Good illustrations of self publicity and soaring ambition are the private space programmes of oligarchs Elon Musk (SpaceX), Jeff Bezos (Blue Origin) and Richard Branson (Virgin Galactic). For a cool US$65 million a ‘civilian’ can get a trip to the International Space Station on SpaceX; a one-hour suborbital flight on Blue Origin will cost US$300,000, with luck having Bezos as a companion; a reservation on Virgin Galactic for a 1 hour trip to the ‘edge of space’ (~100 km up) now costs US$624,000. It’s a tourist trip for the very, very rich only … but even the long-dead can go … or bits of them. On 8 September 2023 aboard Virgin Galactic flight Tim Nash, a South African billionaire had in his pocket a sturdy tube containing a thumb bone of Homo nalediand the collarbone of Australopithecus sediba. Nash reportedly said afterwards, “I am humbled and honoured to represent South Africa and all of humankind as I carry these precious representations of our collective ancestors”.
Reconstructed head of a somewhat annoyed Homo naledi. Credit: John Gurche, Mark Thiessen, National Geographic.
Nash was entrusted with these unique fossils by Lee Berger, Professor in Palaeoanthropology at Witwatersrand University, South Africa and a National Geographic Explorer-in-Residence. Berger recovered fossils of both species from limestone caves in the UNESCO World Heritage Site grandly named the Cradle of Humankind near Johannesburg. He is no stranger to controversy, and this venture cooked up with Nash seems to aim at promotion of South African achievements rather than having any scientific purpose. It has backfired spectacularly (see: McKie, R. 2023. ‘Callous, reckless, unethical’: scientists in row over rare fossils flown into space. The Observer, 22 October 2023). Comments from the anthropological world, six national and international bodies and perhaps the leading hominin specialist Professor Chris Stringer of the Natural History Museum in London include the words and phrases “callous”, “unethical”, “extraordinarily poorly thought-out”, “a publicity stunt”, “reckless” and “utterly irresponsible”. The caper breaks the South African, indeed international, scientific rule that fossils can only be allowed to travel for scientific purposes, applied consistently by similarly hominin-rich African countries such as Ethiopia, Kenya and Tanzania.
But, Hey, that’s how you get on in the world … isn’t it?
The theory of plate tectonics had a long gestation. Continental drift, one of its central tenets, was first proposed by the meteorologist Alfred Wegener in 1912. Apart from a few enthusiasts of such a dynamic aspect of geology, such as Alex du Toit and Arthur Holmes, the majority of geoscientists remained with the non-revolutionary fixist ideology of their Victorian predecessors. Wegener’s stumbling block was his proposed driving mechanism – polflucht (flight from the poles) – which assumed that supercontinents had formed in polar regions to be subject to centrifugal force resulting from Earth’s rotation. This broke them apart to be driven towards the Equator. Such a mechanism being easily invalidated, most contemporary geologists preferred to ‘throw Wegener’s baby out with the bathwater’. Yet every piece of his evidence that continents had moved around and most of his ideas about the nature of their movements were steadily verified and amplified over the next six decades, which attracted more curious and flexible scientists. What is now the central paradigm of the Earth Sciences had to wait for a set of major discoveries in the 1950s and ‘60s enabled by emerging technologies, such as the magnetometers used by Fred Vine and Drummond Matthews to discover sea-floor magnetic striping and thus sea-floor spreading. Their breakthrough presented a plausible mechanism for continental drift and launched a near frenzy of collaborative research among a global milieu of young geoscientists, one of whom being W. Jason Morgan.
W. Jason Morgan outside the Department of Earth Sciences, Princeton University. (Credit: Denise Applewhite, Princeton University)
His initial interest was in the great fracture zones on the floors of the Atlantic and Pacific Oceans. He grasped that each of them was very nearly a great circle. This was a central key to unifying seafloor spreading and continental drift – to move across a spherical surface every point on the seafloor had to follow such a path. Morgan recognised that the fracture zones could only result from rigid plates having to fracture to accommodate that motion. Using spherical geometry he was able to link together ridges, trenches and these huge transform faults with poles of rotation and triple junctions to predict plate motions in a quantitative manner. That insight provided a key to active earthquakes, mountain belts and volcanoes. His scientific unification was a result of genius: in just a few weeks Morgan established the fundamentals of what became known as plate tectonics.
W. Jason Morgan was one of the revolutionaries who made geology dynamic and launched its resurrection from the boring province of damp field workers in anoraks tramping across tracts of extremely puzzling rocks and structures, noses to the ground. He died at the age of 87 on 31 July 2023.
White Sands National Park in New Mexico, USA is notorious for being adjacent to the site at which the first nuclear weapon was tested (code name Trinity) on 16 July 1945. Four weeks later two such bombs killed between 129,000 and 226,000 people at Hiroshima (6 August 1945) and Nagasaki (9 August 1945). The area is one of spectacular geology, the white sand being made of gypsum (CaSO4) grains precipitated from lake water supplied by rivers that had dissolved the mineral from Permian evaporites in the surrounding mountains. Subsequent wind erosion created a large, white dune field: the main attraction. Though a national park that has been proposed for UNESCO World Heritage Centre, the park itself is surrounded by military installations including the nuclear test site.
Gypsum sand dunes in White Sands National Park USA. (Credit: Wikipedia)
As in most evaporite basins, the White Sands’ gypsum sediments built up layer-by-layer through deposition of clays during successive inundations followed by evaporation of CaSO4 rich water. Animals crossing the basin were likely to leave trackways, which subsequent sedimentary cycles could preserve in stratigraphic order. Examples had been found in the early 20th century, revealing the former presence of the late-Pleistocene megafauna: Columbian mammoths, ground sloths, ancient camels, dire wolves, lions, and sabre-toothed cats. One set of dire wolf prints found in the 2010s contained seeds that yielded a radiocarbon age of 18 ka. More recently, 61 human footprint tracks turned up in layers that also displayed signs of megafauna crossing the lake flats, in one case showing convincing signs of hunters having followed a giant ground sloth (Bennett, M.R. 2021 and 13 others 2021. Evidence of humans in North America during the Last Glacial Maximum. Science, v. 373, p. 1528-1531; doi: 10.1126/science.abg7586). Interestingly, many of the human tracks seem to have been made by teenagers and children with only a few made by adults. Dating of seeds in the sediment layers – and in some footprints – yielded 23 to 21 ka radiocarbon ages. This evidence suggested human occupation of New Mexico long before those who left Clovis-style artifacts around 13 ka and others who preceded them. However, the seeds that were dated are those of an aquatic grass (Ruppia cirrhosa), which may have absorbed older carbon from groundwater permeating the evaporite sediments. Being robust, the seeds could also have been transported by wind back and forth from plants that lived before the animals and humans left their marks in the saline flats. Such is the importance of the White Sands fossil trackways that a team of US and British geologists, some of whom authored Bennett et al. 2021, have sought to refute doubts of their antiquity (Pigati, J.S. and 10 others 2023. Independent age estimates resolve the controversy of ancient human footprints at White Sands. Science, v. 382, p. 73-75; DOI: 10.1126/science.adh5007).
Human footprints (arrowed) preserved on three sediment surfaces of the White Sands clay-gypsum sequences; i.e. at three times in their depositional sequence. (Credit: from Pigati et al.; Fig 1)
The researchers cut trenches into the layered clay-gypsum to reveal human footprints on three successive surfaces at the site where Ruppia seeds had provided very old, but disputed ages. They supplemented the earlier evidence by 14C dating of pollen grains blown into the prints from terrestrial plants and optically stimulated luminescence ages (time of last exposure to sunlight) of detrital quartz grains in the evaporites. The pollen dating gave ages from 23.4 to 22.6 ka, the minimum quartz OSL age being 21.5 ka. Similar ages from three different methods are pretty convincing evidence that humans were active in New Mexico during the Last Glacial Maximum (LGM), and that absorption of older carbon from groundwater had not affected the Ruppia seeds.
The Asia to America migration, which led these hunters to what the abundant megafauna trackways suggest were rich pickings around the White Sands palaeo-lake, must have been earlier still. High-latitude North America was almost certainly a vast, frigid desert for thousands of years leading up to the LGM. Another implication of the remarkable finds in the gypsum beds is that migration most probably involved a coastal or even a maritime route along the Eastern Pacific shore to reach more habitable lower latitudes.
The Cretaceous-Palaeogene mass extinction is no longer an event that polarises geologists’ views between a slow volcanic driver (The Deccan large igneous province) and a near instantaneous asteroid impact (Chicxulub). There is now a broad consensus that both processes were involved in weakening the Late Cretaceous biosphere and snuffing out much of it around 66 Ma ago. Yet is still no closure as regards the details. From a palaeontologist’s standpoint the die-off varied dramatically between major groups of animals. For instance, the non-avian dinosaurs disappeared completely while those that evolved to modern birds did not. Crocodiles came through it largely unscathed unlike aquatic dinosaurs. In the seas those animals that lived in the water column, such as ammonites, were far more affected than were denizens of the seafloor. But much the same final devastation was visited on every continent and ocean. However, lesser and more restricted extinctions occurred before the Chicxulub impact.
Scientists from Norway, Canada, the US, Italy, the UK and Sweden have now thrown light on the possibility that climate change during the last half-million years of the Cretaceous may have been eroding biodiversity and disrupting ecosystems (Callegaro, S. et al. 2023. Recurring volcanic winters during the latest Cretaceous: Sulfur and fluorine budgets of Deccan Traps lavas. Science Advances, v. 9, article eadg8284; DOI: 10.1126/sciadv.adg8284). Almost inevitably, they turned to the record of Deccan volcanism that overlapped the K-Pg event, specifically the likely composition of the gases that the magmas may have belched into the atmosphere. Instead of choosing the usual suspect carbon dioxide and its greenhouse effect, their focus was on sulfur and fluorine dissolved in pyroxene grains from 15 basalts erupted in the 10 Formations of the Deccan flood-basalt sequence. From these analyses they were able to estimate the amounts of the two elements in the magma erupted in each of these 10 phases.
Exposed section through a small part of the Deccan Traps in the Western Ghats of Maharashtra, India. (Credit: Gerta Keller, Princeton University)
The accompanying image of a famous section through the Deccan Traps SE of Mumbai clearly shows that 15 sampled flows could reveal only a fraction of the magmas’ variability: there are 12 flows in the foreground alone. The mountain beyond shows that the pale-coloured sequence is underlain by many more flows, and the full Deccan sequence is about 3.5 km thick. Clearly, flood-basalt volcanism is in no way continuous, but builds up from repeated lava flows that can be as much as 50 m thick. Each of them is capped by a red, clay-rich soil or bole – from the Greek word bolos (βόλος) meaning ‘clod of earth’. Weathering of basalt would have taken a few centuries to form each bole. Individual Deccan flows extend over enormous areas: one can be traced for 1500 km. At the end of volcanism the pile extended over roughly 1.5 million km2 to reach a volume of half a million km3.
Fluorine is a particularly toxic gas with horrific effects on organisms that ingest it. In the form of hydrofluoric acid (HF) – routinely used to dissolve rock – it penetrates tissue very rapidly to react with calcium in the blood to form calcium fluoride. This causes very severe pain, bone damage and other symptoms of skeletal fluorosis. The 1783-4 eruption of the Laki volcanic fissure in Iceland emitted an estimated 8,000 t of HF gas that wiped out more than half the domestic animals as a result of their eating contaminated grass. The famine that followed the eruption killed 20 to 25% of Iceland’s people: exhumed human skeletons buried in the aftermath show the distinctive signs of endemic skeletal fluorosis. This small flood-basalt event had global repercussions, as the Wikipedia entry for Laki documents. Volcanic sulfur emissions in the form of SO2 gas react with water vapour to form sulphuric acid aerosols in a reflective haze. If this takes place in the stratosphere as a result of powerful eruptions, as was the case with the 1991 Pinatubo eruption in the Philippines, the high-altitude haze lingers and spreads. This results in reduced solar warming: a so-called ‘volcanic winter’. In the Pinatubo aftermath global temperatures fell by about 0.5°C during 1991-3. Unsurprisingly, volcanic sulfur emissions also result in acid rainfall. Moreover, inhaling the sulphur-rich haze at low altitudes causes victims to choke as their respiratory tissues swell: an estimated 23,000 people in Britain died in this way when the 1783-4 Laki eruption haze spread southwards Sara Calegaro and colleagues found that the fluorine and sulfur contents of Deccan magmas fluctuated significantly during the eruptive phases. They suggest that fluorine emissions were far above those from Laki, perhaps leading to regional fluorine toxicity around the site of the Deccan flood volcanism but not extinctions. Global cooling due to sulphuric acid aerosols in the stratosphere is suggested to have happened repeatedly, albeit briefly, as eruption waxed and waned during each phase. Magmas rich in volatiles would have been more likely to erupt explosively to inject SO2 to stratospheric altitudes (above 10 to 20 km). The authors do not attempt to model when such cooling episodes may have occurred: data from only 15 levels in the Deccan Traps do not have the time-resolution to achieve that. They do, however, show that this large igneous province definitely had the potential to generate ‘volcanic winters’ and toxic episodes. Time and time again ecosystems globally and regionally would have experienced severe stress, the most important perhaps being disruption of the terrestrial and marine food chains.
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.
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.
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.
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’.
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. antecessoror 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.
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 (Paramylodonharlani); and yesterday’s camel (Camelopshesternus).
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
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.
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