bodies in the Solar System showing vastly more. The Moon is a fine example, especially as it has been Earth’s companion since it formed 4.5 billion years ago (Ga) after the proto-Earth collided with a now vanished planet about the size of Mars. The relative ages of lunar impact structures combined with radiometric ages of the surfaces that they hit has allowed the frequency of collisions to be assessed through time. Applied to the sizes of the craters such data can show how the amount of kinetic energy inflicted on the lunar surface has changed with time. During what geologists refer to as the Hadean Eon (before 4 Ga), the moon underwent continuous bombardment that reached a crescendo between 4.1 and about 3.8 Ga. Thereafter impacts tailed off. Always having been close to the Moon, the Earth cannot have escaped the flux of objects experienced by the lunar surface. Because of Earth’s much greater gravitation pull it was probably hit by more objects per unit area. Apart from some geochemical evidence from Archaean rocks (see: Tungsten and Archaean heavy bombardment; July 2002) and several beds of 3.3 Ga old sediment in South Africa that contain what may have been glassy spherules there are no signs of actual impact structures earlier than a small crater dated at around 2.4 Ga in NE Russia.

Shatter cones in siltstone near Marble Bar in the Pilbara Province: finger for scale. Credit: Kirkland et al.; Fig 2a

Now a group of geologists from Curtin University, Perth Western Australia, and the Geological Survey of Western Australia have published their findings of indisputable signs of an impact site in the northern part of Western Australia (Kirkland, C.L. et al. 2025. A Paleoarchaean impact crater in the Pilbara Craton, Western Australia. Nature Communications, v. 16, article 2224; DOI: 10.1038/s41467-025-57558-3). In fact there is no discernible crater at the locality, but sedimentary strata show abundant evidence of a powerful impact in the form of impact-melt droplets in the form of spherules together with shatter cones. These structures form as a result of sudden increase in pressure to 2 to 30 GPa: an extreme that can only be generated in underground nuclear explosions, and thus likely to bear witness to large asteroid impacts. The shocked rocks are immediately overlain by pillow lavas dated at 3.47 Ga, making the impact the earliest known. It has been speculated that impacts during the Archaean and Hadean Eons helped create conditions for the complex organic chemistry that eventually to the first living cells. Considering that entry of hypervelocity asteroids into the early Earth’s atmosphere probably caused such compression that temperatures were raised by adiabatic heating to about ten times that of the Sun’s surface, their ‘entry flashes’ would have sterilised the surface below; the opposite of such notions. Impacts may, however, have delivered both water and simple, inorganic hydrocarbons. Together with pulverisation of rock to make ‘fertiliser’ elements (e.g. K and P) more easily dissolved, they may have had some influence. Their input of thermal energy seems to me to be of little consequence, for decay of unstable isotopes of U, Th and K in the mantle would have heated the planet quite nicely and continuously from Year Zero onwards.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Bone tools widened hominins foraging options 1.5 Ma ago

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Hominins have been making and using stone tools since at least 3.4 Ma, as shown by cut marks on bones and stone artefacts themselves. I use the sack term ‘hominin’ because the likely makers and users of the oldest tools are either australopithecines or paranthropoids, there being no fossils designates to the genus Homo of late-Pliocene age. So it might seem  un-newsworthy to report that the oldest tools deliberately made from bone are now known to occur in 1.5 Ma old sediments from the famous sedimentary sequence at Olduvai Gorge in Tanzania (de la Torre, I and 8 others2025. Systematic bone tool production at 1.5 million years ago. Nature, v. 639; DOI: 10.1038/s41586-025-08652-5). To be clear, there is abundant evidence that hominins had used bones, especially sturdy long bones, for digging perhaps, much earlier in hominin history. Again, paranthropoids have been implicated in their use. The bones found at Olduvai actually show signs of manufacture into useful objects prior to their use: they show clear signs of knapping to produce points and blades. The bones are among the sturdiest known from the Pleistocene, being from elephants and hippos. Before de la Torre and colleagues found what is essentially a bone-tool factory, it was thought that systematic use of bones in such a sophisticated manner only arose between 400 to 250 ka ago among early Homo in Europe. Sadly, fossils of whoever made the tools were not found at the site. Once again, paranthropoids as well as early Homo  are known to have cohabited the area at that time.

‘Front, back and side’ views of a 1.5 Ma old tool made from an elephant humerus – its upper foreleg. The scale bar represents 5 cm. (Credit: de la Torre et al.; Fig 3a)

Bifacial Acheulean stone artefacts first appear in the rock record about 300 ka before these bone tools were made. So one idea that the authors put forward is that the same kind of stone knapping technique was transferred to the more abundant massive bones of the East African Pleistocene megafauna (in the absence or rarity of suitable blocks of stone?). But it remains unclear whether or not such tools were simply selected from very large bones smashed to get at their nutritious marrow. The first possibility implies a cultural shift, whereas the latter points simply to expedience. The authors are at pains to point out that the curious million-year gap in the record of bone tools may be ascribed either to the disappearance of bone technology or simply to archaeologists who worked elsewhere having not regarding bone fragments as the products of skills. That applies equally to earlier times, when bones were indeed used, though with not so much in the way of a ‘mental template’. As de la Torre et al. conclude ‘Future research needs to investigate whether similar bone tools were already produced in earlier times, persisted during the Acheulean and eventually evolved into Middle Pleistocene bone bifaces similar in shape, size and technology to their stone counterparts’.

Direct measurements of ancient atmospheric composition

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For decades, research into the composition of the Earth’s early atmosphere depended on indirect means. An example is the preservation of water-worn grains of sulphides and uranium oxides in coarse terrestrial sediments older than about 2,200 Ma. Their survival on the continental surface suggested that the atmosphere before then had vanishingly low O2. Such grains would have otherwise been broken down by oxidation reactions. Younger sediments simply do not contain such detrital grains. This suggested the appearance of an oxidising atmosphere around 2.2 Ga ago: the Great Oxygenation Event. The greenhouse gases – carbon dioxide and methane – are also difficult to estimate directly, especially in the Precambrian. Once plants colonised the land surface, their photosynthesis depended on inhaling and exhaling air through stomata on the surface of leaves (see: Ancient CO2 estimates worry climatologists; January 2017). The number of stomata per unit area of a leaf surface is expected to increase with lowering of atmospheric CO2 and vice versa, which has been observed in plants grown in different air compositions. By comparing stomatal density in fossilised leaves of modern plants back to 800 ka allows the change to be calibrated against the record of CO­2 inside air bubbles trapped in ice-cores. This proxy method has given a guide to CO2 variations through the Cenozoic, Mesozoic and upper Palaeozoic Eras. However, the reliability of extinct plant leaves as proxies is suspect.

A fluid inclusion (about 0.2 mm) trapped in a crystal of halite (NaCl). Credit: alchetron.com

Is it possible to find air trapped by other means than in glacial ice? It may be. Tiny pockets of liquid and gas – fluid inclusions – are often found in minerals that crystallised at the Earth’s surface. The most common are crystals of salt (NaCl) and carbonates from ancient lake deposits. A 2019 study revealed that Late Triassic carbonates from Colorado, USA record an increase of atmospheric oxygen levels from 15 to 19% about 215 Ma ago over a period of just 3 million years as dinosaurs first spread into North America, then at equatorial latitudes in the Pangaea supercontinent. This sudden increase in the availability of oxygen may also be linked to the trend towards larger and larger dinosaurs worldwide.  Going further back in time trace-metal chemistry of 1,400 Ma old marine sediments from China indicates oxygenated water that suggests an atmospheric oxygen level greater than 4% of that at present. Small as that might seem, it would have been sufficient to sustain animal respiration about half a billion years before the first evidence for the earliest animals. Further work on ancient salt and carbonate deposits confirms much higher oxygen levels  than geochemists have expected previously.

Source: Voosen, P, 2025. Earth’s rocks hold whiffs of air from billions of years ago. Science, v.387, articlezhst73x; DOI: 10.1126/science.zhst73x

Life’s origins: a new variant on Darwin’s “warm little pond”

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In 1871 Charles Darwin wrote to his friend Joseph Hooker, a botanist:

“It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (& oh what a big if) we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts, light, heat, electricity &c present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter wd be instantly devoured, or absorbed, which would not have been the case before living creatures were formed.”

There have been several attempts over the last 150 years, starting with Miller and Urey in 1952, to create physical analogues for this famous insight (See:  The origin of life on Earth: new developments). What such a physico-chemical environment on the early Earth could have been like has also been a fertile topic for discussion: literally warm pools at the surface; hot springs; seawater around deep-ocean hydrothermal vents; even droplets in clouds in the early atmosphere. Attention has recently moved to Darwin’s original surface pools through examination of modern ones. The most important content would be dissolved phosphorus compounds, because that element helps form the ‘backbone’ of the helix structure of RNA and DNA. But almost all natural waters today have concentrations of phosphorus that are far too low for such linkages to form by chemical processes, and also to produce lipids that form cell membranes and the ATP (adenosine triphosphate) so essential in all living metabolism. Phosphorus availability has been too low for most of geological time simply because living organisms are so efficient at removing what they need in order to thrive.

Mono Lake in semi-arid eastern California – a ‘soda lake’- is so concentrated by evaporation that pillars of carbonate grow above its surface

For the first life to form, phosphorus would somehow have had to be concentrated in watery solution as phosphate ions – [PO ₄]³⁻. The element’s source, like that of all others in the surface environment, is in magmas and the volcanic rocks that they form. Perhaps early chemical weathering or reactions between lavas and hydrothermal fluids could have released phosphate ions to solution from a trace mineral present in all lavas: the complex phosphate apatite (Ca10(PO4)6(OH,F,Cl)2). But that would still require extreme concentration for it to be easily available to the life-forming process. In January 2024 scientists at the University of Washington in Seattle, USA (Haas, S. et al. 2024. Biogeochemical explanations for the world’s most phosphate-rich lake, an origin-of-life analog. Nature Communications, v. 5, article 28; DOI: 10.1038/s43247-023-01192-8) showed that the highest known concentrations of dissolved phosphorus occur in the so called “soda lakes” that are found in a variety of modern environments, from volcanically active continental rifts to swampy land. They contain dissolved sodium carbonate (washing soda) at very high concentrations so that they are extremely alkaline and often highly salty. Usually, they are shallow and have no outlet so that dry weather and high winds evaporate the water. Interestingly, the streams that flow into them are quite fresh, so soda lakes form where evaporation exceeds annual resupply of rainwater.

The high evaporation increases the dissolved content of many ions in such lakes to levels high enough for them for them to combine and precipitate calcium, sodium and magnesium as carbonates. In some, but not all soda lakes, such evaporative concentration also increases their levels of dissolved phosphate ions higher than in any other bodies of water. That is odd, since it might seem that phosphate ions should combine with dissolved calcium to form solid calcium phosphate making the water less P-rich.  Haas et al. found that lakes which precipitate calcium and magnesium together in the form of dolomite (Ca,Mg)CO3 have high dissolved phosphate. Removal of Ca and other metal ions through bonding to carbonate (CO3) deprives dissolved phosphate ions in solution of metal ions with which they can bond. But why has dissolved phosphate not been taken up by organisms growing in the lakes: after all, it is an essential nutrient. The researchers found that some soda lakes that contain algal mats have much lower dissolved phosphate – it has been removed by the algae. But such lakes are not as salty as those rich in dissolved phosphate. They in turn contain far less algae whose metabolism is suppressed by high levels of dissolved NaCl (salt). Hass et al.’s hypothesis has now been supported by more research on soda lakes.

In an early, lifeless world phosphate concentrations in alkaline, salty lakes would be controlled by purely inorganic reactions. This strongly suggests that ‘warm little soda lakes’ enriched in dissolved sodium carbonate by evaporation, and which precipitated dolomite could have enabled phosphorus compounds to accumulate to levels needed for life to start. They might have been present on any watery world in the cosmos that sustained volcanism.

See also: Service, R.F. 2025. Early life’s phosphorus problem solved? Science, v. 387, p. 917; DOI: 10.1126/science.z78227f; Soda Lakes: The Missing Link in the Origin of Life? SciTechDaily, 26 January 2024. .

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Modelling climate change since the Devonian

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A consortium of geoscientists from Australia, Britain and France, led by Andrew Merdith of the University of Adelaide examines the likely climate cooling mechanisms that may have set off the two great ‘icehouse’ intervals in the last 541 Ma (Merdith, A.S. et al. 2025. Phanerozoic icehouse climates as the result of multiple solid-Earth cooling mechanisms. Science Advances, v. 11, article eadm9798: DOI: 10.1126/sciadv.adm9798). They consider the first to be the global cooling that began in the latter part of the Devonian culminating in the Carboniferous-Permian icehouse. The second is the Cenozoic global cooling to form the permanent Antarctic ice cap around 34 Ma and culminated in cyclical ice ages on the northern continents after 2.4 Ma during the Pleistocene. They dismiss the 40 Ma long, late Ordovician to early Silurian glaciation that left its imprint on North Africa and South America –  then combined in the Gondwana supercontinent. The data about two of the parameters used in their model – the degree of early colonisation of the continents by plants and their influence on terrestrial weathering are uncertain in that protracted event.  Yet the Hirnantian glaciation reached 20°S at its maximum extent in the Late Ordovician around 444 Ma to cover about a third of Gondwana: it was larger than the present Antarctic ice cap. For that reason, their study spans only Devonian and later times.

Fluctuation in evidence for the extent of glacial conditions since the Devonian: the ‘ice line’ is grey. The count of glacial proxy occurrences in each 10° of latitude through time is shown in the colour key. Credit: Merdith et al., Fig 2A.

Merdith et al. rely on four climatic proxies. The first of these comprises indicators of cold climates, such as glacial dropstones, tillites and evidence in sedimentary rocks of crystals of hydrated calcium carbonate (ikaite – CaCO3.6H2O) that bizarrely forms only at around 0°C . From such occurrences it is possible to define an ‘ice line’ linking different latitudes through geological time. Then there are estimates of global average surface temperature; low-latitude sea surface temperature; and estimates of atmospheric CO2. The ‘ice-line’ data records an additional, long period of glaciation in the Jurassic and early Cretaceous, but evidence does not extend to latitudes lower than 60°. It is regarded by Merdith et al. as an episode of ‘cooling’ rather than an ‘icehouse’. Their model assesses sources and sinks of CO2­ since the Devonian Period.

The main natural source of the principal greenhouse gas CO2 is degassing through volcanism expelled from the mantle and breakdown of carbonate rock in subducted lithosphere. Natural sequestration of carbon involves weathering of exposed rock that releases dissolved CO2 and ions of calcium and magnesium.   A recently compiled set of plate reconstructions that chart the waxing and waning of tectonics since the Devonian Period allows them to model the tectonically driven release of carbon over time, with time scales on the order of tens to hundreds of Ma. The familiar Milanković forcing cycles on the order of tens to hundreds of ka are thus of no significance in Merdith et al.’s  broader conception of icehouse episodes  Their modelling shows high degassing during the Cretaceous, modern levels during the late Palaeozoic and early Mesozoic, and low emissions during the Devonian. The model also suggests that cooling stemmed from variations in the positions and configuration of continents over time.  Another crucial factor is the tempo of exposure of rocks that are most prone to weathering. The most important are rocks of the ocean lithosphere incorporated into the continents to form ophiolite masses. The release of soluble products of weathering into ocean basins through time acts as a fluctuating means of ‘fertilising’ so that more carbon can be sequestered in deep sediments in the form of organisms’ unoxidised tissue and hard parts made of calcium carbonates and phosphates. Less silicate weathering results in a boost to atmospheric CO2.

Only two long, true icehouse episodes emerge from the empirical proxy data, expressed by the ‘ice-line’ plots. Restricting the modelling to single global processes that might be expected to influence degassing or carbon sequestration produces no good fits to the climatic proxy data. Running the model with all the drivers “off” produces more or less continuous icehouse conditions since the Devonian. The model’s climate-related outputs thus imply that many complex processes working together in syncopation may have driven the gross climate vagaries over the last 400 Ma or so. A planet of Earth’s size without such complexity would throughout that period have had a high-CO2 warm climate. According to Andrew Merdith its fluctuation from greenhouse to icehouse conditions in the late Palaeozoic and the Cenozoic were probably due to “coincidental combination of very low rates of global volcanism, and highly dispersed continents with big mountains, which allow for lots of global rainfall and therefore amplify reactions that remove carbon from the atmosphere”.

Geological history is, almost by definition, somewhat rambling. So, despite despite the large investment in seeking a computed explanation of data drawn from the record, the outcome reflects that in a less than coherent account. To state that many complex processes working at once may have driven climate vagaries over the last 400 Ma or so, is hardly a major advance: palaeoclimatologists have said more or less the same for a couple of decades or more, but have mainly proposed single driving mechanisms. One aspect of Merdith et al.’s  results seems to be of particular interest. ‘Icehouse’ conditions seem to be rare events interspersed with broader ice-free periods. We evolved within the mammal-dominated ecosystems on the continents during the latest of these anomalous climatic episodes. And we and those ecosystems now rely on a cool world. As the supervisor of the project commented, ‘Over its long history, the Earth likes it hot, but our human society does not’.

Readers may like to venture into how some philosophers of science deal with a far bigger question; ‘Is intelligent life a rare, chance event throughout the universe?’ That is, might we be alone in the cosmos? In the same issue of Science Advances is a paper centred on just such questions (Mills, D.B. et al. 2025. A reassessment of the “hard-steps” model for the evolution of intelligent life. Science Advances, v. 11, article eads5698; DOI: 10.1126/sciadv.ads5698). It stems from cosmologist Brandon Carter’s ‘Anthropic Principle’ first developed at Nicolas Copernicus’s 500th birthday celebrations in 1973. This has since been much debated by scientists and philosophers – a gross understatement as it knocks the spots off the Drake Equation. To take the edge off what seems to be a daunting task, Mills et al. consider a corollary of the Anthropic Principle, the ‘hard steps model’. That, in a nutshell, postulates that the origin of humanity and its ability to ponder on observations of the universe required a successful evolutionary passage through a number of hard steps. It predicts that such intelligence is ‘exceedingly rare’ in the universe. Icehouse conditions are respectable candidates for evolutionary ‘hard steps’, and in the history of Earth there have been five of them.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

A cure for the Great British Pothole Plague?

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Anyone who read the manifestos of the mainstream political parties in the UK – there may not be many who did – would have been amused to see that all promised to resolve the plague of potholes in the countries roads, both major and minor. For decades road users have been alarmed when hitting a pothole and in some cases had damage inflicted on their vehicles, and in the case of those on two wheels, on themselves. The RAC (Royal Automobile Club) has estimated that there are, on average, six potholes per mile on Britain’s roads: the greatest density in Europe. The AA (Automobile Association) estimated that almost £0.6 billion was spent in 2024 repairing pothole-damaged vehicles. This is not a new phenomenon. Before the advent of turnpike trusts in the late 18th century, which maintained roads travelled by Britain’s mail coach services, it was not uncommon to encounter potholes up to two metres deep. Legend has it that on one such route through northern Nottinghamshire two coach horses fell into a pothole and drowned. Scottish engineer, John Loudon McAdam invented a solution around 1820: crushed stone laid on the road surface in slightly convex layers, the topmost being bonded with stone dust. This ‘macadam’ surface created cambered highways that drained rainwater to the sides and downwards. Modern roads are still based on that principle, with the addition of tar or bitumen to the top layer to produce a hard, impermeable surface, which also prevents aggregate and dust being sucked from the surface by fast moving vehicles.

A spore of the club moss Lycopodium

So, why the potholes? Several reasons: increased traffic; heavier vehicles; less maintenance; patching rather than resurfacing. Most important: the materials and the weather. Dry, hot weather softens the bitumen and drives out volatile hydrocarbons making the bitumen less plastic. The pounding of tyres in cooler weather fractures the now stiffened bitumen, mainly at microscopic scales. Wetting of the tarmac seeps water into the microfractures. The formation of ice films jacks opens the microfractures and produces more in the cold stiff bitumen, eventually to separate the particles of aggregate in the asphalt. The wearing course begins to crumble so that aggregate grains escape and scatter. Thus weakened, the top layer breaks up into larger fragments and a pit forms to join up with others so that a pothole forms and grows. Wheels of traffic bounce when they cross a pothole, the shock of which causes the centre of degradation to shift and create more cavities. Simply filling the existing potholes merely serves to create new ones: a vicious cycle that can only be broken by complete resurfacing: the traffic cones come out!.

All this has been known for well over a century by civil engineers. Around the start of the 21st century – maybe slightly earlier – it dawned on engineers that the critical problem was degradation of bitumen. A petroleum derivative, occurring naturally as surface seeps in some oilfields, bitumen is chemically complex: a combination of asphaltenes and maltenes (resins and oils). Deterioration of bitumen through evaporation, oxidation and exposure to ultraviolet radiation decreases the maltene content and stiffens the binding agent in asphalt. So the earliest attempts at reducing pothole formation centred on rejuvenation by periodically adding substitutes for maltenes to road surfaces. Diesel (gas-oil) works, but is obviously hazardous. More suitable are vegetable oils such as waste cooking oils or those produced by pyrolysis of cotton, straw, wood waste and even animal manure. The problem is getting the rejuvenators into existing asphalt surfaces: clearly, simply spraying them on the surface seems a recipe for disaster! A solution that dawned on engineers around 2005 was to make bitumen that is ‘self-healing’.

Schematic of the production of microcapsules from club moss spores to contain sunflower oil to be used in self-healing asphalt (Credit: Alpizar-Reyes, E. et al. 2022)

Simply mixing rejuvenators into bitumen during asphalt manufacture will not do the trick, for the result would be a weakened binding agent at the outset. For the last 15 years researchers have sought means of adding rejuvenators in  porous capsules, to release them as microfractures begin to form: on demand, as it were. There have been dozens of publications about experiments that found ‘sticking points’. However, in early 2025 what seems to be a viable breakthrough splashed in the British press. It was made by an interdisciplinary team of scientists from King’s College London and Swansea University, in collaboration with scientists in Chile. They chemically treated spores of Lycopodium club mosses to perforate their cell walls and clear out their contents to be replaced by sunflower oil, an effective bitumen rejuvenator. Experiments showed that such microcapsules released the oil to heal cracks in aged  bitumen samples in around an hour. Mixed into bitumen to be added to asphalt they would remain ‘dormant’ until a microfracture formed in their vicinity released it, thereby making the asphalt binder self healing.

Will such an advance finally resolve the pothole plague? It may take a while …

See: Alpizar-Reyes, E. et al. 2022. Biobased spore microcapsules for asphalt self-healing. ACS Applied Materials & Interfaces, v. 14, p. 31296-31311; DOI: 10.1021/acsami.2c07301

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Early hominin dispersal in Eurasia

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Evidence from Dmanisi in Georgia that Homo erectus may have been the first advanced hominin to leave Africa about 1.8 Ma ago was a big surprise (see: First out of Africa? November 2003). Remains of five individuals included one skull of an aged person who face was so deformed that he or she must have been cared for by others for many years. So, a second surprise from Dmanisi was that human empathy arose far earlier than most people believed. Since 2002 there has been only a single further find of hominin bones of such antiquity, at Longgudong in central China. For the period between 1.0 and 2.0 Ma eight other sites in Eurasia have yielded hominin remains. If finds of stone tools and evidence of deliberate butchery – cut marks on prey animals’ bones – are accepted as tell-tale signs, the Eurasian hominin record is considerably larger, and longer,. There are 11 Eurasian sites that have yielded such evidence – but no hominin remains – that are older than Longgudong: in Russia, China, the Middle East, North Africa and northern India. The oldest, at Masol in northern India is 2.6 Ma old. In January 2025 the earliest European evidence for hominin activity was reported from Grăunceanu in Romania (Curran, S.C. and 15 others 2025. Hominin presence in Eurasia by at least 1.95 million years ago. Nature Communications, v. 16, article 836; DOI: 10.1038/s41467-025-56154-9) in the form of animal bones showing clear signs of butchery, as well as stone tools, but no hominin fossils.

Animal bones showing cut marks from the 1.95 Ma old Grăunceanu site in Romania. (Credit: Curran et al. 2025, Figs 2A and C)

There were stone-tool makers who butchered prey in Africa as early as 3.4 Ma ago (see: Stone tools go even further back; May 2015), but without direct evidence of which hominin was involved. Several possible candidates have been suggested: Australopithecus; Kenyanthropus; Paranthropus. The earliest known African remains of H. erectus have been dated at around 2.0 Ma. So, all that can be said with some certainty about the pre-2 Ma migrants to Eurasia, until fossils of that antiquity are found, is that they were hominins of some kind: maybe advanced australopithecines, paranthropoids or early humans. Those from Longgudong and Dmanisi probably are early Homo erectus, and 2 others (1.7 and 1.6 Ma) from China have been designated similarly. Younger, pre-1.0 Ma Eurasian hominins from Israel, Indonesia, Spain and Turkey are currently un-named at the species level, but are allegedly members of the genus Homo.

So, what can be teased from the early Eurasian hominin finds? Some certainly travelled thousands of kilometres from their assumed origins in Africa, but none penetrated further north than about 50°N. Perhaps they could not cope with winters at higher latitudes, especially during ice ages. To reach as far as eastern and western Eurasia suggests that dispersal following exit from Africa would have taken many generations. There is no reason to suppose continual travel; rather the reverse, staying put in areas with abundant resources while they remained available, and then moving on when they became scarce. Climate cycles, first paced at around 40 ka (early Pleistocene) then at around 100 ka (mid Pleistocene and later), would have been the main drivers for hominin population movements, as it would have been for game and vegetation.

After about 3 Ma the 40 ka climate cyclicity evolved to greater differences in global temperature between glacial and interglacial episodes, and even more so after the mid Pleistocene transition to 100 ka cycles (see Wikipedia entry for the mid-Pleistocene Transition). Thus, it seems likely that chances of survival of dispersed bands of hominins decreased over hundreds of millennia. Could populations have survived in particularly favourable areas; i.e. those at low latitudes? If so did both culture and the hominins themselves evolve? Alternatively, was migration in a series of pulses out of Africa and then dispersal in all directions, most ending in regional extinction? Almost certainly, pressures to leave Africa would have been driven by climate, for instance by increased aridity as global temperatures waned and sea-level falls made travel to Eurasia easier. There may also have been secondary, shorter migrations within Eurasia, again driven by environmental changes. Without more data from newly discovered sites we can go little further. Within the 35 known, pre-1 Ma hominin sites there are two clusters: southern and central China, and the Levant, Turkey and Georgia. Could they yield more developments? A 2016 article in Scientific American about Chinese H. erectus finds makes particularly interesting reading in this regard.

Neanderthals and the elusive Denisovans began to establish permanent Eurasian ranges, after roughly 600 ka ago. Both groups survived until after first contact with waves of anatomically modern humans in the last 100 ka, with whom some interbred before vanishing from the record. However, evidence from the DNA of both groups suggests an interesting possibility. Before the two groups split genetically, their common ancestors (H. heidelbergensis or H. antecessor?) apparently interbred with genetically more ancient Eurasian hominins (see Wikipedia entry for Neanderthal evolution). This intriguing hint suggests that more may be discovered when substantial remains of Denisovans – i.e. more than a few teeth and small bones – are discovered and yield more DNA. My guess is such a future development will stem from analysis of early hominin remains in China, currently regarded as H. erectus. See China discovers landmark human evolution fossils. Xinhua News Agency 9 December 2024)

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

The origin of life on Earth: new developments

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Debates around the origin of Earth’s life and what the first organism was like resemble the mythical search for the Holy Grail. Chivalric romanticists of the late 12th and early 13th centuries were pretty clear about the Grail – some kind of receptacle connected either with the Last Supper or Christ’s crucifixion – but never found it. Two big quests that engage modern science centre on how the chemical building blocks of the earliest cells arose and the last universal common ancestor (LUCA) of all living things. Like the Grail’s location, neither is likely to be fully resolved because they can only be sought in a very roundabout way: both verge on the imaginary. The fossil record is limited to organisms that left skeletal remains, traces of their former presence, and a few degraded organic molecules. The further back in geological time the more sedimentary rock has either been removed by erosion or fundamentally changed at high temperatures and pressures. Both great conundrums can only be addressed by trying to reconstruct processes and organisms that occurred or existed more than 4 billion years ago.

Artistic impression of the early Earth dominated by oceans (Credit: Sci-news.com)

In the 1950s Harold Urey of the University of Chicago and his student Stanley Miller mixed water, methane, ammonia and hydrogen sulfide in lab glassware, heated it up and passed electrical discharges through it. They believed the simple set-up crudely mimicked Hadean conditions at the Earth surface. They were successful in generating more complex organic chemicals than their starting materials, though the early atmosphere and oceans are now considered to have been chemically quite different. Such a ‘Frankenstein’ approach has been repeated since with more success (see Earth-logs April 2024), creating 10 of the 20 amino acids plus the peptide bonds that link them up to make all known proteins, and even amphiphiles, the likely founders of cell walls. The latest attempt has been made by Spanish scientists at the Andalusian Earth Sciences Institute, the Universities of Valladolid and Cadiz, and the International Physics Centre in San Sebastian (Jenewein, C. et al 2024. Concomitant formation of protocells and prebiotic compounds under a plausible early Earth atmosphere. Proceedings of the National Academy of Sciences, v. 122, article 413816122; DOI: 10.1073/pnas.241381612).

Biomorphs formed by polymerisation of HCN (Credit: Jenewein, C. et al 2024, Figure 2)

Jenewein and colleagues claim to have created cell-like structures, or ‘biomorphs’ at nanometre- and micrometre scale – spheres and polyp-like bodies – from a more plausible atmosphere of CO2 , H2O, and N2. These ‘protocells’ seem to have formed from minutely thin (150 to 3000 nanometres) polymer films built from hydrogen cyanide that grew  on the surface of the reaction chamber as electric discharges and UV light generated HCN and more complex ‘prebiotic’ chemicals. Apparently, these films were catalysed by SiO2 (silica) molecules from the glass reactor. Note:  In the Hadean breakdown of olivine to serpentinite as sea water reacted with ultramafic lavas would have released abundant silica. Serpentinisation also generates hydrogen. Intimate release of gas formed bubbles to create the spherical and polyp-like ‘protocells’. The authors imagine the Hadean global ocean permanently teeming with such microscopic receptacles. Such a veritable ‘primordial soup’ would be able to isolate other small molecules, such as amino acids, oligopeptides, nucleobases, and fatty acids, to generate more complex organic molecules in micro-reactors en route  to the kind of complex, self-sustaining systems we know as life.

So, is it possible to make a reasonable stab at what that first kind of life may have been? It was without doubt single celled. To reproduce it must have carried a genetic code enshrined in DNA, which is unique not only to all species, but to individuals. The key to tracking down LUCA is that it represents the point at which the evolutionary trees of the fundamental domains of modern life life – eukarya (including animals, plants and fungi), bacteria, and archaea – converge to a single evolutionary stem. There is little point in using fossils to resolve this issue because only multicelled life leaves tangible traces, and the first of those was found in 2,100 Ma old sediments in Gabon (see: The earliest multicelled life; July 2010). The key is using AI to compare the genetic sequences of the hugely diverse modern biosphere. Modern molecular phylogenetics and computing power can discern from their similarities and differences the relative order in which various species and broader groups split from others. It can also trace the origins of specific genes that provides clues about earlier genetic associations. Given a rate of mutation the modern differences provide estimates of when each branching occurred. The most recent genetic delving has been achieved by a consortium based at various institutions in Britain, the Netherlands, Hungary and Japan  (Moody, E.R.R. and 18 others 2024. The nature of the last universal common ancestor and its impact on the early Earth system. Nature Ecology & Evolution, v.8, pages 1654–1666; DOI: 10.1038/s41559-024-02461-1).

Moody et al have pushed back the estimated age of LUCA to halfway through the Hadean, between 4.09 to 4.33 billion years (Ga), well beyond the geologically known age of the earliest traces of life (3.5 Ga). That age for LUCA in itself is quite astonishing: it could have been only a couple of hundred million years after the Moon-forming interplanetary collision. Moreover, they have estimated that Darwin’s Ur-organism had a genome of around 2 million base pairs that encoded about 2600 proteins: roughly comparable to living species of bacteria and archaea, and thus probably quite advanced in evolutionary terms. The gene types probably carried by LUCA suggest that it may have been an anaerobic acetogen; i.e. an organism whose metabolism generated acetate (CH3COO−) ions. Acetogens may produce their own food as autotrophs, or metabolise other organisms (heterotrophs). If LUCA was a heterotroph, then it must have subsisted in an ecosystem together with autotrophs which it consumed, possibly by fermentation. To function it also required hydrogen that can be supplied by the breakdown of ultramafic rocks to serpentinites, which tallies with the likely ocean-world with ultramafic igneous crust of the Hadean (see the earlier paragraphs about protocells). If an autotroph, LUCA would have had an abundance of CO2 and H2 to sustain it, and may have provided food for heterotrophs in the early ecosystem. The most remarkable possibility discerned by Moody et al is that LUCA may have had a kind of immune system to stave off viral infection.

The carbon cycle on the Hadean Earth (Credit: Moody et al. 2024; Figure 3e)

The Hadean environment was vastly different to that of modern times: a waterworld seething with volcanism; no continents; a target for errant asteroids and comets; more rapidly spinning with a 12 hour day; a much closer Moon and thus far bigger tides. The genetic template for the biosphere of the following four billion years was laid down then. LUCA and its companions may well have been unique to the Earth, as are their descendants. It is hard to believe that other worlds with the potential for life, even those in the solar system, could have followed a similar biogeochemical course. They may have life, but probably not as we know it  . . .

See also: Ball, P. 2025. Luca is the progenitor of all life on Earth. But its genesis has implications far beyond our planet. The Observer, 19 January 2025.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook