The earliest upright ape

Two decades ago the world of palaeoanthropologists was in turmoil with the publication of an account of a new find in Chad (see: Bonanza time for Bonzo; July 2002). A fossil cranium, dubbed Sahelanthropus tchadensis (nicknamed Toumaï­ or ‘hope of life’ in the Goran language), appeared like a cross between a chimpanzee and an australopithecine. The turmoil erupted partly because of its age: Upper Miocene, around 7 Ma old. Such an antiquity was difficult to reconcile with the then accepted ~5 Ma estimate for the evolutionary split between humans and chimpanzees, based on applying a ‘molecular clock’ approach to the difference between their mtDNA. The other point of contention was the size of Sahelanthropus’s canine teeth: far too large for australopithecines and humans, but more appropriate for a gorilla or chimp.

Cast of the reconstructed skull of Sahelanthropus tchadensis. (Credit: Didier Descouens, University of Toulouse)

In the absence of pelvic- and foot bones, or signs of the foramen magnum where the spinal cord enters the skull – crucial in distinguishing habitual bipedalism or being an obligate quadruped – encouraged the finders of a 6.1 to 5.7 Ma-old Kenyan hominin Orrorin tugenensis to insist that its skeletal remains – several teeth, fragments of a lower jaw, a thigh bone, an upper arm and of a finger and thumb but no cranial bones – were of ‘the earliest human ancestor’. In Orrorin’s favour were smaller canine teeth than those of later australopithecines. At the time of the dispute, centred mainly on absence of crucial evidence, doyen of hominin fossils Bernard Wood of George Washington University and an advocate of ‘untidy’ evolution, suggested that both early species may well have been evolutionary ‘dead ends’ (see: A considered view; October 2002). And there the ‘muddle’ has rested for 20 years.

In 2002 not only a cranium of Sahelanthropus had been unearthed. Three lower jaw bones and a collection of teeth suggested that as many as 5 individuals had been fossilised. A partial leg bone (femur) and three from forearms (ulna) cannot definitely be ascribed to Sahelanthropus but, in the absence of evidence of any other putative hominin species, they may well be. It has taken two decades for these remains to be analysed to a standard acceptable to peer review (Daver, G. et al. 2022. Postcranial evidence of late Miocene hominin bipedalism in Chad. Nature v. 608, published online; DOI: 10.1038/s41586-022-04901-z). The authors present convoluted anatomical evidence that Toumaï­’s femur, which had been gnawed by a porcupine and lacks joints at both ends, suggesting that it was indeed suited to upright walking. Yet the arm bones hint that it may have been equally comfortable in tree canopies. Yet it does look very like an ape rather than a hominin.

Much the same conclusion has been applied to Australopithecus afarensis, indeed its celebrated representative ‘Lucy’ met her end through falling out of a large tree ~3.2 Ma ago (see: Lucy: the australopithecine who fell to Earth?; September 2016). So, dual habitats may have been adopted by hominins long after they emerged. Yet Au afarensis was capable of trudging through mud as witnessed by the famous footprints at Laetoli in Tanzania. Only around 3 Ma has reasonably convincing evidence for upright walking similar to ours been discovered in Au africanus. The full package of signs from pelvis and foot for habitual bipedalism dates to 2 Ma ago in Au sediba. Even this latest known australopithecine seems to have had a gait oddly different from that of members of the genus Homo.

So, in many respects the benefits of full freeing of the hands to develop manipulation of objects, as first suggested by Freidrich Engels, may have had to await the appearance of early humans. Earlier hominins almost certainly did make tools of a kind, but the revolutionary breakthrough associated with humanity was more than 5 million years in the making.

See also: Callaway, E. 2022. Seven-million-year-old femur suggests ancient human relative walked upright. Nature (News)24 August 2022;

Handwerk, B. 2022. Seven Million Years Ago, the Oldest Known Early Human Was Already Walking. Smithsonion Magazine, 24 August 2022 (click the link ‘published today in Nature’ in 2nd paragraph to access complimentary PDF of Daver et al)

New dating questions previous ideas about early hominins

The Sterkfontein cave 40 km northwest of Johannesburg in South Africa first sprang to the attention of scientists in 1936, with the discovery there of an adult hominin skull. This showed clear affinities with the discovery 400 km to the SW in 1924 of the fossil skull of a juvenile primate, which Raymond Dart claimed to be ancestral to modern humans, naming it Australopithecus africanus. Sterkfontein has since yielded more than 500 hominin fossils, many of which are Au. africanus.

Limestone cave deposits are difficult to date precisely, unlike sediments that are interbedded with volcanic rocks, the most amenable material being that deposited by water flowing through the cave to form flowstone or speleothem. Using the U-Pb method of radiometric dating yielded an age of between 2.1 to 2.6 Ma for flowstone that cements the breccia in which the Au. africanus fossils occur. Clearly, the flowstone formed after burial so that was a minimum age for them, awaiting the use of a different chronological tool to suggest when burial of the bones took place

The face of an Australopithecus africanus: ‘Mrs Ples’. (Credit University of Zurich)

An almost complete skeleton of another australopithecine found in another part of the Sterkfontein cave system was dated in 2015 by a different approach. This used the decay of 10Be and 26Al isotopes that high-energy cosmic rays produce in quartz grains while they are exposed at the surface. Burial of irradiated sedimentary grains protects them from such bombardment, and the two isotopes  then steadily decay at a known rate. Quartz grains associated with this specimen (fondly known as ‘Little Foot’) turned out to be far older than the flowstone U-Pb age, with a cosmogenic burial age of about 3.7 Ma. Its much greater antiquity prompted scientists to regard ‘Little Foot’ as a different species – Au. prometheus – despite being similar to Au. africanus.

Since that success, much the same team from South Africa, the US and France has been working on sedimentary grains buried with the abundant Au. africanus specimens from Sterkfontein (Granger D.E. et al. 2022. Cosmogenic nuclide dating of Australopithecus at Sterkfontein, South AfricaProceedings of the National Academy of Sciences, v. 119, article e2123516119; DOI: 10.1073/pnas.2123516119). Their newly published efforts show that “Little Foot’s” burial took place between 3.41 and 3.49 Ma, more than a million years earlier than suggested by the flowstone U-Pb dating and just ~200 ka younger than the ‘Little Foot’ skeleton. More surprising is that Au. africanus lived during the same period (3.4 to 3.7 Ma) as did Au. afarensis – the species to which ‘Lucy’ belonged – 3500 km to the north in Ethiopia.

So it is no longer justifiable  to suggest that the first known human species (Homo habilis ~2.3 to 1.65 M) is either a more ‘advanced’ australopithecine or a direct descendant from that genus, for the new dating opens a million-year gap in the history of human evolution. That age range does contain stone tools but no plausible candidates for an australopithecine-human evolutionary connection. One of the most recently suggested link is Au. sediba (see: Another candidate for earliest, direct human ancestor, October 2011; and Australopithecus sediba: is she or is she not a human ancestor? April 2013). The snag with that candidate is that the well-established age (2.0 Ma) of known specimens falls in the middle of the range for H. habilis. The two may have been cohabiters of Africa but are very different.

The million years that separated Au. africanus together with afarensis from H. habilis is the period when the defining character of humans, tool making, evolved. So the hunt is on for hominins associated with stone tools in that huge stratigraphic gap. One of the drawbacks with famous sites, such as the ‘Cradle of Humankind’ that includes Sterkfontein, is that they almost become clichés so that scientists return to them again and again, while the key that they seek may well lie elsewhere.

Wider traces of the elusive Denisovans

We know that when anatomically modern humans (AMH) arrived in Asia they shared the landscape with ‘archaic’ humans that had a much longer pedigree. In 2010 an individual’s little-finger bone dated to around 30 to 49 ka old was found in the Denisova Cave in central Siberia (at 50°N). It yielded a full genome that was distinctly different from those of AMH and Neanderthals (see: Other rich hominin pickings; May 2010). Four other fossils found subsequently in the Denisova Cave contained similar DNA. Checking the DNA of living humans and fossil Neanderthal remains revealed that the newly discovered human group had interbred with both. In the case of AMH, segments of Denisovan DNA are found in the genomes of indigenous people living in East and South Asia, Australia, the Pacific Islands and the Americas, at levels of 0.2%, rising to 6% in Melanesian people of Papua-New Guinea. But such introgressions have not been found in Europeans (but see below), suggesting that the Denisovans were restricted to Asia.

There have been suggestions that at least some of the ‘archaic’ human remains found widely and abundantly in China may have been Denisovans; although they might equally be of Homo erectus. But none of the Chinese fossils have been subjected to gene sequencing – those found in caves outside tropical and sub-tropical climates might retain DNA just as well as Neanderthal and even older remains in temperate Europe. Yet a partial lower jaw discovered in a cave on the Tibetan Plateau (at 35°N) did yield proteins that had close affinities to those recovered from Siberian Denisovans. Now similar analyses have been performed on an abnormally large molar found in a cave in Northern Laos, showing that it too is most likely to be from a young (as suggested by its being little worn), possibly female (it lacks male-specific peptides), Denisovan. The locality lies at about 20°N, far to the south of the other two Denisovan sites (Demeter, F. et al. A Middle Pleistocene Denisovan molar from the Annamite Chain of northern Laos. Nature Communications, v. 13, article 2557; DOI: 10.1038/s41467-022-29923-z). Sparse as the evidence is, Denisovans were able to tolerate climate differences across 30 degrees of latitude.

A probable Denisovan molar from 164 to 131 ka old cave sediments in northern Laos. (credit: Demeter, et al.; Fig. 2)

The Wikipedia entry for Denisovans is a mine of additional information. For instance, detailed analysis of the roughly 5% of their genome that indigenous people of New Guinea carry suggests that the two groups may have interbred there as late as 30 ka. Since Both New Guinea and Australia were until 8 thousand years ago part of the Sahul landmass when sea level was low during the last ice age, these inferences add tropical occupancy to the Denisovan range. Does this suggest that Papuans and indigenous Australians migrated with Denisovans, or had the latter crossed the sea from Timor earlier and independently, after moving from Asia by ‘hopping’ from island to island through eastern Indonesia? There is a possibility that Denisovans could even have survived in Sahul until as late as 14.5 ka. Even more odd, modern Icelandic people are unique among Europeans in having detectable traces of Denisovan DNA. However, rather than having been directly shared between Denisovans and ancestral Scandinavians – a possibility – it may have been carried by Neanderthal-Denisovan hybrids migrating westwards from Siberia with whom the Icelanders’ ancestors interbred. There are other interesting points in the Wikipedia entry. One is that the consistently lower Denisovan ancestry in living East Asians compared with people of Oceania, may indicate two separate waves of eastward migration by AMH. The latter may have arrived first, had greater contact with Denisovans and then moved on across seaways to remain isolated from the later migrants.

Finally, something that puzzles me as a non-geneticist. If both Denisovans and Neanderthals died out as genetically distinct groups tens of millennia ago how could the genetic traces of interbreeding with AMH have been retained at such high levels until the present; i.e. through thousands of generations? Each of us carries a 50% deal of genes from our parents. Then with each subsequent generation the proportion is diluted, so that we inherit 25% from grandparents, 12.5 % from great-grandparents and so on. Yet Papuans still have 5 to 6 percent of Denisovan DNA: much the same holds for Europeans’ Neanderthal heritage. Does such a high level of retention of this ancestry suggest that a large proportion of the earliest migrating AMH individuals stemmed from generation to generation interbreeding on a massive scale? Did the ‘newcomers’ and ‘locals’ band eventually together almost completely to merge genetically, or am I missing something … ? Probably

Did earliest modern humans in Europe share a cave with Neanderthals?

The cave of Grotte Mandrin in the Rhône Valley, France. (Credit: Slimak et al Fig 1c)

Since 1999 a cave (Grotte Mandrin) on the west flank of the lower Rhône valley in sothern France has been revealing archaeological remains from 3 metres of sediment that can be divided into 12 distinct layers (Slimak, L. and 22 others 2022. Modern human incursion into Neanderthal territories 54,000 years ago at Mandrin, France. Science Advances, v. 8, article eabj9496; DOI: 10.1126/sciadv.abj9496). Tens of thousands of objects have been recovered, mostly from a layer just below midway in the sequence, which is dominated by small (<1 cm), ‘standardised’ stone points that are also found at other sites in the local area. This veritable industry – dubbed the ‘Neronian’ from the nearby Grotte de Néron – seems to have been focussed hereabouts. Older artefacts in layers F and G are considered to be Mousterian, that is generally ascribed to late Neanderthals. Horse, bison and deer bones suggest that these were the main source of animal protein for the cave’s occupants. The site also contained a few objects that show simple decoration. The way in which the Neronian points were produced resembles the working of similar artefacts in Lebanon by anatomically modern humans (AMH) about 45 ka ago; so it is possible that the technology had spread westward with the earliest AMH migrants into Europe. Yet precise radiocarbon and optically stimulated luminescence dating of the Grotte Mandrin site suggests that the sediment accumulated between 84 to 44 thousand years ago. The Mousterian/Neanderthal objects occur in layers F and G between 79 and 57 ka, whereas the Neronian layer E spans 56.8 to 51.7.

Grotte Mandrin has yielded very few hominin remains, except for 9 teeth in layers C to G. Those from C, D, F and G showed clear Neanderthal dental features. However, shape analysis of one damaged, deciduous (infant) molar from Layer E suggests that it matches Upper Pleistocene AMH dental morphology. That seems to place Grotte Mandrin as by far the oldest AMH occupation site in Europe, up 11 thousand years earlier than the 45 to 43 ka AMH site at Bacho Kiro in Bulgaria. To some extent that tallies with the tiny tooth’s association with a prolific, standardised and delicate industry new to the area: probably points for small projectiles. Neanderthals re-occupied the site in Layers D to B, yet in the upper part of layer B, from 44.1 to 41.5, there is a return of Neronian-like points, probably made by AMH.

A curious detail from layer E (not reported in this paper) is the occurrence of soot trapped in thin, annually deposited layers of carbonate on the cave walls. Fragments of the sooty speleothem continually fell onto the cave floor to be incorporated into the sediments. The base of layer E that contains Neronian, possibly AMH artefacts and the top of layer F that shows preceding Neanderthal occupation, contain such sooty speleothem fragments. Precise dating of them is claimed to suggest a very short period of transition between the two kinds of occupants: perhaps only a few years. Neanderthals and AMH may not have met in the cave, but may well have been co-occupants of the surrounding area at the same time.

A great deal of effort over more than two decades has gone into this publication, and several of its findings have caused quite a stir. Because permanent AMH occupation of the Levant began at least 55 ka ago, there is no reason to suppose that AMH migrating along the northern shores of the Mediterranean could not have arrived a little earlier in what is now southern France. What has been emphasised in the broad media is the exchange of a Neanderthal to an AMH population in the Grotte Mandrin, as if it was done in a friendly, indeed neighbourly spirit (!). That hinges on the ultra-precise dating of the sooty speleothem fragments to reveal just a few years between the Neanderthals doing a ‘flit’ and the AMH starting a ‘squat’ in the vacant premises to set up a cottage industry. The time of the replacement before present is, in fact, very close to the limit at which radiocarbon dating is feasible, almost all 14C formed at that time having decayed away since then. There can be no doubt that layer E did mark a major change in sophistication of stone technology, but was it really an AMH development? The only definite evidence is the single deciduous molar, and that is damaged to such an extent that an independent dental paleoanthropologist who has specialised in distinguishing AMH from Neanderthal dentition isn’t convinced. But,surely, DNA from the tooth would resolve the issue. The paper notes that trial extraction and sequencing of 6 horse teeth from layer E failed to yield results, which suggests degradation of genetic teril. So the team did not commit the tooth to sequencing, which would have further damaged it. Finally, four separate groups occupying what certainly looks like a nice little cave over the course of about 40 thousand years is hardly a surprise. Many caves throughout Europe and southern Africa show evidence of multiple occupancy. After all, before 11 ka all humans and their forebears were of necessity foragers and migrants; just think of how many times your neighbours have changed since you moved in …

See also: Price, M. 2022. Did Neanderthals and modern humans take turns living in a French cave? Science, v, 375, p. 598-599; DOI: 10.1126/science.ada1114

Holocene migrations of people into Britain

People assigned to a variety of human species: Homo sapiens H. neanderthalensis (Swanscombe, 400 ka and several later times ) H heidelbergensis (Boxgrove, ca 500 ka, )H. antecessor (Happisburgh, ca 950 ka) – have left signs of their presence in Britain. Human occupancy has largely depended on climate. Around 9 times since the first known human presence here, much of Britain was repeatedly buried by glacial ice to become a frigid desert for tens of thousands of years. Between 180 and 60 ka only a couple of flint artefacts found in road excavations in Kent hint at Neanderthal visitors. For most of the Late Pleistocene the archipelago seems to have been devoid of humans. Arguably, Europe’s first known anatomically modern humans occupied several caves in Devon, Derbyshire and South Wales as early as around 43 ka, while climate was cooling, only to abandon Britain during the Last Glacial Maximum (24 to 18 ka ago). As climate warmed again thereafter, sporadic occupation by Late Palaeolithic hunter-gatherers occurred up to the sudden onset of the frigid Younger Dryas (12.9 ka). Once warming returned quickly 11,700 years ago, sea level was low enough for game and hunter gatherers to migrate to Britain; this time for permanent occupancy. Bones of the earliest known of these Mesolithic people have yielded DNA and a surprise: they were dark skinned and so far as we can tell remained so until the beginning of Neolithic farming in Britain around 6100 years ago. The DNA of most living Britons with pale skins retains up to 10% of inheritance from these original hunter gatherers.  Much the same is known from elsewhere in NW Europe. In the early Holocene it was possible to walk across what is now the southern North Sea thanks to Doggerland. Following a tsunami at around 8.2 ka this rich area of wetland vanished, so that all later migration demanded sea journeys.  

Mesolithic people remained in occupation of the British Isles for another two millennia. A wealth of evidence, summarised nicely in Ray, K. & Thomas, J. 2018, Neolithic Britain, Oxford University Press, suggests that there was a lengthy period of overlap between Mesolithic and Neolithic occupation around 4100 BCE. The main difference between the two groups was that Neolithic communities subsisted on domesticated grains and animals, while those of the Mesolithic consumed wild resources. Cultural clues in archaeological finds, however, suggest a lot in common, such as the erection of various kinds of monuments. Posts of tree trunks, sometimes arranged in lines, were raised in the Mesolithic and lines of probably ritual pits were dug. Both ‘traditions’ continued into the Neolithic and evolved to stone monuments, to which were added burials of different kinds. It is worth noting that Stonehenge was developed on a site that held much earlier, large totem-pole like posts, with a nearby spring that had hosted regular gatherings of Mesolithic people. Signs of Mesolithic occupation in Britain extend just as widely as do those of Neolithic practices. A study of DNA from 7 Mesolithic skeletons and 67 of early Neolithic age (Brace, S. and 20 others 2019. Population Replacement in Early Neolithic Britain. Nature Ecology & Evolution, v. 3, p. 765-771; DOI: 10.1038/s41559-019-0871-9) revealed that early Neolithic people did not wipe out the genetic make-up (either by complete displacement or annihilation) of their predecessors. About 20 to 30% of Neolithic DNA was inherited from them; as would be expected from assimilation of a probably much smaller number of hunter-gatherers into a larger population  of  immigrants who brought farming and herding from Asian Turkey (Anatolia). Such ‘hybrid’ genetics was widespread in Europe and they are referred to as the Early European Farmers (EEF). As Ray and Thomas suggest, aspects of Mesolithic culture may have been adopted by the newcomers across the British Isles from Orkney to Wiltshire.

Around 2400 BCE the earliest Neolithic ceremonial site at Brodgar on Orkney was destroyed to the accompaniment of an enormous feast that consumed several hundred cattle. At about the same time several men, whose tooth geochemistry indicated an origin in the European Alps, were buried on Salisbury Plain together with the earliest metal artefacts known from Britain (copper knives), the accoutrements of archery and distinctive, bell-shaped pottery beakers. Stonehenge was ‘remodelled’ shortly afterwards, with the addition of its giant trilithons, four of which were later adorned with carvings of metal axes and daggers. The Early Bronze (or Chalcolithic) Age had arrived! A 2018 study of ancient DNA from Bronze Age burials in Europe suggested a far more drastic swamping of Neolithic genetic heritage by the ‘Beaker people’ (Olalde, I. and a great many others 2018. The Beaker phenomenon and the genomic transformation of northwest Europe. Nature, v. 555, p. 190-196; DOI: 10.1038/nature25738). The skeletons from Britain analysed by Olalde et al. apparently suggested that, within a few hundred years, up to 90% of the Neolithic gene pool had been removed from the British population. Who were these people who used metals and the distinctive Bell Beakers, where did they come from and what did they do?

The closest match to the British and western European Bronze Age DNA was that associated with the Yamnaya people from the steppes of SE Ukraine and Southern Russia who had developed a culture centred on herding. They had also adopted the wheel from people of the Mesopotamian plains and had domesticated the horse for riding and pulling carts: ideal for their semi-nomadic lifestyle and for moving en masse. After 3000 BCE they spread into Europe, as widely recorded by their distinctive beakers and the presence of their DNA in the genomes of later Europeans. Their burials – in ‘kurgans’ – resembled the round barrows that appeared on Salisbury Plain and elsewhere during the Bronze Age. The DNA replacement data from 2018 were limited and held few clues to how it happened. One possibility for such a dramatic change could be a violent takeover that drove down the population of British Neolithic people. To address the broader influence of migration in more detail and over a loner time span, a team led by the Universities of York and Vienna, and Harvard Medical School (Patterson, N. and a great many others 2021. Large-scale migration into Britain during the Middle to Late Bronze Age. Nature, early online release; DOI: 10.1038/s41586-021-04287-4) used ancient DNA from 793 individuals excavated in Britain (416 individuals) and continental Europe (377) from Bronze- to Iron Age sites (2300 to ~100 BCE).

The proportion of Early European Farmers DNA in British individuals from the Bronze Age (2400 BCE) to the Iron Age (750 BCE to 43 CE). Note the ‘fuzzy’ nature of the data, and that the decline in EEF in British individuals was not as great as earlier analyses had shown. Remarkably, the ‘Amesbury Archer’, who brought the first metals to Britain, had a higher proportion of EEF ancestry than the Early Bronze-Age average. (Credit: Patterson et al. Fig. 3)

The new data from Britain suggest that the migrants, who crossed the Channel later in the Bronze Age, were of mixed ethnicity, but most carried EEF genes. The influence of earlier migrants from the Yamnaya heartlands is present, but so too are relics of Mesolithic ancestry. Interestingly, the British data show a much larger increase in the genes associated with lactase persistence, which marks the ability of adults to digest milk, than was apparent in the wider European population (50% compared with about 7% in Eastern Europeans of the time). Whatever the impact of the first influx of metal-using people – it may have been culturally decisive in Britain – by the end of the Bronze Age the EEF ‘signature’ had increased in peoples’ genomes. Rather than some kind of invasion, the influx was more likely to have been a sustained movement of people to Britain over several hundred years By the Iron Age, almost half the ancestry of Britain, particularly in England and Wales, was once again predominantly of EEF origin (around 40% of the mixture), but culture had become completely different. There are even suggestions that the influx brought with it the beginnings of Celtic languages. Yet the data leave a great deal of further analysis to be undertaken.

See also: Drury, S.A. 2019. Genetics and the peopling of Britain: We are all hybrids, People and Nature; Ancient DNA Analysis Reveals Large Scale Migrations Into Bronze Age Britain, SciTechDaily, 28 December 2021.

Some Homo naledi news

In 2015 the remains of about 15 hominins, new to science, were found in a near-inaccessible South African cave (See: The ‘star’ hominin of South Africa;  September 2015), that number having risen to more than 24 at the time of writing. The ‘star’ status of Homo naledi (named after the cave’s name Naledi meaning star in the local Sotho language) arose partly from an extraordinary barrage of promotion by the organisers of the expedition that unearthed them (probably to boost fundraising). But it was indeed one of the most extraordinary discoveries in palaeoanthropology. The remains were recovered by a team of women archaeologists who small and lithe enough to wriggle through a maze of extremely narrow cave passages. The bones in the remote chamber were complete, with no sign of physical trauma, except gnawing by snails and beetles. Few hominin fossils were found in the more accessible parts of the cave. One likely explanation was that a living H. naledi group had deliberately carried the bodies through the cave system for burial – at less than 1.5 m tall with a slender build they could have done this far more easily than the modern excavators. A plausible alternative is that a group of H. naledi scrambled deep into the cave on being panicked by large predators, and suffocated as CO2 built-up to toxic levels.

Map of the Rising Star cave system in Gautong Province South Africa. The yellow dot marks the chamber where Homo naledi fossils were first found; the red one is the site of a new discovery. (Credit: Elliott et al 2021, PaleoAnthropology. Issue 1.64, Fig. 1)

Initially, the bones were estimated to be 2 Ma old. The fossils are so well-preserved that most aspects of their functional anatomy are known in great detail, such as the articulation of their hands and feet. Although not a single tool was found in the cave deposit, to get into the far reaches of the labyrinthine cave system they must have lit the way with firebrands. The anatomy of H. naledi is far more advanced than that of contemporary H. habilis. The discoverers speculated that the group may have been a species that gave direct rise to the later H. ergaster and erectus, and ultimately us. Alternatively, the individuals’ diminutive size suggested parallels with much later H. floresiensis and H. luzonensis from the other side of the world. Much of this hype was later blunted by more reliable geochronology indicating an age of between 236 ka and 335 ka: i.e. about the time when anatomically modern humans were already roaming Africa. A more plausible conclusion, therefore, is that H. naledi was one of at least 6 hominin groups that co-occupied the late-Pleistocene world: i.e. similar to H. floresiensis.

Now the partial skull and half a dozen teeth of an immature H. naledi has been recovered from another remote chamber in the cave system (Brophy, J.K. et al. 2021. Immature Hominin Craniodental Remains From a New Locality in the Rising Star Cave System, South Africa. PaleoAnthropology. Issue 1.64; DOI: 10.48738/2021.iss1.64). Fossils of young humans are rare, their bones being thinner and much more fragile than those of adults, so the skull had to be reconstructed from 28 fragments. Unlike the older individuals from the main chamber, there are no other bones associated with the skull. Oddly, the supposedly young H. naledi’s brain volume (between 480 to 610 cm3) is between 90 to 95 % that of adults. A possible explanation for this degree of similarity is that these beings reached maturity far more quickly than do anatomically modern humans. The evidence for youth is based on close dental similarity with those of other ‘immature’ specimens from the main bone deposit, and most importantly that two of the teeth are demed to be deciduous (‘milk’) teeth. Yet the ‘milk’ teeth show severely chipped enamel as do the permanent teeth of more mature specimens, to the extent of being unique in the fossil record of hominins. Clearly, their diet was sand-rich.

Shortly after publication in the journal PaleoAnthropology during early November 2021 the world’s media leapt on the two papers rorting these new finds. Yet it is hard to judge why it was deemed by science journalists to have truly popular appeal. It actually adds very little to the H. naledi story, apart from specialised anatomical description. Despite the skull being bereft of the rest of the individual’s body, the authors ‘…regard it as likely that some hominin agency was involved in the deposition of the cra­nial material’.  Perhaps the ‘star’ status was rekindled because the press release from the University of the Witwatersrand used the word ‘child’ again and again – a sure fire way of getting wide attention. The published papers properly refers to it as an ‘immature hominin individual’, which it undoubtedly is.  The same sort of attention came the way of Raymond Dart from a small skull of Australopithecus africanus found in 1924 by workers in a limestone quarry – he called it ‘the Taung Child’. Of course, H. naledi is one of the best-preserved hominins known. But how does its current newsworthiness rank above H. floresiensis? Now, that was a surprise, but the hype about that tiny human has died down. And when H. naledi was originally deemed to be 2 Ma old, it too was astonishing. But since its true, quite young age was determined, it too is no longer such a big deal.

Interestingly, South African scientists self-proclaimed the name ‘Cradle of Humankind’ for the area in Gautung Province close to Johannesburg, which is rich in limestone caves and has a long history of fossil hominin discoveries since Raymond Dart’s Taung Child. But the earliest anatomically modern human remains are from Jebel Irhoud in Morocco, and the oldest known hominin fossils are from Chad, and most advances in early hominin evolution have stemmed from Ethiopia, Kenya and Tanzania.   The fossiliferous part of Gautung Province rightly has World Heritage status, but not under that name. Instead it is called more accurately ‘Fossil Hominid Sites of South Africa”

See also: Partial skull of a child of Homo naledi: Insight into stages of life of remarkable species. Science Daily, November 2021.

A cometary air-burst over South America 12 thousand years ago

Earth-logs has previously covered quite a few hypotheses involving catastrophic astronomical events of the past, often returning to them as new data and ideas emerge. They range from giant impacts, exemplified in the mass extinction at the K-Pg boundary to smaller-scale events that may have coincided with important changes in climate, such as the sudden onset of the Younger Dryas, and a few that have been suggested as agencies affecting local human populations such as the demise of Sodom by a cosmogenic air-burst. Some of the papers that spurred the Earth-pages posts have been widely regarded in the geoscience community. Yet there have been others that many have doubted, and even condemned. For instance, data used by the consortium that suggested an extraterrestrial event triggered the frigid millennium of the Younger Dryas (YD) have been seriously and widely questioned. A sizeable number of the team that were under close scrutiny in 2008 joined others in 2019 to back the YD air-burst hypothesis again, using similarly ‘persuasive’ data from Chile. Members of the original consortium of academics also contributed to the widely disputed notion of a cosmic air-burst having destroyed a Bronze Age urban centre in Jordan that may, or may not, have been the site of the Biblical Sodom. Again, they cited almost the ‘full monty’ of data for high-energy astronomical events, but again no crater or substantial melt glass, apart from tiny spherules. Now another paper on much the same theme, but none of whose authors contributed to those based on possibly ‘dodgy’ data, has appeared in Geology (Schultz, P.H. et al. 2021. Widespread glasses generated by cometary fireballs during the Late Pleistocene in the Atacama Desert, Chile. Geology, published online November 2, 2021; doi: 10.1130/G49426.1).

Peter Schultz of Brown University, USA and colleagues from the US and Chile make no dramatic claims for death and destruction or climate destabilisation, and simply report a fascinating discovery. In 2012 one of the authors, Nicolas Blanco of the Universidad Santo Tomás in Santiago, Chile, found slabs made of glassy material up to half a metre across. They occurred in several 1 to 3 km2 patches over a wide area of the Atacama Desert. Resting on Pleistocene glacio-fluvial sediments, they had been exposed by wind erosion of active sand dunes. The glass is dark green to brown and had been folded while still molten. For the glass slabs to be volcanic bombs presupposes a nearby volcano, but although Chile does have volcanoes none of the active vents are close enough to have flung such large lumps of lava into the glass-strewn area. The glassy material also contains traces of vegetation, and varies a great deal in colour (brown to green). Its bulk chemical composition suggests melting of a wide variety of surface materials: quite unlike volcanic glasses.

Chilean glass occurrence: panorama of large glass fragments in the Atacama Desert; a specimen of the glass; thin section of glass showing bubbles and dusty particles (Credit: Schultz et al. 2021; Figs 1B, 2D and 2C)

Microscopic examination of thin sections of the glasses also reveals nothing resembling lava, except for gas bubbles. The slabs are full of exotic fragments, some of which closely resemble mineral assemblages found in meteorites, including nickel-rich sulfides embedded in ultramafic material. Others are calcium-, aluminium- and titanium-rich inclusions, such as corundum (Al2O3) and perovskite (CaTiO3), thought to have originated as very-high temperature condensates from the pre-solar nebula: like the celebrated ‘white inclusions’ in the Allende meteorite. Some minute grains resemble dust particles recovered by the NASA Stardust mission to Comet 81P/Wild-2 which returned samples to Earth in 2006. Zircon grains in the glasses, presumed to be locally derived, have been decomposed to zirconium oxide (baddeleyite), suggesting melting temperatures greater than 1670°C: far above the highest temperature found in lavas (~1200°C). Interestingly, the green-yellow silica glass strewn over the Sahara Desert around the southern Egypt-Libya border also contains baddeleyite and cometary dusts, together with anomalously high platinum-group elements and nanodiamonds that are not reported from the Chilean glass. Much prized by the elite of pharaonic Egypt and earlier makers of stone tools, the Saharan glass is ascribed to shock heating of the desert surface by a cometary nucleus that exploded over the Sahara. Unsurprisingly, Schultz et al. come to the same conclusion.

Any object entering the Earth’s atmosphere does so at speeds in excess of our planet’s escape velocity (11.2 km s-1). Not only does that result in heating by friction with the air, but much of the kinetic energy of hypersonic entry goes into compressing air through shock waves, especially with objects larger than a few tens of metres. Such adiabatic compression can produce temperatures >>10 thousand °C. Hence the ‘fireballs’ associated with large meteorites. With very large air-bursts the flash of radiant energy would be sufficient to completely melt surface materials in microseconds, though rugged topography could protect areas shadowed from the air-burst by mountains, perhaps explaining the patchy nature of the glass occurrences.  (Note: the aforementioned papers on the YD and Sodom ‘air-bursts’ do not mention large glass fragments, whereas some surface melting would be expected). Some of the Chilean glass contains carbonised remnants of vegetation. Radiocarbon dating of four samples show that the glass formed at some time between 16.3 to 12.1 ka. Yes, that does include the age of the start of the YD (12.9 ka) and human migrants had established themselves in northern Chile and coastal Peru after 14.2 ka. Yet the authors, perhaps wisely, do no more than mention the coincidence, as well as that with the disappearance of South American Pleistocene megafaunas – more severe than on any other continent. With a very distinctive product, probably spanning a far larger area of South America, and attractive to humans as an ornament or a resource for sharp tools, expect follow-up articles in the future.

See also: http://www.sci-news.com/space/atacama-desert-comet-10247.html, Science News, 8 November 2021; Vast patches of glassy rock in Chilean desert likely created by ancient exploding comet, Eureka Alert, 2 November 2021.

A new, ‘bureaucratised’ hominin – Homo bodoensis

Palaeoanthropologists are in a bit of a muddle about the early humans of the Middle Pleistocene (~780 to 130 ka), namely Homo heidelbergensis and H. rhodesiensis. The first was defined in 1907 based on a massive lower jaw or mandible (but no cranium) found near Heidelberg in Germany. Fourteen years later a massively browed cranium (but no mandible) turned up near Kabwe in what is now Zambia (then Northern Rhodesia). That specimen became, in true colonialist fashion, H. rhodesiensis. Since then scientists have unearthed more such highly ‘robust’, ‘archaic’ remains in Africa, Asia and especially Europe: including at least 28 individuals in the Sima de los Huesos (‘pit of bones’), part of the World Heritage Site in the Atapuerca mountains of northern Spain. Do these widespread fossils really represent just two species or do specimens just happen to fit within two broadly similar morphological types? These days, most scientists experience discomfort with a reference to the legacy of Cecil Rhodes, so several sacks full of bones were metaphorically lumped into H. heidelbergensis. So widely dispersed are their sources and their ages covering such a wide span of time that the specimens might be expected contain a diverse range of genetic signatures. Yet only a single specimen from northern Spain, dated around 400 ka, has yielded DNA. The Sierra de Atapuerca provided an even more archaic European dated between 1.2 to 0.8 Ma (Early Pleistocene), from which dental proteins have been extracted. Comparative proteomics have encouraged H. antecessor to be considered as a possible common ancestor for anatomically modern humans (AMH), Neanderthals and Denisovans … and H. heidelbergensis.

A new, simplified model for the evolution of the genus Homo over the last 2 million years (Credit: Roksandic et al Fig 1)

A group of palaeoanthropologists has proposed a way to clear such muddy waters (Roksandic, M. & Radović, P. et al. 2021. Resolving the “muddle in the middle”: The case for Homo bodoensis sp. nov.. Evolutionary Anthropology, v. 30, early-release article 21929; DOI: 10.1002/evan.21929). Their device is to abolish the two previous species and lump together many human remains from the Middle Pleistocene of Africa into a new species named after the Bodo site in the Awash Valley of Ethiopia. It was there that a human cranium bearing characteristics similar to all the African specimens was found in 1976. Originally it was allocated to H. heidelbergensis, but now the composite group of archaic Middle Pleistocene Africans is proposed to be assigned to H. bodoensis. This composite species is also reckoned by the authors to be the ancestor of all surviving, anatomically modern humans. European examples of H. heidelbergensis are to be slotted into an early population of Neanderthals. Since the Denisovans of Asia are only known by DNA from tiny skeletal fragments, the taxonomic rearrangement logically should assign Asian archaic humans to early members of that mysterious but well-defined group. But a spanner in the works is that the sole example of H. heidelbergensis DNA (mitochondrial) – from northern Spain – more closely resembles Denisovans than it that of Neanderthals (see: Mitochondrial DNA from 400 thousand year old humans; Earth-logs December 2013).

There is also a bit of a problem with H. antecessor. There aren’t many specimens, and they are all from Atapuerca. Yet they are a plausible candidate, according to the proteomic analyses, for the most recent common ancestor (MRCA) of all subsequent humans (whatever taxonomists care to call them). But they do not fit in the taxonomic model suggested by Roksandic et al., who reject them as MRCA, on grounds that they are European. They consign them to an anomalous ‘spur’ that petred out in Spain while the real action was in Africa. So what happens if a cranium that bears close similarity to both H antecessor and H. bodoensis pops out of African Early Pleistocene sediments (older than about 700 ka)? There is at least one candidate from ~1 Ma sediments in Eritrea (Abbate, E. and 16 others 1998. A one-million-year-old Homo cranium from the Danakil (Afar) Depression of Eritrea. Nature, v. 393, p. 458-460; DOI: 10.1038/30954), which is said to display ‘a mixture of characters typical of H. erectus and H. sapiens’. And there are others of that antiquity from Ethiopia.

Since the time of Charles Darwin there have been taxonomists who were (and are) either habitual ‘lumpers’ or ‘splitters’. There are more with a propensity for splitting because a new species carries the name of its initiator into posterity! So I expect the paper by Roksandic et al. to raise a cloud of academic dust. Yet taxonomic lumping has its stand-out species in the field of human evolution – H. erectus. A great many ‘archaic-looking’ human remains from the period after ~1.9 Ma until as recently as 200 ka have been dubbed ‘Erects’, giving the group an unsurpassed survival span of over a million years. A few early examples from Africa have been ‘split’ away to give H. ergaster, on taxonomic grounds that some palaeoanthropologists do not fully accept. Yet there are signs of later diversity that ‘splitters’ have, so far, not dared to slice-off from the mainstream consensus. So common are these ‘Erect’ fossils in China, that it is almost state policy that it was they who gave rise to living Han Chinese people! The lumpers are likely to hold sway in the absence of ancient DNA sequencing, which may never be possible outside temperate climates or for ages greater than that of the Spanish H. antecessor. With the knowledge that several anatomically very distinct hominin groups occupied the Earth together at several times in the last 300 ka – think H. floresiensis and H. naledi – it seems likely that the proposed pan-African H. bodoensis may not reflect past reality and the hypothesis needs considerably more testing

Earliest Americans and Denisovan art

It was Mary Leakey’s jaw-dropping discovery in the 1970s of the footprints of two adult Australopithecus afarensis and an accompanying juvenile in 3.6 Ma-old volcanic ash at Laetoli, Tanzania that provided the oldest palpable evidence of a bipedal hominin species. Just seeing a high-resolution image of this now legendary trackway made me determined to call my book on Earth and human evolution Stepping Stones: the Making of our Homeworld. Human footprints have figured several times in Earth-logs articles. A jumble of footprints in 1.0 to 0.78 Ma old Pleistocene interglacial sediments at Happisbugh on England’s Norfolk coast marks the presence there of Homo antecessor: the earliest known, northern Europeans. In The first volcanologists (March 2003) I noted the discovery of evidence that Neanderthal children played in 350 ka volcanic ash on the Roccamonfina volcano in Italy. The emotion generated by seeing such relics has never left me. Two similarly important proofs of human presence emerged in September 2021.

Footprints thought to have been made by children and teenagers between 23 and 21 thousand years ago in lake shore muds at White Sands, New Mexico. (Credit Bennett et al. 2021)

Since 2011 a variety of evidence has accumulated that the Americas began to be populated by anatomically modern humans before what had long been assumed to be the ‘first arrivals’: the Clovis people who made finely-worked stone spear points first found in 13 ka-old sediments in New Mexico. To the pre-Clovis artefacts that suggested earlier immigrations have been added indisputable signs of human presence even earlier than anticipated. They were uncovered in lake sediments beneath the gypsum sand dunes of White Sands National Park in New Mexico. The site is not far from where Robert Oppenheimer exclaimed to himself ‘Now I am become Death, the destroyer of worlds’ after he witnessed his creation, the first detonation of a nuclear weapon on 9 July 1945. These lake sediments have yielded thousands of human and animal footprints over the years, but the latest have been dated at between 23 to 21 ka (Bennett, M.R. 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). As with the Happisburgh and Roccamonfina human trackways, size analysis suggests that they were made mainly by children and teenagers! Other animal trackways show that the lake edge was teeming with game at the height of the last Ice Age: abundant food for hunter-gatherers generally results in lots of free time. So maybe these early American people were having fun too. When ice sheets were at their maximum extent sea level had fallen, leaving the Bering Strait dry. The broad Beringia land-bridge made the Americas accessible from Eurasia. Whatever objections have previously been raised as regards human penetration south from Alaska during the Last Glacial Maximum, the White Sands find sweeps them away; people overcame whatever obstacles there were.

Travertine outcrop covered with hand- and footprints at Quesang on the Tibetan Plateau (Credit: Zhang et al., Fig. 1c)

Much older footprints and handprints, preserved in a biogenic carbonate (travertine) deposit from the Tibetan Plateau – more than 4,000 metres above sea level – are reported in an article soon to be published by Elsevier (Zhang, D.D. and 17 others 2021. Earliest parietal art: hominin hand and foot traces from the middle Pleistocene of Tibet, Science Bulletin v 66 online; DOI: 10.1016/j.scib.2021.09.001). Travertine forms when calcium carbonate is precipitated from lime-rich spring water onto films of algae or bacteria. At first it is soft and spongy, hardening as more carbonate is precipitated and solidifying when dried out to form a porous rock. People made a jumble of prints when they pressed their hands and feet into the originally spongy biofilm. Three-dimensional images of the slab provide the basis for interpreting how the prints were made. There are 5 handprints and 5 footprints. From comparing their sizes with modern humans’ feet and hands, it seems that the handprints were made by a single 12-year-old, and the footprints by a child of about 7. Although the travertine layer would have been steep and slippery none of the prints show signs of falling or sliding. They seem to have been deliberately placed close to one another, with suggestions that at least one thumb was wiggled. The authors argue that the prints are a form of art similar to the hand stencils commonly seen on Palaeolithic cave walls. It could be that a couple of kids took delight in leaving signs that they had been there, ‘messing around’: but still an art form. What is especially exciting is their age, between 169 and 226 ka. The children are unlikely to have been anatomically modern humans, who first reached Tibet only a little before 21 ka. One alternative is that they were Denisovans (see: Denisovan on top of the world, May 2019.

See also: Bennett, M.R. 2021.  Fossil footprints prove humans populated the Americas thousands of years earlier than we thought. The Conversation, 23 September 2021. 2021Metcalf, T. 2021. Art or not? Ancient handprints spark debate. NBC News, 16 September 2021.

Opportunities for anatomically modern humans to have left Africa

Key ages of early H. sapiens, Neanderthals and Denisovans (credit: Delson, 2019; Fig. 1)

For almost 2 million years humans have migrated long distances, the earliest example of a move out of Africa being the Georgian Homo erectus specimens (see: First out of Africa? November 2003). As regards H. sapiens – anatomically modern humans (AMH) – the earliest fossils, found at Jebel Irhoud in Morocco, are about 300 ka old. By 260 ka they were present at several sites that span the African continent. The first sign of AMH having left Africa are fossils found at Mislaya in Israel and Apidima in Greece – dated to 177 and 210 to 170 ka respectively – and 125 ka-old tools tentatively attributed to AMH in the Arabian Peninsula (see: Arabia : staging post for human migrations?, September 2014). There is also genetically dated evidence of geneflow from Homo sapiens into Neanderthal DNA between 130 to 250 ka ago. The evidence for an early ‘Out of Africa’ migration by AMH is concrete but very sparse, a fuller story of our permanently colonising all habitable parts of the world only emerging for times after about 65 ka.

It is easy to appreciate that the main hindrance for palaeo-anthropological research into human migration centres on the issue of where to look for evidence, a great many discoveries owing more to luck than to a strategic approach. And, of course, once interesting sites are found researchers congregate there. There is a limited number of active palaeoanthropologists of whom only a proportion engage regularly in field exploration. And there is also an element of the old gold prospectors adage: ‘If you want to find elephants, go to elephant country’! But there are other issues connected with discoveries. When was it possible for AMH to make transcontinental journeys and what routes would have been feasible from time to time? Robert Beyer of the Cambridge University with scientists from New Zealand, Estonia and the UK have devised a rational approach to the questions of optimum times and routes for major migration (Beyer, R.M., et al. 2021. Climatic windows for human migration out of Africa in the past 300,000 yearsNature Communications, v.  12, article 4889; DOI: 10.1038/s41467-021-24779-1). Just two routes out of Africa have been considered feasible: by crossing the Strait of Bab el Mandab from Djibouti and southern Eritrea to the Yemen, and following the Nile northwards to access Eurasia via the Levant. The first depends to some extent on how wide the Strait was; depending on sea level fluctuations, it has varied from 4 to 20 km during the last 300 ka. Exit by way of both routes would also have depended on vegetation, game and drinking water supplies that varying amounts of rainfall would have supported.

Assessing the feasibility of crossing the southern Red Sea at different times is fairly easy. Sea level fluctuates according to the amount of water locked in the ice caps of Antarctica and Greenland and on the land glaciated during ice ages in northern North America and Scandinavia. Oxygen isotopes in Pleistocene sea-floor sediments and today’s ice caps reveal that variation. Being one of the world’s most important seaways the bathymetry of the Red Sea is known in considerable detail. At present the minimum sea distance needed to cross the Strait of Bab el Mandab is about 21 km. At the lowest sea levels during the Pleistocene the sea journey was reduced to slightly less than 5 km, which would not have required sophisticated boats or seafaring skills. There is evidence that AMH and earlier humans occupied the western shore of the Red Sea to use its rich marine resources, but none for boats or for habitation of the Yemeni coastline. However, calculations by Beyer et al. of sea level fluctuations during the last 300 ka show that for more than half that time the sea crossing was less than 7 km thanks to a shallow continental shelf and a very narrow stretch of deep water. Clearly the varying width of the Strait is not a useful guide to windows of opportunity for migration via that route. Except for warm interglacials and a few interstadials, people could have crossed at any time provided that the ecosystems on either side could sustain them.

Annual precipitation during each millennium of the Late Pleistocene for the two most likely out-of-Africa routes. The double green lines show the lower level of tolerance for hunter gatherers. The percentage of decades during which ANH could sustain themselves is colour-coded in blues. (Credit: Beyer et al. Fig 2)

Turning to climatic fluctuations, especially that of rainfall, Beyer et al. first estimated the lowest rainfall that hunter-gatherers can survive from the distribution of surviving groups according to annual precipitation and the biomass of grazing prey animals in their habitats. The lower limit is about 90 mm per year. Using the climate record for the Late-Pleistocene from proxies, such as oxygen isotopes, in global climate modelling produces a series of high-resolution ‘time-lapse snapshots’ of conditions in the geographic areas of interest – the Nile-Levant route and that from the Horn of Africa to Yemen. The results are expressed as the percentage of decades in each thousand-year interval that hunter-gatherers could sustain themselves under prevailing climatic conditions in the two regions. What seems clear from the figure (above) is that the southern, Bab el Mandab route had considerable potential for AMH migrants. The northern one looks as if it was more risky, as might be expected from today’s dominant aridity away from the Mediterranean and Gulf coasts. The northern route seems to have been just about feasible for these periods: 245-230; 220-210; 206-197; 132-94; 85-82; ~75 and ~72 ka. The climatic windows for possible migration via the southern route are: ~290; 275-240 (with optimums at ~273, ~269, ~246 and ~243); 230-210; 203-200; 182-145; 135-118; 112; 107; 70-30; 18-13 ka. The well documented beginning of major AMH migration into Eurasia was around 75 to 60 ka, which the southern route would most favour on climatic grounds. Yet before that there are many possibilities involving either route. Any AMH finds outside Africa before 250, and between 190-133 ka seem almost certain to have been via the southern route, based on arid conditions in the north. But, of course, there would have been other factors at play encouraging or deterring migration via either route. So perhaps not every climatic opportunity was exploited.

Beyer and colleagues have provided a basis for plenty of discussion and shifts in focus for future palaeo-anthropological work. One thing to bear in mind is that different humans may also have taken up the opportunities; for example, some Neanderthals are now suspected to have migrated back to Africa in the last 300 ka.

See also: Groucutt, H.S  and 22 others 2021. Multiple hominin dispersals into Southwest Asia over the past 400,000 years. Nature, ; DOI: 10.1038/s41586-021-03863-y