Author: zooks777

Former Senior Lecturer at British Open University, research in remote sensing of arid lands, groundwater exploration, Precambrian tectonics and geochemistry

A way for early humans to leave Africa for Eurasia via the Middle East

Published on by zooks777Leave a comment

Without seafaring skills and sturdy boats, ancient humans had only two options to leave Africa for Eurasia: by crossing the Straits of Bab el Mandab at the southern end of the Red Sea and from the Nile delta to the Levant at its northern end. Both would have been difficult. The first route demanded extremely low sea level drawn down by continental ice accumulation to narrow the sea crossing, the earliest in the last glacial cycle being around 70 ka ago. The northern route, with no sea crossing, was potentially achievable throughout the history of the genus Homo. But that way is beset to the north and east by deserts with large tracts that today lack natural water sources. To leave Africa by that route seems the most obvious, being reached along the well-watered Nile valley or the Red Sea coast with its abundant marine resources. Yet moving eastwards to Arabia and further would have required climatic windows of opportunity to ensure well-watered corridors: it would be impossible today without an infrastructure of wells; and edible resources are extremely sparse. Remains of anatomically modern humans (AMH) as old as 200 ka and others in the period between 130 to 85 ka have been found around the eastern shores of the Mediterranean. Either of the two routes could have led them there during periods of increased humidity, perhaps in a series of migratory pulses. In the case of an exodus across the Straits of Bab el Mandab, people could have moved northwards along the Red Sea coast of modern Yemen and Arabia to the Levant. However, the record is patchy, and there is no direct fossil evidence to suggest they went further, into southern Asia or Europe in these earlier times. Each early venture may also have ended in extinction.  The first presence of AMH in Asia and Europe, seems to have been tens of thousand years later: about 75 ka and 45 ka, respectively, so far as we know.

Left: Satellite image of the Arabia and the Levant, showing the possible northern (red) and southern migration routes (blue) and sites that yielded various palaeoclimatic signs of formerly wet areas, Homo sapiens fossils and stone tools (see key). Right colour-coded map of topographic elevation for the study area in the Levant with sites that reveal palaeoclimatic and anthropological information. (Credit: Abbas et al., Fig 1)

Research in the Arabian Peninsula has early recorded human presence from discarded stone artefacts at widely scattered sites, as far east as the UAE and Oman, but whether these were carried by AMH or other human groups is uncertain. Yet geological research suggests that even in the presently forbidding Empty Quarter of Saudi Arabia there were from time to time abundant springs, river networks and even lakes: occasionally climate changes made much of Arabia habitable. Researchers from the University of Southampton (UK) and Shantou University (China), together with colleagues in Jordan, Australia and the Czech Republic have documented further evidence for ‘green’ episodes on the Jordan Plateau – part of the currently hyperarid  Arabian interior (Abbas, M. and 10 others 2023. Human dispersals out of Africa via the Levant. Science Advances, v.9, article eadi6838; DOI: 10.1126/sciadv.adi6838).

Three sites in Jordan reveal wetland sediments incised by now dry channels or wadis, one of which yielded stone tools Luminescence dating of wetland sediment grains shows the times when they were last exposed to sunlight: some between 86 to 65 ka, others between 57 to 43 ka. Together with data from the rest of Arabia the sites help roughly to define routes that would have permitted human migration, though not the actual directions that early AMH might have travelled or their destinations – if any. They may just have wandered around surviving on the resources that they found during short periods of amenable local climate, and vegetation much as do desert dwellers today. Actually to exit Arabia to southern Asia would require migration around what is now the Persian Gulf, where relevant data are lacking and likely to remain so while poor security for research prevails. To get to Europe would require a much more intricate journey through large mountainous tracts to reach the shores of the Black Sea.

See also: Early human migrants followed lush corridor-route out of Africa. Science Daily. 4 October 2023

Sudden climate change: a warning from 8 millennia ago

Published on by zooks777Leave a comment

Mesolithic hunter-gatherers in Britain must have had a very hard time around 8.2 thousand years age. The whole area around the North Atlantic experienced sudden climatic cooling of around 3.3°C together with drought that lasted about 70 years. To make things worse shortly afterwards, coasts around the North were devastated by a tsunami generated by a submarine landslide off western Norway. That event exceeded the maximum coast ‘run up’ of both the 26 December 2004 Indian Ocean tsunami and that in NW Japan on 11 March 2011. Doggerland, then in the central North Sea was devastated by a catastrophic event of a few days duration. It littered the seabed with the bones of its megafauna and even Mesolithic tools recovered by trawlers from its surviving relic the shallow Dogger Bank. It seems the tsunami arrived just as climate was warming back to ‘normal’ Holocene conditions: for many foragers, surely, a last straw.

The cooling episode has been attributed to perturbation of the Atlantic Meridional Overturning Circulation (AMOC) as a result of meltwater discharge during the deglaciation of the Laurentide Ice Sheet (see: Just when you think it’s going to turn out alright… November 2009).The event may have unfolded in a similar fashion to the trigger for the Younger Dryas and the succession of warming-cooling episodes known as Dansgaard-Oeschger events that interrupted the otherwise relentless global cooling towards the last glacial maximum (see: Review of thermohaline circulation; February 2002). The physics that set off such climatic ‘hiccups’ is that freshening of surface seawater reduces its density, so that it cannot sink to be replaced by denser saline water ‘dragged’ northwards from warmer latitudes. That currently takes the form of the Gulf Stream with its warming influence, particularly in the eastern North Atlantic and even beyond Norway’s North Cape, responsible for much warmer winters than at similar latitudes on the western side. The culprit  had long been suggested to be the drainage of a huge lake dammed by the ice sheet that covered most of eastern Canada during late stages of deglaciation. Seemingly the best candidate was Lake Agassiz trapped by the early Holocene ice front in Manitoba – the largest proglacial lake known anywhere.

Colour coded topographic elevation of North America showing the maximum extent of Lake Agassiz and four possible routes for its drainage: north-west to the Arctic Ocean via the Mackenzie River; south to the Gulf of Mexico via the Mississippi valley; east to the North Atlantic via the Great Lakes and St Laurence River; north to the North Atlantic via Hudson Bay. (Credit: ©Sheffield University)

The present landforms of central Canada show evidence for several outflow directions at different times, Including to the northwest to reach the Arctic Ocean at the onset of the Younger Dryas. Until recently there was little detailed evidence for the flow volume and timing of its drainage around 8 to 9 ka. Providing the details in the context of the short-lived event around 8.2 ka requires accurate data over a mere 200 years able to reveal a change in sea level to a precision of better than a few tens of centimetre. Any site on the shores of the North Atlantic would do, provided it satisfies these criteria. Geographers from universities in York, Leeds, Sheffield and Oxford, UK selected the small estuary of the River Ythan in NE Scotland. There, a continuous sand unit just above fine-grained intertidal tidal muds marks the knife-sharp time datum of the Storegga tsunami (Rush, G. et al. 2023. The magnitude and source of meltwater forcing of the 8.2 ka climate event constrained by relative sea-level data from eastern Scotland. Quaternary Science Advances, v. 12, article 100119; DOI: 10.1016/j.qsa.2023.100119).

Cores of the intertidal sediments from beneath the present Ythan salt marsh contain plant remains that yielded precise radiocarbon dates at several stratigraphic levels from which to derive an age-depth model for the age range of interest. The buried sediments are also rich in marine microfossils (foraminifera and diatoms) that thrive in estuaries at a variety of depths.  These enabled fluctuations in relative sea level during the build-up of the intertidal sediments to be constrained at unprecedented resolution and precision for a three thousand year period from 9.5 to 6.5 ka. The authors show that there were two episodes of rapid sea-level rise over that time: between 8.53 and 8.37 ka (~2.4 m at 13 mm yr-1) and 8.37 to 8.24 ka (~ 0.6 m at 4 mm yr-1) – these would have been global increases in sea level.

Despite its vast size, it turns out that Lake Agassiz would have been unable to result in sea-level rises of that magnitude so quickly merely through outflow. Rush et al. suggest that the huge  and rapid addition of fresh water to the North Atlantic involved flow of lake water towards Hudson Bay, beneath the ice sheet, causing it to collapse and melt, followed by completion of Lake Agassiz’s emptying in the second stage. It took a long drawn-out ‘freshening’ of the North Atlantic surface water ultimately to shut down the Atlantic Meridional Overturning Circulation, thereby depriving high latitudes of its east-side warming effect by the Gulf Stream.

Sea level has been rising since the early 20th century mainly through the melting of Greenland’s ice cap together with a substantial amount of thermal expansion while global climate has been warming. Between 1901 and 2018 the rise has amounted to 15 to 25 cm at a rate of 1 to 2 mm yr-1. The AMOC is possibly weaker now than at any time during the last millennium (Zhu, C. et al. 2023. Likely accelerated weakening of Atlantic overturning circulation emerges in optimal salinity fingerprint. Nature Communications, v. 14, article 1245; DOI: 10.1038/s41467-023-36288-4). Yet increases in freshening of the northernmost parts of the North Atlantic are now being added to by annual increases in the melting of polar sea ice, which is salt-free. The AMOC may be approaching a tipping point, because warming is accelerating over Greenland at around 1.5°C each year: faster than most of the rest of the world. In 2021 it rained for the first time ever recorded at the ice cap’s summit (3.2 km above sea level). A ‘perturbation’ of the AMOC would add chaos to the dominantly linear view of global warming taken by climatologists. That could launch frigidity and drought at mid northern latitudes as it did eight millennia ago: the opposite of what is currently feared.

See also: Unlocking Ancient Climate Secrets – Melting Ice Likely Triggered Climate Change Over 8,000 Years Ago. Scitechdaily 16 September 2023.

Direct signs of what caused the Palaeocene-Eocene thermal maximum

Published on by zooks777Leave a comment

Until about 56 Ma ago North America and Europe were connected: one of the last relics of the Pangaea supercontinent. Oxygen isotopes and magnesium/calcium ratios in the tests of both surface- and bottom-dwelling foraminifera suggest that around that time global mean surface temperature increased by about 5 to 6°C within 10 to 20 thousand years. The rate of global warming was comparable to that currently being induced by human activities. The Palaeocene-Eocene thermal maximum (PETM) is seen by climatologists as a dreadful warning of times to come in the not so distant future. The PETM event marks the most dramatic biological changes since the mass extinction at the Cretaceous-Palaeogene boundary 10 million years earlier. They included the rapid expansions of mammals and land plants and major extinction of deep-water foraminifera. The PETM also coincided with an equally profound excursion in the δ13C of carbon-rich strata of that age, whose extreme negative value marks the release of a huge mass of previously buried organic carbon into the atmosphere. It was probably methane, much more potent at delaying heat loss to space than carbon dioxide – methane has more than 80 times the warming effect of carbon dioxide. Since CH4 is soon oxidised to CO2 and H2O estimates of atmospheric greenhouse gas levels are generally expressed in terms of CO2. The PETM release was equivalent to about 4.4 x 1013metrictons over 50 ka; on average 0.24 gigatons per year compared with 0.51 Gt from energy-related sources in 2022.

During the Palaeocene, areas around the present North Atlantic were subject to basaltic continental volcanism before the rifting that opened the North Atlantic from 62 to 58 Ma. Magmatism, dominated by intrusions, began again at the Palaeocene-Eocene boundary from 56 to 54 Ma, linked to the start of continental rifting. Both episodes suggest a rising mantle plume. Once the rift had truly opened volcanism became restricted to the mid Atlantic ridge and a mantle plume remains active beneath Iceland. After geoscientists became aware of the PETM and its coincidence with North Atlantic igneous activity many palaeoclimatologists suggested methane release from organic-rich sediments heated by intrusion of basaltic sills below the opening seaway (but see 2022 post on alternative hypotheses). As with so many extreme geological events, choosing a most-likely scenario depends ultimately on tangible evidence. A convincing sign has been demonstrated dramatically in a recent study by a multinational team of geophysicists, oceanographers, geochemists, palaeontologists and sedimentologists (Berndt, C. and 35 others 2023. Shallow-water hydrothermal venting linked to the Palaeocene–Eocene Thermal Maximum. Nature Geoscience, v. 16, p. 803–809; DOI: 10.1038/s41561-023-01246-8).

Three-dimensional view of seismic reflection data off western Norway. The greytone lower part is a vertical ‘slice’. The coloured part shows the depth variation of sediments that fill hydrothermal vent systems beneath a horizontal unconformity. (Credit: Berndt et al, Fig 1b)

The breakthrough by Berndt et al. stemmed from a detailed 3-D seismic survey off the coast of Norway. It revealed an unconformity at the P-E boundary beneath which were clear signs of hundreds of large pockmarks, up to 80 m deep. Seismic reflection from older sediments beneath the unconformity showed the distinctive presence of intrusive sills of igneous rocks. The consortium drilled 20 boreholes into the seabed beneath the survey area. Five of them penetrated crater-like features to yield cores through the sediments that had filled them. The fills were muds, which were interleaved beds of volcanic ash in the sequences marking the P-E boundary suggesting an igneous influence. Organic remains in the muds established the depositional timing of several distinct layers and also gave clues to their depositional conditions. Those spanning the 50 ka of the PETM were dominated by plant debris, pollen and spores, together with abundant marine diatoms that live in very shallow water. Laminations in the muds dip radially inwards towards the deeper parts of some craters to define funnel-like structures. In others the sediments have been domed upwards. The sediments and their structures closely resemble those in blow-out craters formed during petroleum drilling accidents and in onshore maar volcanoes produced by sudden explosive eruptions on land. The pockmarks formed suddenly, to be filled by mobilised mud and volcanic ash.

The evidence points to explosive vents formed by massive degassing of deeper sediments induced by igneous intrusions. Such systems are common around active ocean-floor rifts: ‘black-‘ and ‘white smokers’, but those off Norway formed in shallow water. That has an important bearing on their potency during the PETM. Deep hydrothermal systems may emit methane, but it is oxidised to CO2 in seawater. Those very close to the surface vent their gas almost directly into the atmosphere before such oxidation can consume methane. Intrusive sills also underlie the eastern continental margin of Greenland, so such explosive hydrothermal vents may have been widespread during the initial rifting of the North Atlantic’.

An evolutionary bottleneck and the emergence of Neanderthals, Denisovans and modern humans

Published on by zooks777Leave a comment

The genetic diversity of living humans, particularly among short, repetitive segments of DNA, is surprisingly low. As they are passed from generation to generation they have a high chance of mutation, which would be expected to create substantial differences between geographically separated populations. In the late 1990s and early 2000s some researchers attributed the absence of such gross differences to the human gene pool having been reduced to a small size in the past, thereby reducing earlier genetic variation as a result of increased interbreeding among survivors. They were able to assess roughly when such a population ‘bottleneck’ took place and the level to which the global population fell. Genetic analysis of living human populations seemed to suggest that around 74 ka ago the global human population fell to as little as 10 thousand individuals. A potential culprit was the catastrophic eruption of the Toba supervolcano in Sumatra around that time, which belched out 800 km3 of ash now found as far afield as the Greenland and Antarctic ice caps. Global surface temperature may have fallen by 10°C for several years to decades. Subsequent research has cast doubt on such a severe decline in numbers of living hummans; for instance archaeologists working in SE India found much the same numbers of stone tools above the Toba ash deposit as below it (see: Toba ash and calibrating the Pleistocene record: December 2012). Other, less catastrophic explanations for the low genetic diversity of modern humans have also been proposed. Nevertheless, environmental changes that placed huge stresses on our ancestors may repeatedly have led to such population bottlenecks, and indeed throughout the entire history of biological evolution.

An improved method of ‘back-tracking’ genetic relatedness among living populations, known as fast infinitesimal time coalescence or ‘FitCoal’, tracks genomes of individuals back to a last common ancestor. In simple language, it expresses relatedness along lineages to find branching points and, using an assumed mutation rate, estimates how long ago such coalescences probably occurred. The more lineages the further back in time FitCoal can reach and the greater the precision of the analysis. Moreover it can suggest the likely numbers of individuals, whose history is preserved in the genetics of modern people, who contributed to the gene pool at different branching points. Our genetics today are not restricted to our species for it is certain that traces of Neanderthal and Denisovan ancestry are present in populations outside of Africa. African genetics also host ‘ghosts’ of so-far unknown distant ancestors. So, the FitCoal approach may well be capable of teasing out events in human evolution beyond a million years ago, if sufficient data are fed into the algorithms. A team of geneticists based in China, Italy and the US has recently applied FitCoal to genomic sequences of 3154 individual alive today (Hu, W.and 8 others 2023. Genomic inference of a severe human bottleneck during the Early to Middle Pleistocene transition. Science, v. 381, p. 979-984; DOI I: 10.1126/science.abq7487). Their findings are startling and likely to launch controversy among their peers.

Their analyses suggest that between 930 and 813 ka ago human ancestors passed through a population bottleneck that involved only about 1300 breeding individuals. Moreover they remained at the very brink of extinction for a little under 120 thousand years. Interestingly, the genetic data are from people living on all continents, with no major differences between the analyses for geographically broad groups of people in Africa and Eurasia. Archaeological evidence, albeit sparse, suggests that ancient humans were widely spread across those two continental masses before the bottleneck event. The date range coincides with late stages of the Mid-Pleistocene climatic transition (1250 to 750 ka) during which glacial-interglacial cycles changed from 41 thousand-year periods to those that have an average duration of around 100 ka. The transition also brought with it roughly a doubling in the mean annual temperature range from the warmest parts of interglacials to the frigid glacial maxima: the world became a colder and drier place during the glacial parts of the cycles.

Genomes for Neanderthals and Denisovans suggest that they emerged as separate species between 500 and 700 ka ago. Their common ancestor, possibly Homo heidelbergensis, H. antecessor or other candidates (palaeoanthropologists habitually differ) may well have constituted the widespread population whose numbers shrank dramatically during the bottleneck. Perhaps several variants emerged because of it to become Denisovans, Neanderthals and, several hundred thousand years later, of anatomically modern humans. Yet it would require actual DNA from one or other candidate for the issue of last common ancestor for the three genetically known ‘late’ hominins to be resolved. But Hu et al. have shown a possible means of accelerated hominin evolution from which they may have emerged, at the very brink of extinction.

Oxygen-isotope record and global temperature changes over the last 5 million years, green lines showing the times dominated by 41 and 100 ka climatic cycles. The mid-Pleistocene climatic transition is shown in pink (Credit: Robert A Rohde)

There is a need for caution, however. H. erectus first appeared in the African fossil record about 1.8 Ma ago and subsequently spread across Eurasia to become the most ‘durable’ of all hominin species. Physiologically they seem not to have evolved much over at least a million years, nor even culturally – their biface Acheulean tools lasted as long as they did. They were present in Asia for even longer, and apparently did not dwindle during the mid-Pleistocene transition to the near catastrophic levels as did the ancestral species for living humans. The tiny global population suggested by Hu et al. for the latter also hints that their geographic distribution had to be very limited; otherwise widely separated small bands would surely have perished over the 120 ka of the bottleneck event. Yet, during the critical period from 930 to 813 ka even Britain was visited by a small band of archaic humans who left footprints in river sediments now exposed at Happisburgh in Norfolk. Hu et al. cite the scarcity of archaeological evidence from that period – perhaps unwisely – in support of their bottleneck hypothesis. There are plenty of other gaps in the comparatively tenuous fossil and archaeological records of hominins as a whole.

The discovery of genetic evidence for this population bottleneck is clearly exciting, as is the implication that it may have been the trigger for evolution of later human species and the stem event for modern humans. Hopefully Hu et al’s work will spur yet more genetic research along similar lines, but there is an even more pressing need for field research aimed at new human fossils from new archaeological sites.

See also: Ashton, N. & Stringer, C. 2023. Did our ancestors nearly die out? Science (Perspectives), v. 381, p. 947-948; DOI: 10.1126.science.adj9484.

Ikarashi, A. 2023. Human ancestors nearly went extinct 900,000 years ago. Nature, v. 621; DOI: 10.1038/d41586-023-02712-4

Di Vicenzo, F & Manzi, G. 2023. An evolutionary bottleneck and the emergence of Neanderthals, Denisovans and modern humans. Homo heidelbergensis as the Middle Pleistocene common ancestor of Denisovans, Neanderthals and modern humans. Journal of Mediterranean Earth Sciences, v, 15, p. 161-173; DOI: 10.13133/2280-6148/18074

When and why did the North American Pleistocene megafauna collapse?

Published on by zooks777Leave a comment

The US city of Los Angeles, originally known as El Pueblo de Nuestra Señora la Reina de los Ángeles (The Town of Our Lady the Queen of the Angels), was founded in 1781 by 44 Spanish settlers. It remained a small cattle-centred town after the annexation of California from Mexico by the USA in 1847. Once it was reached by the transcontinental Southern Pacific railroad in 1876 it had the potential for growth. But it took the discovery of oil within its limits in 1892 for its population to increase rapidly. The Los Angeles City Oil Field became the top producer in California with 200 separate oil companies crammed cheek by jowl by 1901. Now only one remains, producing just 3.5 barrels per day. That crude oil was there for the taking was pretty obvious as bitumen seeps had long been exploited by native people and the original Spanish colonists. The oilfield was developed near one such seep: the Rancho La Brea tar pits.

Rancho La Brea tar pit and derricks of the Los Angeles City Oil Field in 1901

By 1901 perfectly preserved bones of a huge variety of animals – 231 vertebrate species – as well as plants and invertebrates began to be collected from the continually roiling pond of bitumen. Thousands of specimens have been collected since then, both predators and prey of all sizes. Famous for mastodons and sabre-toothed cats, La Brea is a repository of almost the entire western Californian fauna through much of the Late Pleistocene: before about 100 ka the area lay beneath the Pacific Ocean. Tar pits are traps for unwary animals of any kind, especially as shallow water often hides the danger. Carnivores seeking easy, abundant food end up trapped too.

Because of the anaerobic nature of bitumen, bacterial decay is suppressed. Many of the bones still contain undegraded collagen: the most abundant protein in mammals, which can be dated using the radiocarbon method. So, despite the lack of stratigraphy in the tar pits, it is possible to track the history of the ecosystem by painstaking dating of individual fossils (OKeefe, F.R and 18 others 2023. Pre–Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift. Science, v. 381, article eabo3594; DOI: 10.1126/science.abo3594). Robin OKeefe and colleagues dated 169 specimens of eight large mammal species most commonly found in the bitumen: sabre-toothed cat (Smilodon fatalis); dire wolf (Aenocyon dirus); coyote (Canis latrans); American lion (Panthera atrox); ancient bison (Bison antiquus); western horse (Equus occidentalis); Harlan’s ground sloth (Paramylodon harlani); and yesterday’s camel (Camelops hesternus).

The authors focussed on precisely dated specimens spanning the 15.6 to 10.0 ka time range. This would allow the disappearance times of individual species to be compared with stages in the rapid change in the Californian climate during post glacial maximum warming, those during the Younger Dryas abrupt cooling (12.9 to 11.7 ka) and the earliest Holocene warming that succeeded it. The first to go extinct were the camels and giant sloths about 13.6 ka ago. At 13.2 ka the other mammals declined very rapidly, the two remaining herbivores vanishing more quickly than the four predators. By 12.9 ka the only surviving species of the chosen eight was the coyote. So seven members of the Pleistocene mammalian megafauna became extinct before the onset of the Younger Dryas cold millennium.

Part of the team examined pollen from a core through sediments deposited in a lake 100 km south of La Brea. They found that flora, and probably climate, had not changed at the time of camel and sloth extinctions around 13.6 ka. However a 300 year period between 13.2 and 12.9 ka witnessed a collapse in deciduous tree species while conifers, grasses and drought-tolerant shrubs increased. A woodland ecosystem had been replaced by semi-arid chaparral. Another feature of the lake-bed sediments was that charcoal fragments increased explosively during that 300-year episode that ended both the woodland ecosystem and the megafauna that exploited it: undoubtedly three centuries of regular wildfires. What remained was the chaparral ecosystem based on drought-tolerant, fire-adapted plants.

Were the megafauna collapse and a change in ecology results of a climatic harbinger for the Younger Dryas cool millennium, or some other cause? Interestingly, tangible evidence for the Clovis hunting culture of North America, which has long been implicated in the faunal ‘extirpation’, does not appear until 12.9 ka, and in California neither does any implicating other human groups. Yet evidence is accumulating for much earlier entry of humans into North America. Occupation sites are very rare on land, but human presence here and there implies such earlier migration, probably along the west coast that avoided the frigid interior further north than California. The question posed by OKeefe ­et al. is, ‘Were the fires ignited by humans over a 300 year period just before the Younger Dryas’? It remains to be confirmed … First human arrivals coinciding with evidence for wildfires in Australia, New Zealand and a few other areas do suggest that it is a possibility. There needs to be a motive, such as producing lush clearings in forest to attract game, or removing cover to make hunting easier. In this case, the fires immediately preceded a global climatic downturn with terrestrial drying, so they may have had natural causes: the potentially incendiary chaparral flora had been increasing steadily beforehand and decreased rapidly after the evidence for wildfires

See also: Price, M. 2023.  Death by fire. Science, v. 381, p. 724-727; DOI: 10.1126/science.adk3291

A book on archaeology, radiocarbon dating, ancient DNA, and how modern humans evolved

Published on by zooks777Leave a comment

Since 2001 Tom Higham, now Professor of Scientific Archaeology at the University of Vienna, helped develop new ways of refining radiocarbon dating at Oxford University’s Research Lab for Archaeology and the History of Art. Specifically his lab learned how to remove contamination of ancient samples by recent carbon and to reduce the detection limit of their accelerator mass spectrometer for the 14C atoms that remained from when they were in living organisms. The Oxford Radiocarbon Accelerator Unit pushed sample dates to the absolute limit of the method: around 50 thousand years. Being among the very best, the ORAU had a path beaten to its doors by archaeologists from across the world keen to get the most believable dates for their samples. Equally, Higham engaged in the field work itself and in the interpretation of other data from sites, such as ancient DNA. An outcome of Higham’s energetic efforts over two decades is his book The World Before Us: How Science is Revealing a New Story of Our Human Origins (paperback edition 2022, Penguin Books,ISBN-10: ‎0241989051). One reviewer commented ‘The who, what, where, when and how of human evolution’.

The World Before Us is not only comprehensive and eminently clear for the lay-reader, but it is more exciting than any science book that I have read. For the moment, it is the latest ‘word’ on early, anatomically modern humans and on the closely related Neanderthals and Denisovans. Its core is about how these three key groups ‘rubbed along’ once they met  in the Late Pleistocene. As an amateur interested in palaeoanthropology, I have tried to keep pace with all the developments in the field since 2001 through Earth-logs, but Higham shows just how much I have missed that is important to the human story. If you have followed my many posts on human evolution and migrations with interest, read his book for a great deal more and a coherent story of how things stand.

News about when subduction began

Published on by zooks777Leave a comment

Tangible signs of past subduction take the form of rocks whose mineralogy shows that they have been metamorphosed under conditions of high pressure and low temperature, and then returned to the surface somehow. Ocean-crust basaltic rocks become blueschist and eclogite. The latter is denser than mantle peridotite so that oceanic lithosphere can sink and be recycled. That provides the slab-pull force, which is the major driver of plate tectonics. Unfortunately, neither blueschists nor eclogites are found in metamorphic complexes older than about 800 Ma. This absence of direct proof of subduction and thus modern style plate tectonics has resulted in lively discussion and research seeking indirect evidence for when it did begin, the progress of which since 2000 you can follow through the index for annual logs about tectonics. An interesting new approach emerged in 2017 that sought a general theory for the evolution of silicate planets, which involves the concept of ‘lid tectonics’. A planet in a stagnant-lid phase has a lithosphere that is weak as a result of high temperatures: indeed so weak and warm that subduction was impossible. Stagnant-lid tectonics does not recycle crustal material back to its source in the mantle and it simply builds up the lithosphere. Once planetary heat production wanes below a threshold level that permits a rigid lithosphere, parts of the lid can be driven into the mantle. The beginnings of this mobile-lid phase and thus plate tectonics of some kind involves surface materials in mantle convection: the may be recycled.

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)

A group of geochemists from China, Canada and Australia have sought evidence for recycled crustal rocks from silicon and oxygen isotopes in the oldest large Archaean terrane, the  4.0 Ga old Acasta Gneiss Complex in northern Canada (Zhang, Q. and 10 others 2023. No evidence of supracrustal recycling in Si-O isotopes of Earth’s oldest rocks 4 Ga ago. Science Advances, v.9, article eadf0693; DOI: 10.1126/sciadv.adf0693). Silicon has three stable isotopes 28Si, 29Si, and 30Si. As happens with a number of elements, various geochemical processes are able to selectively change the relative proportions of such isotopes: a process known as isotope fractionation. As regards silicon isotopes used to chart lithosphere recycling, the basic steps are as follows: Organisms that now remove silicon from solution in seawater to form their hard parts and accumulate in death as fine sediments like flint had not evolved in the Archaean. Because of that reasonable supposition it has been suggested that seawater during the Archaean contained far more dissolved silicon than it does now. Such a rich source of Si would have entered Archaean oceanic crust and ocean-floor sediments to precipitate silica ‘cement’. The heaviest isotope 30Si would have left solution more easily than the lighter two. Should such silicified lithosphere have descended to depths in the mantle where it could partially melt the anomalously high 30Si would be transferred to the resulting magmas.

Proportions of 30Si in zircons, quartz and whole rock for Acasta gneisses (coloured), other Archaean areas (grey) and Jack Hills zircons (open circles. Vertical lines are error bars. (Credit: simplified from Zhang et al. Fig 1)

Stable-isotope analyses by Zhang et al. revealed that zircon and quartz grains and bulk rock samples from the Acasta gneisses, with undisturbed U-Pb ages, contain 30Si in about the same proportions relative to silicon’s other stable isotopes as do samples of the mantle. So it seems that the dominant trondhjemite-tonalite-granodiorite (TTG) rocks that make up the oldest Acasta gneisses were formed by partial melting of a source that did not contain rocks from the ocean crust. Yet the Acasta Gneiss Complex also contains younger granitic rocks (3.75 to 3.50 Ga) and they are significantly more enriched in 30Si, as expected from a deep source that contained formerly oceanic rocks. A similar ‘heavy’ silicon-isotope signature is also found in samples from other Archaean terranes that are less than 3.8 Ga old. Thus a major shift from stagnant-lid tectonics to the mobile-lid form may have occurred at the end of the Hadean. But apart from the Acasta Gneiss Complex only one other, much smaller Hadean terrane has been discovered, the 4.2 Ga Nuvvuagittuq Greenstone Belt. It occupies a mere 20 km2 on the eastern shore of Hudson Bay in Canada, and appears to be a sample of Hadean oceanic crust. It does include TTG gneisses, but they are about 3.8 Ga old and contain isotopically heavy silicon. So it seems unlikely that testing this hypothesis with silicon-isotope data from other Hadean gneissic terranes will be possible for quite a while, if at all.

Geochemical evidence for the origin of eukaryotes

Published on by zooks777Leave a comment

Along with algae, jellyfish, oak trees, sharks and nearly every organism that can be seen with the naked eye, we are eukaryotes. The cells of every member of the Eukarya, one of the three great domains of life, all contain a nucleus – the main location of genetic material – and a variety of other small bodies known as organelles, such as the mitochondria of animals and the chloroplasts of plant cells. The vast bulk of organisms that we can’t see unaided are prokaryotes, divided into the domains of Bacteria and Archaea. Their genetic material floats around in their cells’ fluid. The DNA of eukaryotes shares some stretches with prokaryotes, but no prokaryotes contain any eukaryote genetic material. This suggests that the Eukarya arose after the Bacteria and Archaea, and also that they are a product of evolution from prokaryotes, probably by several combining in symbiotic relationships inside a shared cell membrane. Earth-logs has followed developments surrounding this major issue since 2002, as reflected in some of the posts linked to what follows. 

While prokaryotes can live in every conceivable environment at the Earth’s surface and even in a few kilometres of crust beneath, the vast majority of eukaryotes depend on free oxygen for their metabolism. Logically, the earliest of the Eukarya could only have emerged when oxygen began to appear in the oceans following the Great Oxidation Event around 2.4 billion years ago. That is more than a billion years after the first prokaryotes had left their geological signature in the form of curiously bulbous, layered carbonate structures (stromatolites), probably formed by bacterial mats. The oldest occur in the Archaean rocks of Western Australia as far back as 3.5 Ga, and disputed examples have been found in the 3.7 Ga Isua sediments of West Greenland. The oldest of them are thought to have been produced through the anoxygenic photosynthesis of purple bacteria (See: Molecular ‘fossils’ and the emergence of photosynthesis; September 2000), suggested by organic molecules found in kerogen from early Archaean sediments. Later stromatolites (<3.0 Ga) have provided similar evidence for oxygen-producing cyanobacteria.

Acritarchs are microfossils of single-celled organisms made of kerogen that have been found in sediments up to 1.8 billion years old. Features protruding from their cell walls distinguish them from prokaryote cells, which are more or less ‘smooth’: acritarchs have been considered as possible early eukaryotes. Yet the oldest undisputed eukaryote microfossils – red and green algae – are much younger (about 1.0 Ga). A means of estimating an age for the crown group from which every later eukaryote organism evolved – last eukaryotic common ancestor (LECA) – is to use an assumed rate of mutation in DNA to deduce the time when differences in genetics between living eukaryotes began to diverge: i.e. a ‘molecular clock’. This gives a time around 2 Ga ago, but the method is fraught with uncertainties, not the least being the high possibility of mutation rates changing through time. So, when the Eukarya arose is blurred within the so-called ‘boring billion’ of the early Proterozoic Eon. A way of resolving this uncertainty to some extent is to look for ‘biomarker’ chemicals in the geological record that provide a ‘signature’ for eukaryotes.

A new study has been undertaken by a group of Australian, German and French scientists to analyse sediments ranging in age from 635 to 1640 Ma from Australia, China, Asia, Africa, North and South America (Brocks, J.J and 9 others 2023. Lost world of complex life and the late rise of the eukaryotic crown. Nature, v. 618, p. 767–773; DOI: 10.1038/s41586-023-06170-w; contact for PDF). Their chosen biomarkers are sterols (steroids) that regulate eukaryote cell membranes. Some prokaryotes also synthesise steroids but all of them produce hopanepolyols (hopanoids), which eukaryotes do not. The key measures for the presence/absence of eukaryote remains in ancient sea-floor sediments is thus the relative proportions of preserved steroids and hopanoids, together with those for the breakdown products of both – steranes and hopanesthat are, crudely speaking, carbon ‘skeletons’ of the original chemicals.

Proportions of biomarkers in sediments from present to 1.64 Ga. Cholesteroids – reds; ergosteroids – blues; stigmasteroids – greens; protosteroids magentas, hopanoids – yellows; unsampled – grey. Snowball glaciations are shown in pale blue. (Credit: Simplified from Figure 3 in Brocks et al.)

Interpretation of the results by Jochen Brocks and colleagues is complicated, and what follows is a summary based partly on an accompanying Nature News & Views article(Kenig, F. 2023. The long infancy of sterol biosynthesis. Nature, v. 618, p. 678-680; DOI: 10.1038/d41586-023-01816-1). The conclusions of Brocks et al. are surprising. First, the break-down products of steroids (saturated steranes) that can be attributed to crown eukaryotes (left on the figure above) are only present in sediments going back to about 200 Ma before the first Snowball Earth event (~900 Ma). Before that only hopanes formed by hopanoid degradation are present: a suggestion that LECA only appeared around that time – the authors suggest sometime between 1 and 1.2 Ga. That is far later than the time when eukaryotes could have emerged: i.e. once there was available oxygen after the Great Oxidation Event (~2.4 to 2.2 Ga). So what was going on before this? The authors broke new ground in analysis of biomarkers by being able to detect signs of the presence of actual hopanoids and steroids of several different kinds. Steroids were present as far back as 1.6 Ga in the oldest sediments that were analysed.

Steroids of crown eukaryotes are represented by cholesteroids, ergosteroids and stigmasteroids. All three are present throughout the Phanerozoic Eon and into the time of the Ediacaran Fauna that began 630 Ma ago. In that time span they generally outweigh hopanoids, thus reflecting the dominance of eukaryotes over prokaryotes. Back to about 900 Ma, only cholesteroids are present, together with archaic forms that are not found in living Eukarya, termed protosteroids.  Before that, only protosteroids are found. Moreover, these archaic steroids are not present in sediments that follow the Snowball Earth episodes (the Cryogenian Period).

Thus, it is possible that crown group eukaryotes – and their descendants, including us – evolved from and completely replaced an earlier primitive form (acritarchs?) at around the time of the greatest climatic changes that the Earth had experienced in the previous billion years or more. Moreover, the Cryogenian and Ediacaran Periods seem to show a rapid emergence of stigmasteroid- and ergosteroid production relative to cholesteroid: perhaps a result of explosive evolution of the Eukarya at that time. The organisms that produced protosteroids were present in variable amounts throughout the Mesoproteroic. Clearly there need to be similar analyses of sediments going back to the Great Oxygenation Event and the preceding Archaean to see if the protosteroid producers arose along with increasing levels of molecular oxygen. The ‘boring billion’ (2.0 to 1.0 Ga) may well be more interesting than previously thought.

Early modern human fossils from a Laotian cave and the eastward ‘out-of-Africa’ migration

Published on by zooks7771 Comment

Finding human fossils in SE Asia is rare because its tropical climate generally results in decomposition of bones. Up to now the oldest known anatomically modern human (AMH) found beyond the Middle East is from Australia and has been dated to 65 ka. Other, less convincing candidates for the earliest appearance of AMH in Asia are scattered teeth found in Chinese caves that yielded dates of up to 139 ka: their assignment to AMH and the reliability of their dating are disputed. Now a large team of scientists from the USA, Germany, Australia, South Africa, France, Denmark and Laos have unearthed convincing but fragmented AMH bones among a jumble of diverse animal fossils in sediment flooring Tam Pà Ling cave  in northern Laos (Friedline, S.E. and 30 others 2023. Early presence of Homo sapiens in Southeast Asia by 86–68 kyr at Tam Pà Ling, Northern Laos. Nature Communications, v. 14, article 3193; DOI: 10.1038/s41467-023-38715-y). Several dating techniques reveal ages of the AMH samples that range from 46 to 77 ka, and potentially as far back as 86 ka. It is conceivable that the oldest are from the population that subsequently reached Australia. Far to the west of Laos in Greece, Israel and Arabia an earlier AMH presence goes back as far as 90 to 210 ka. Moreover, palaeoclimatic studies suggest many opportunities for eastward migration since 290 ka ago that AMH emigrants may have exploited. Once beyond regions around Arabia and the Gulf, which were periodically hyperarid, the journey to the rest of Asia was probably continuously habitable throughout the last two glacial-interglacial cycles.

Entrance to Tam Pà Ling cave in northern Laos (credit: Demeter et al.; Fig S1)

Another aspect of the AMH record in southern and SE Asia is that the individuals represented seem to have been anatomically very varied (Demeter, F. et al. 2023. Early Modern Humans and Morphological Variation in Southeast Asia: Fossil Evidence from Tam Pa Ling, Laos. PLOS ONE, v. 10, article e0121193. DOI:10.1371/journal.pone.0121193). This may suggest that migration was by significantly different groups at different times. Oddly, the earliest known examples have more ‘modern’ characteristics than younger ones that appear somewhat ‘archaic’. The age of the fossils conflicts with the 60 ka age reconstructed from genetic evidence for the main diffusion across Eurasia and Australasia. One possibility is that there were several pre-60 ka migrations, descendents of these early populations having been replaced or assimilated by a later, larger numbers of AMH migrants. At 74 ka the Sumatran Toba supervolcano erupted about 2,800 km3 of ash to blanket a vast area and cause global cooling that could have more than decimated migrating AMH groups. Alternatively the 60 ka ‘genetics’ date is not correct, as suggested by the minimum date of 65 ka for the earliest Australians. Such a conflict of evidence will surely spur further excavation: as one researcher observed about Laos, ‘There are thousands of caves to explore’.

See also: Coleman, J. 2023. Laos cave fossils prompt rethink of human migration map. Nature, v.618; DOI: 10.1038/d41586-023-01903-3; Ashworth, J. 2023. Fossils reveal early modern humans in southeast Asia 77,000 years ago. Natural History Museum’s Discover, 15 June 2023.

Did Precambrian BIFs ‘fall’ into the mantle to trigger mantle plumes?

Published on by zooks7772 Comments

How the Earth has been shaped has depended to a large extent on a very simple variable among rocks: their density. Contrasts in density between vast rock masses are expressed when gravity attempts to maintain a balance of forces. The abrupt difference in elevation of the solid surface at the boundaries of oceans and continents – the Earth’s hypsometry – stems from the contrasted densities of continental and oceanic crust: the one dominated by granitic rocks (~2.8 t m-3) the other by those of basaltic composition (~ 3.0 t m-3). Astronomers have estimated that Earth’s overall density is about 5.5 t m-3 – it is the densest planet in the Solar System. The underlying mantle makes up 68% of Earth’s mass, with a density that increases with depth from 3.3 to 5.4 t m-3 in a stepwise fashion, at a number of discontinuities, because mantle minerals undergo changes induced by pressure. The remaining one third of Earth’s mass resides in the iron-nickel core at densities between 9.5 to 14.5 t m-3. Such density layering is by no means completely stable. Locally increased temperatures in mantle rocks reduce their density sufficiently for masses to rise convectively to be replaced by cooler ones, albeit slowly. By far the most important form of convection affecting the lithosphere involves the resorption of oceanic lithosphere plates at destructive margins, which results in subduction. This is thought to be due to old, cold oceanic basalts undergoing metamorphism as pressure increases during subduction. They are transformed at depth to a mineral assemblage (eclogite) that is denser (3.4 to 3.5 t m-3) than the enveloping upper mantle. That density contrast is sufficient for gravity to pull slabs of oceanic lithosphere downwards. This slab-pull force is transmitted through oceanic lithosphere that remains at the surface to become the dominant driver of modern plate tectonics. As a result, extension of the surface oceanic lithosphere at constructive margins draws mantle upwards to partially melt at reduced pressure, thus adding new basaltic crust at mid-ocean rift systems to maintain a form of mantle convection. Seismic tomography shows that active subducted slabs become ductile about 660 km beneath the surface and below that no earthquakes are detected. Quite possibly, the density of the reconstituted lithospheric slab becomes less than that of the mantle below the 660 km discontinuity. So the subducted slab continues by moving sideways and buckling in response to the ‘push’ from its rigid upper parts above. But it has been suggested that some subducted slabs do finally sink to the core-mantle boundary, but that is somewhat conjectural.

Typical banded iron formation

There are sedimentary rocks whose density at the surface exceeds that of the upper mantle: banded iron formations (BIFs) that contain up to 60% iron oxides (mainly Fe2O3) and have an average density at the surface of around 3.5 t m-3. BIFs formed mainly in the late Archaean and early Proterozoic Eons  (3.2 to 1.0 Ga) and none are known from the last 400 Ma. They formed when soluble iron-2 (Fe2+) – being added to ocean water by submarine hydrothermal activity –was precipitated as Fe3+ in the form of iron oxide (Fe2O3) where oxygen was present in ocean water. With little doubt this happened only in shallow marine basins where cyanobacteria that appeared about 3.5 Ga ago had sufficient sunlight to photosynthesise. Until about 2.4 Ga the atmosphere and thus the bulk of ocean water contained very little oxygen so the oceans were pervaded by soluble iron so that BIFs were able to form wherever such biological activity was going on. Conceivably (but not proven), that BIF-forming biochemical reaction may even have operated far from land in ocean surface water, slowly to deposit Fe2O3 on the deep ocean floor. After 2.4 Ga oxygen began to build in the atmosphere after the Great Oxidation Event had begon. That time was also when the greatest production of BIFs took place. Strangely, the amount of BIF in the geological record fell during the next 600 Ma to rise again to a very high peak at 1.8 Ga. Since there must have been sufficient soluble iron and an increasing amount of available oxygen for BIFs to form throughout that ‘lean’ period the drop in BIF formation is paradoxical. After 1.0 Ga BIFs more or less disappear. By then so much oxygen was present in the atmosphere and from top to bottom in ocean water that soluble iron was mostly precipitated at its hydrothermal source on the ocean floor. Incidentally, modern ocean surface water far from land contains so little dissolved iron that little microbiological activity goes on there: iron is an essential nutrient so the surface waters of remote oceans are effectively ‘wet deserts’.

Plots of probability of LIPs and BIFs forming at the Earth’s surface during Precambrian times, based on actual occurrences (Credit: Keller, et al., modified Fig 1A)

Spurred by the fact that if a sea-floor slab dominated by BIFs was subducted it wouldn’t need eclogite formation to sink into the mantle, Duncan Keller of Rice University in Texas and other US and Canadian colleagues have published a ‘thought experiment’ using time-series data on LIPs and BIFs compiled by other geoscientists (Keller, D.S. et al. 2023. Links between large igneous province volcanism and subducted iron formations. Nature Geoscience, v. 16, article; DOI: 10.1038/s41561-023-01188-1.). Their approach involves comparing the occurrences of 54 BIFs through time with signs of activity in the mantle during the Palaeo- and Mesoproterozoic Eras, as marked by large igneous provinces (LIPs) during that time span. To do this they calculated the degree of correlation in time between BIFs and LIPs. The authors chose a minimum area for LIPs of 400 thousand km2 – giving a total of 66 well-dated examples. Because the bulk of Precambrian flood-basalt provinces, such as occurred during the Phanerozoic, have been eroded away, most of their examples are huge, well-dated dyke swarms that almost certainly fed such plateau basalts. Rather than a direct time-correlation, what emerged was a match-up that covered 74% of the LIPs with BIFs that had formed about 241 Ma earlier. They also found a less precise correlation between LIPs associated with 241 Ma older BIFs and protracted periods of stable geomagnetic field, known as ‘superchrons’. These are thought by geophysicists to be influenced by heat flow through the core-mantle boundary (CMB).

The high bulk density of BIFs at the surface would be likely to remain about 15 % greater than that of peridotite as pressure increased with depth in the mantle. Such slabs could therefore penetrate the 660 mantle discontinuity. Their subduction would probably result in their eventually ‘piling up’ in the vicinity of the CMB. The high iron content of BIFs may also have changed the way that the core loses heat, thereby triggering mantle plumes. Certainly, there is a complex zone of ultra-low seismic velocities (ULVZ) that signifies hot, ductile material extending above the CMB. Because BIFs’ high iron-content makes them thermally highly conductive compared with basalts and other sediments, they may be responsible. Clearly, Keller et al’s hypothesis is likely to be controversial and they hope that other geoscientists will test it with new or re-analysed geophysical data. But the possibility of BIFs falling to the base of the mantle spectacularly extends the influence of surface biological processes to the entire planet. And, indeed, it may have shaped the later part of its tectonic history having changed the composition of the deep mantle. The interconnectedness of the Earth system also demands that the consequences – plumes and large igneous provinces – would have fed back to the Precambrian biosphere. See also: Iron-rich rocks unlock new insights into Earth’s planetary history, Science Daily, 2 June 2023