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

Massive hominin skull from China: is it a Denisovan?

In 1933 labourers unearthed a very large skull during the construction of a bridge near Harbin, Northern China. At that time, the area was under occupation by Imperial Japanese forces. To keep it out of the invaders’ hands the skull was quickly wrapped in a cloth and hidden in an old well. It was only in 2018 that the original finder’s grandson recovered it to pass on to archaeologists at  Hebei Geo University. It lacks a lower jawbone, so technically it is a cranium, but is very well preserved. The face has very large brow ridges – generally taken as a primitive feature – but also some more modern features. With a 1,420 ml brain case, it is significantly larger than most modern human crania. Apparently, it is of an adult male. As well as a big head, he had a large nose, as do Neanderthals: a possible adaptation to very cold conditions. Without waiting to see if the bones might yield DNA, five of the team who examined the cranium claimed it as a new species, Homo longi or ‘Dragon Man’; i.e. distinct from modern humans and Neanderthals and all known older hominins (Ni, X. et al. 2021. Massive cranium from Harbin establishes a new Middle Pleistocene human lineage in China. The Innovation, v. 2, article 100130; DOI: 10.1016/j.xinn.2021.100130. Ji, Q. et al. 2021. Late Middle Pleistocene Harbin cranium represents a new Homo species. The Innovation, v. 2, article 100132; DOI: 10.1016/j.xinn.2021.100132). They based this phylogenetic interpretation on morphology alone. At least one of the team, Chris Stringer a leading hominin palaeoanthropologist at The Natural History Museum in London, demurred. The cranium is not unique and bears close similarity to another from the central Chinese province of Shaanxi, which was found in the late 1970s. In fact there are three other Chinese crania that resemble that from Harbin, although they are less well preserved.

All-sided views of the Harbin cranium. (Credit: Ni et al., Fig 2)

Dating the fossil was not easy, as the site where labourers discovered him was destroyed during construction of the bridge. Researchers used a variety of geochemical analyses, including from sediment stuck in his nasal cavity, to derive a likely stratigraphic profile from which the cranium may have been excavated. The best fit is with Middle Pleistocene sediments in the Harbin area. Uranium-series dating of the bone suggests that it is older than 146 ka (Shao, Q.  et al. 2021. Geochemical locating and direct dating of the Harbin archaic human craniumThe Innovation, v. 2, article 100131; DOI: 10.1016/j.xinn.2021.100131). So it is likely that this man and his companions did not cohabit China with anatomically modern humans, who arrived no more than about 50 ka ago. The highly robust nature of all the similar crania suggests that the individuals must have been large and physically active. Like the Neanderthals, they had adapted to harsh conditions over several hundred thousand years of repeated climate change. Even today, winters in northern China average around -16°C, and far inland conditions are semi-arid to arid. For them to migrate would have involved traversing some of highest, bleakest passes in the world. These people evolved to survive extreme climatic and environmental change, much as did the Neanderthals in West Asia and Europe. By comparison anatomically modern humans evolved in the more stable environments of Africa and the Middle East, surviving only the last ice age once they had migrated northwards. Those who made it to northern Siberia and crossed the Bering Strait via Beringia around the last glacial maximum did evolve physical traits that helped them survive, but minor ones compared with the earlier humans.

So what do these Chinese fossils represent? Using cranial features alone to propose distinct species smacks of the techniques of 19th and early 20th century anatomical anthropologists, albeit with powerful statistical analysis. We know that anatomically modern humans carry genetic signatures of interbreeding with at least two known ‘species’ with whom they cohabited Eurasia – Neanderthals and Denisovans. Indeed, traces in  the DNA of living African and Eurasian humans hint at other unknown and probably very ancient ‘ghost’ populations. Genetic, physical and probably cultural differences did not deter repeated interbreeding with these ‘others’. To be frank, erecting new human ‘species’ these days seems to serve little purpose. ‘Dragon Man’ is just as likely to represent the Denisovans as the fully sequenced DNA from a couple of bones from caves in Siberia and Tibet. The latter matched stretches of the DNA from living people of East Asia and parts of the Pacific. There are no other such live genetic tracers awaiting a different candidate to fill the role that we know Neanderthals and Denisovans to have filled. That may yet change, but the first job for the mainly Chinese consortium of scientists is to get genetic material from these crania and sequence it, or invite other highly successful palaeogeneticists who would leap at the opportunity.

See also: Jones, N. 2021. Mysterious skull fossils expand human family tree — but questions remain. Nature, v. 595, p. 50; DOI: 10.1038/d41586-021-01738-w

Sample I. 2021. Massive human head in Chinese well forces scientists to rethink evolution. The Guardian, 25 June 2021.

The early signs of counting and arithmetic?

Three earlier articles in Earth-logs originally focussed on what I supposed to be ‘ancient abstract art’.  One highlighted a clam shell that bears carefully etched V-shapes found at the type locality for Asian Homo erectus at Trinil on the Solo River, Java, dated between 430 and 540 ka. Another is about parallel lines etched on a piece of defleshed bone from China dated at 78 to 123 ka, which may be a Denisovan artefact. The most complex is a piece of ochre found in the coastal Blombos Cave 300 km east of Cape Town, South Africa in association with tools ascribed to early modern humans who lived there about 73 ka ago. Fascinating as they seemed at the time, they may hold much greater significance about early-human cognitive powers than about mere decoration. That is thanks to recent evaluation of other simple artefacts made of lines and notches by anthropologists, cognitive scientists and psychologists. Their work is summarised in a recent Nature Feature by Colin Barras (Barras, C. 2021. How did Neanderthals and other ancient humans learn to count? Nature, v. 594, p. 22-25; DOI: 10.1038/d41586-021-01429-6). The European Research Council recently allocated a €10 million grant to foster research into ‘when, why and how number systems appeared and spread’.

Examples of ancient ‘abstract’ art. Top – V-shaped features inscribed on 430-540 ka freshwater clam from Java; Middle – parallel lines etched through red ochre to show white bone, from a possible Denisovan site in China; Bottom – complex inscription on a tablet of iron-rich silcrete from South Africa

Straight lines and patterns made from them are definitely deliberate, whatever their antiquity. In recent times, such devices have been used by artists to render mental images, moods and thoughts as simplified abstractions: hence ‘abstract’ art, such as that of Piet Mondrian and Kazimir Malevich. The term also applies to the dribbles and drabbles of Jackson Pollock and many more styles. But these works are a very recent evolutionary development out of earlier schools of art. So deliberate geometric shapes and arrangements of lines that are many millennia old cannot simply be termed ‘abstract art’. It is certainly not easy to see how they evolved into the magnificence of Palaeolithic figurative cave art that started at least 40 thousand years ago; Yet they are not ‘doodles’. Being so deliberate suggests that they represented something to their makers. The question is, ‘What?’

The research summarised by Barras is mainly that of Francisco d’Errico of The University of Bordeaux, France and colleagues from Canada and Italy (d’Errico, F. et al. 2018. From number sense to number symbols. An archaeological perspective. Philosophical Transactions of the Royal Society B, v. 373, article 2160518; DOI: 10.1098/rstb.2016.0518). They focused their work on two remarkable artefacts. The oldest (72 to 60 ka), from a cave near Angoulême in France, is a fragment of a hyena’s thigh bone that carries nine notches. It is associated with stone tools almost certainly made by Neandethals. The other, from the Border Cave rock shelter in KwaZulu-Natal in South Africa, is a 44 to 42 ka old baboon’s shin bone, which carries a row of 29 prominent notches, and a number of less distinct, roughly parallel scratches. The rock shelter contains remains of anatomically modern humans and a very diverse set of other artefacts that closely resemble some used by modern San people.

Top: notched hyena femur bone fragment associated with Neanderthal tools from SW France. Bottom: notched baboon shin bone from Border Cave, South Africa. Scale bars(Credit: F. d’Errica and L. Backwell)

Microscopic examination of the notches made by a Neanderthal suggest that all 9 notches were cut at the same time, using the same stone blade. Those on the Border Cave shin bone suggest that they were made using four distinctly different tools on four separate occasions. Are both objects analogous to tally sticks; i.e. to count or keep a record of things as an extension to memory? There are other known examples, such as a 30 ka-old  wolf’s radial bone from the Czech Republic having notches in groups of five, suggesting a record of counting on fingers. Yet very similar devices, made in recent times by the original people of Australia, were not used for keeping count, but to help travellers commit a verbal message to memory enabling them to recount it later.

Do read Barras’s summary and the original paper by d’Errico et al. to get an expanded notion of the arguments being debated. They emerge from the truly novel idea that just because the makers of such objects lived tens or even hundreds of thousands of years ago that doesn’t make them intellectually lacking. Imagining in the manner of Victorian scientists that ancient beings such as Neanderthals and H. erectus must have been pretty dim is akin to the prejudice of European colonialists that people of colour or with non-European cultures were somehow inferior, even non-human. To me it is admirable that the European Research Council has generously funded further research in this field at a time when research funding in the UK, especially for the disciplines involved, has been decimated by those who demanded an exit from the EU.

The older Trinil and Blombos patterns appear yet more sophisticated. The pattern on the latter looks very like the kind of thing that someone in a prison cell might draw to keep track of time. It also incorporates the zig-zag element engraved on the Trinil clam shell. Remember that the word ‘Exchequer’ is derived from a tax audit during the reign of Henry I of England that was conducted on a counting board whose surface had a checked pattern

CSI and detecting the presence of ancient humans

Enter a room, even for a few minutes, and dead skin cells will follow you like an invisible cloud to settle on exposed surfaces. Live there and a greyish white, fluffy dust builds up in every room. Even the most obsessive cleaning will not remove it, especially under a bed or on the bathroom floor. Consider a cave as a home, but one without vacuum cleaners, any kind of sanitation, paper tissues, panty liners, nappies or wet wipes. For pre-modern human dwellings can be added snot, fecal matter, sweat, urine, menstrual blood and semen among all the other detritus of living. A modern crime-scene investigator would be overwhelmed by the sheer abundance of DNA from the host of people who had once dwelt there. CSI works today as much because most homes are pretty clean and most people are fastidious about personal hygene as because of the rapidly shrinking lower limit of DNA detection of the tools at its disposal. Except, that is, when someone from outside the home commits a criminal offence: burglary, GBH, rape, murder. We have all eagerly watched ‘police operas’ and in the absence of other evidence the forensic team generally gets its perpetrator, unless they did the deed wearing a hazmat suit, mask, bootees and latex gloves.

Artistic impression of Neanderthal extended-family life in a cave (credit: Tyler B. Tretsven)

Since 2015 analysis of environmental DNA from soils has begun to revolutionise the analysis of ancient ecosystems, including the living spaces of ancient humans (see: Detecting the presence of hominins in ancient soil samples, April 2017). It is no longer necessary to find tools or skeletal remains of humans to detect their former presence and work out their ancestry. DNA sequencing of soil samples, formerly discarded from archaeological sites, can now detect former human presence in a particular layer, as well as that of other animals. In many cases the ‘signal’ pervades the layer rather than occurring in a particular spot, as expected from shed skin cells and bodily fluids. The first results were promising but only revealed mitochondrial DNA. Now the technique has extended to nuclear DNA: the genome (Vernot, B. and 33 others 2021. Unearthing Neanderthal population history using nuclear and mitochondrial DNA from cave sediments. Science, v. 372, article eabf1667; DOI: 10.1126/science.abf1667). Benjamin Vernot and colleagues from 7 countries collected and analysed cave soils from three promising sites with tangible signs of ancient human occupation. Two of them were in Siberia and had previously yielded Neanderthal and Denisovan genomes from bones. The other is part of the Atapuerca cave complex of NW Spain that had not. The Russian caves yielded DNA from more than 60 samples, 30 being nuclear DNA consistent with that from actual Neanderthal and Denisovan bones found in the caves. Galería de las Estatuas cave in Spain presented a soil profile spanning about 40 thousand years from 112 to 70 ka.

Teasing-out nuclear DNA from soil is complicated, from both technical and theoretical standpoints. So being able to match genomes from soil and bone samples in the Russian caves validated the methodology. The Spanish samples could then be treated with confidence. Galería de las Estatuas revealed the presence of Neanderthals throughout its 40 ka soil profile, but also a surprise. The older DNA was sufficiently distinct from that from later levels to suggest that two different populations had used the cave as a home, the original occupants being replaced by another genetically different group around 100 to 115 ka ago. The earlier affinity was with the ancestors of sequenced Neanderthal remains from Belgium, the later with those from Croatia. That time is at the end of the last (Eemian) interglacial episode, so one possibility is a population change driven by climatic deterioration. This success is sure to encourage other re-examinations of caves all over the place. That is, if there is the analyical capacity to perform such painstaking work in greater volume and at greater pace. Like many other palaeo-genomic studies, this one has relied heavily on the analytical facilities pioneered and developed by Svante Paäbo at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Covid has forced genetics to the front page for a year and more. And it has led to many advances in anlytical techniques, particularly in their speed. It would nice to think that a dreadful experience may end-up with positive benefits for understanding the full history of humanity.

Relationships between modern humans and Neanderthals

Before 40 thousand years (ka) ago Europe was co-occupied by Neanderthals and anatomically modern humans (AMH) for between five to seven thousand years; about 350 generations – as long as the time since farming began in Neolithic Britain to the present day. Populations of both groups were probably low given their dependence on hunting and foraging during a period significantly colder than it is now. Crude estimates suggest between 3,000 to 12,000 individuals in each group; equivalent to the attendance at a single English Football League 2 match on a Covid-free winter Saturday afternoon. Moving around Europe south of say 55°N, their potential range would have been around 5 million square kilometres, which very roughly suggests that population density would be one person for every 200 km2. That they would have moved around in bands of, say, 10 to 25 might seem to suggest that encounters were very infrequent. Yet a hybrid Neanderthal-Denisovan female found in Siberia yielded DNA that suggested a family connection with Croatia, 5,000 km away (see: Neanderthal Mum meets Denisovan Dad, August 2018); early humans moved far and wide.

The likely appearances of Neanderthals and anatomically modern humans when they first met between 50 and 40 thousand years ago. (Credit: Jason Ford, New York University)

A sparsely populated land can be wandered through with little fear other than those of predators, sparse resources or harsh climate and lack of shelter. But it still seems incredible for there to have been regular meetings with other bands. But that view leaves out knowledge of good places to camp, hunt and forage that assure shelter, water, game and so forth, and how to get to them – a central part of hunter-gatherers’ livelihoods. There would have been a limited number of such refuges, considerably increasing chances of meeting. Whatever the physiognomic differences between AMH and Neaderthals, and they weren’t very striking, meeting up of bands of both human groups at a comfortable campsite would be cause for relief, celebration, exchanges of knowledge and perhaps individuals of one group to partner members of the other.

As well as that from Neanderthals, ancient DNA from very early European AMH remains has increasingly been teased out. The latest comes from three individuals from Bacho Kiro Cave in Bulgaria dated to between 45.9 to 42.6 ka; among the earliest known, fully modern Europeans. One had a Neanderthal ancestor less than six generations removed (perhaps even a great-great grandparent 60 years beforehand). Because of the slight elapsed time, the liaison was probably in Europe, rather than in the Middle East as previously suggested for insertion of Neanderthal genes into European ancestry. The genetic roots of the other two families stemmed back seven to ten generations – roughly 100 to 150 years (Hajdinjak, M. and 31 others 2021. Initial Upper Palaeolithic humans in Europe had recent Neanderthal ancestryNature, v. 592, p. 253–257; DOI: 10.1038/s41586-021-03335-3). The interpretation of these close relationships stems from the high proportion of Neanderthal DNA (3 to 4 %) in the three genomes. The segments are unusually lengthy, which is a major clue to the short time since the original coupling; inherited segments tend to shorten in successive generations. The groups to which these AMH individuals belonged did not contribute to later Eurasian populations, but link to living East Asians and Native Americans. They seem to have vanished from Europe long before modern times. The same day saw publication of a fourth instance of high Neanderthal genetic content (~3 %) in an early European’s genome, extracted from a ~45 ka female AMH from Zlatý kůň (Golden Horse) Cave in Czechia (Prüfer, K. and 11 others 2021. A genome sequence from a modern human skull over 45,000 years old from Zlatý kůň in Czechia. Nature Ecology & Evolution  DOI: 10.1038/s41559-021-01443-x). In her case, too, the Neanderthal DNA segments are unusually lengthy, but indicate 70 to 80 generations (~2,000 to 3,000 years) had elapsed. Her DNA also suggests that she was dark-skinned and had brown hair and brown eyes. Overall her genetics, too, do not have counterparts in later European AMH. The population to which she belonged may have migrated westwards from the Middle East, where one of her ancestors had mated with a Neanderthal, perhaps as long as 50 ka ago. But that does not rule out her group having been in Europe at that time. A later modern human, dated at 42 to 37 ka, is a young man from the Petştera cu Oase cave in Romania, whose forbears mixed with Neanderthals. His genome contains 6.4% of Neanderthal DNA, suggesting that his Neanderthal ancestor lived a mere 4 to 6 generations earlier, most likely in Europe, and was perhaps one of the last of that group.

The data suggest that once modern humans came into contact with their predecessors in the Middle East and Europe, mixture with Neanderthals was ‘the rule rather than the exception’. Yet their lack of direct relationship to later Europeans implies that AMH colonisation of Europe occurred in successive waves of people, not all of whom survived. As Palaeolithic specialist Chris Stringer of the Natural History Museum in London cautions, of these multiple waves of incomers ‘Some groups mixed with Neanderthals, and some didn’t. Some are related to later humans and some are not’. Even five thousand years after ‘first contact’, relations of modern humans with Neanderthals remained ‘cordial’, to say the least, including with the last few before their extinction.

See also: Gibbons, A. 2021. More than 45,000 years ago, modern humans ventured into Neanderthal territory. Here’s what happened next. Science, v. 372, News article; DOI: 10.1126/science.abi8830. Callaway, E. 2021. Oldest DNA from a Homo sapiens reveals surprisingly recent Neanderthal ancestry. Nature, v. 592, News article; DOI: 10.1038/d41586-021-00916-0. Genomes of the earliest Europeans (Science Daily, 7 April 2021). Bower, B. 2021 Europe’s oldest known humans mated with Neandertals surprisingly often (ScienceNews, 7 April 2021)

Snippet: Early human collection of useless objects

The Ga-Mohana rock shelter in North Cape Province, South Africa (Credit: Jayne Wilkins, University of the Witwatersrand)

We all, especially as kids, have collected visually interesting objects for no particular reason other than they ‘caught our eye’: at the beach; from ploughed fields; river gravel, or at the side of a path. They end up in sheds, attics and mantel shelves. In an online News and Views article at the Nature website Pamela Willoughby discusses the significance of a paper on an archaeological site in the southern Kalahari Desert, North Cape Province South Africa (Willoughby, P.R. 2021. Early humans far from the South African coast collected unusual objects. Nature, v. 323, online News and Views; DOI: 10.1038/d41586-021-00795-5). Jayne Wilkins and co-workers from South Africa, Australia, Canada, Austria and the UK have investigated a rock shelter, with floor deposits going back over 100 thousand years. The researchers have, in a sense, continued the long human habit of seeking objets trouvée by using trowels and sieves to excavate the shelter’s floor sediments. They found a collection of cleavage fragments of white calcite and abundant shards of ostrich shell. Ga-Mohana Hill is still a place that locals consider to have spiritual significance. The authors consider the original collectors to have had no other motive than aesthetic pleasure and perhaps ritual, and that this signifies perhaps the earliest truly modern human behaviour. Yet, in 1925 a cave on the other side of South Africa, in Limpopo Province, yielded a striking example of a possible ‘collector’s piece’ from much earlier times. It is associated with remains of australopithecines and has been dated to around 3 Ma ago (see: Earliest sign of a sense of aesthetics, November 2020).

Source: Wilkins, J. et al.2021. Innovative Homo sapiens behaviours 105,000 years ago in a wetter Kalahari. Nature, v. 323 DOI: 10.1038/s41586-021-03419-0

Magnetic reversal and demise of the Neanderthals?

A rumour emerged last week that the Neanderthals met their end as one consequence of an extraterrestrial, possibly even extragalactic influence. Curiously, it stems from a recent discovery in New Zealand, where of course Neanderthals never set foot and nor did anatomically modern humans, the ancestors of Maori people, until a mere 800 years ago. It started with an ancient log from a kauri tree (Agathis australis), a species that Maoris revere. Found in excavations of boggy ground, the log weighed about 60 tons, so it was a valuable commodity, especially as it is illegal to fell living kauri trees. The wood is unaffected by burial and insect attack, has a regular grain and colour throughout, so is ideal for monumental Maori sculpture. Such swamp kauri also preserves their own life history in annual growth rings, and the log in question has 1700 of them. Using growth rings to chart climate variation gives the most detailed records of the recent past, provided the wood can be dated. Matching growth ring records from several trees of different ages is key to charting local climate with annual precision over several millennia.

An ancient kauri tree log recovered by swampland excavations in New Zealand. (Credit: Jonathan Palmer, in Voosen 2021)

Radiocarbon dating indicates that this particular kauri tree was growing around 42 thousand years ago. That is close to the upper limit for using 14C concentration in organic matter to determine age because the isotope has a short half-life (5730 years). In this case samples of the log would contain only about 0.7 % of its original complement of radioactive carbon. Cosmic rays generate 14C when they hit nitrogen atoms in the atmosphere and it enters COand thus the carbon cycle. Carbon dioxide taken up by photosynthesis to contribute carbon to plants contains only about one part per trillion of 14C. Consequently wood as ancient as that in the kauri log contains almost vanishingly small amounts, yet it can still be measured using mass spectrometry to yield an accurate radiometric age.

The particularly interesting thing about the 42 ka date is that it coincides with the timing of the last reversal of the Earth’s magnetic field, known as the Laschamps event. The kauri tree bears detailed witness through its growth rings to the environmental effects of a decrease in that field to almost zero as the poles flipped. The bulk of cosmic rays are normally deflected away from the Earth by the geomagnetic field, but during a reversal a great many more pass through the atmosphere, the most energetic reaching the surface and the biosphere. The kauri growth rings record fluctuations in the generation of 14C by their passage and thereby the geomagnetic field strength, which was only 6% of normal levels from 42.3 to 41.6 ka (Cooper, A. and 32 others  2021. A global environmental crisis 42,000 years ago. Science, v. 371, p. 811-818; DOI: 10.1126/science.abb8677). This coincided with an unrelated succession of periods of low solar activity and a reduced solar ‘wind’, which also provides some cosmic-rayprotection when activity is at normal levels; a ‘double whammy’. One consequence would have been destruction of stratospheric ozone by cosmic rays and thus increased ultraviolet exposure at ground level.

Combined with the highly precise growth-ring dating, the climatic changes over the 1700 year lifetime of the kauri tree can be linked to other records of environmental change. These include glacial ice- and lake-bed cores together with stalactite layers. Apparently, the Laschamps geomagnetic reversal coincided with abrupt shifts in wind belts and precipitation, perhaps triggering major droughts in the southern continents. Highly plausible, but some of the other speculations are less certain. For instance, some time around 42 ka, but far from well-established, Australia’s marsupial megafauna experienced major extinctions, the Neanderthals disappear from the fossil record and modern humans started decorating caves in Europe (20 ka after they did in Indonesia). In fact, speculation becomes somewhat silly, with suggestions that early Europeans went to live in caves because of increased exposure to UV (they knew, did they, while Neanderthals didn’t?), their painting and, by implication, their entire culture shifting through the shock and awe of mighty displays of the aurora borealis. Just because the number 42 is (or was), according to the late Douglas Adams’s Hitchhiker’s Guide to the Galaxy, ‘the answer to life, the universe and everything’, the authors tag the episode as the ‘Adams Event’. In their summary for The Conversation they include an animation with a quintessential Stephen Fry narrative, which Earth-logs readers can judge for themselves. Perhaps ‘Lockdown Trauma’ has a lot more to answer for, other than upsurges in Zoom conferences, knitting and gourmet experimentation …

See also: Voosen, P. 2021. Kauri trees mark magnetic flip 42,000 years ago. Science, v. 371, p. 766; DOI: 10.1126/science.371.6531.766

And here’s another snippet: Neanderthal link to our brain

Elizabeth Pennisi reports on a ‘Petri-dish’ experiment that substitutes a Neanderthal gene for a modern human one in a culture of human brain tissue. It gives some idea of how our very close relative may have thought differently from us. Pennisi, E. 2021. Neanderthal-inspired ‘minibrains’ hint at what makes modern humans specialScience, online news item; DOI:10.1126/science.abh0331

The ancestry of our opposable thumbs

Since the appearance of smart phones and the explosion of social media our thumbs have found a new niche; typing while holding a mobile. At a desktop keyboard, most of us don’t use thumbs very much, unless we have mastered fast touch typing, but for a huge variety of manual tasks thumbs are essential. The first makers of sophisticated stone tools must have been able to grip between fingers and thumb to manipulate the materials from which they were made and to perform the various stages in creating a razor sharp edge. To do that, as most of us are aware, the tip of the thumb must be capable of touching the tips of all four fingers; an opposable thumb is essential for the ‘precision grip’. Being able to tell when opposable thumbs evolve depends, of course, on finding hand-bone fossils. Being made of many bones disarticulated hands are a lot more fragile than long bones or those of the skull. Complete fossil hands are rare, as are feet, but a number have been found more or less complete. Whichever hominin had evolved opposable thumbs, their potential would have given them a considerable advantage over those that hadn’t.

The main muscles that control the movements of modern human fingers and thumb (Credit: Wikipedia)

Simply comparing the shapes of fossilised bones of fingers and thumbs with those of modern humans and other living primates has, so far, not proved capable of resolving with certainty which hominin groups either did or did not have opposable thumbs. The key lies in the muscles that operate them. It has become commonplace to reconstruct faces and even whole bodies from fairly complete skeletal remains by modelling musculature from the positioning and shape of the points of attachment of muscles to bone. But that become increasingly difficult for the small-scale and intricate attachments in hands. The critical muscle for opposable thumbs is known as the Opponens pollicis (the Latin for thumb is Digitus pollex); a small triangular muscle that operates in conjunction with three others (with pollicis in their Latin names).

Fotios Karakostis and six colleagues from German, Swiss and Greek universities have devised software that can model muscles in 3-D (F.A. Karakostis et al. 2021. Biomechanics of the human thumb and the evolution of dexterityCurrent Biology, v.31,  online; DOI: 10.1016/j.cub.2020.12.041). Based on the anatomy of human and chimpanzee hand muscles and the positions of their attachment to individual bones, they have been able to establish a series of parameters that clearly distinguish the morphological and probably functional characteristics of the thumbs of these living primates. Complete sets of thumb bones from four Neanderthal skeletons show that they were significantly, but only slightly, different from anatomically modern humans. Those from three species of Australopithecus (africanus, sediba and afarensis) lie between ours and chimps’, with significantly closer affinity to chimpanzees. It seems that australopithecines of whatever age were not equipped with opposable thumbs and were possible tool producers and users with the very limited capabilities of modern chimps; holding, pounding and poking. A single set of hominin thumb bones from about two million years ago that were found in the famous Swartkrans Cave in South Africa show just as close affinity in thumb opposability to humans as do Neanderthals. So at 2 Ma there was a hominin species sufficiently dextrous to make and use sophisticated tools. The problem is, the bones are not directly associated with others and have been ascribed by different authors either to H. habilis or Paranthropus robustus. Interestingly, this paranthropoid has also been suggested (controversially) to have been the first known hominin to use fire, and it also used digging sticks. No one has ever suggested that the genus Homo descended from a paranthropoid ancestor or vice versa; these massively jawed beings did coexist with early humans in East Africa for over a million years. The other hominin who left hands in the geological record was Homo naledi; a controversial species because it was found in a barely accessible cave chamber, and took a while to date. This context gave rise to the notions that it was the direct ancestor of humans and that it buried its dead in a special place. However, it turned out to be relative recent, at about 280 ka (see: Homo naledi: an anti-climax; May 2017). Homo naledi does seem to have had opposable thumbs, but there is no associated evidence to suggest either tool making or use.

Fascinating as the methodology outlined by Karakostis et al. is, their findings do not take early human capabilities very much further than what is already known. Tools were made and used as far back as 3.3 Ma ago, and we know that H. habilis was doing this by about 2.6 Ma; i.e. long before the first evidence for opposable thumbs, and who had them first is uncertain. What is clear is that sophisticated tools, such as the bifacial Acheulian artifacts whose manufacture demands great dexterity, only appeared after the potential for nimble dexterity (about 1.8 Ma). The same goes for the first migration out of Africa, at about the same time, which demanded resourcefulness that may have sprung from the ability to manipulate natural materials effectively and carefully

See also: Handwerk B. 2012. How dexterous thumbs may have helped shape evolution two million years ago. (Smithsonian Magazine, 28 January 2021); Bower, B. 2021. Humanlike thumb dexterity may date back as far as 2 million years ago. (Science News, 28 January 2021)