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.

Sunrise Girl-Child: the first American colonist

Thanks to a variety of archaeological finds of tools and animal bones bearing cut marks, together with precise dating, it now seems clear that the Americas began to be colonised as early as the Last Glacial Maximum from tangible evidence from Bluefish Cave in the Yukon territory of Canada and as early as 15.5 ka close to the southern tip of South America in Chile. Although confirmation remains to be found, there is even a possibility that pre-sapiens people had arrived far earlier. Advances in analysis of ancient genetic material help understand the divergence of early colonisers. Y-chromosome DNA from living indigenous men suggests that all early Americans stemmed from 4 separate colonising populations who may have entered by crossing the Beringia land bridge, exposed as a result of glacial fall in global sea level, to follow different routes, including along the Pacific coast. A possible common ancestor of all native Americans emerged in 2013 from the mitochondrial and Y-chromosome DNA of the skeleton of a young man from near Lake Baikal in Siberia who lived about 24 ka ago. At the very start of 2018 an online paper in Nature took the story even further.

This image was first published in the 1 st (18...
The diversity of Native American people. (credit: Wikipedia from a 19th century Norwegian painting)

The remains of a ~6-week-old girl recovered from a site at Upward Sun River in Alaska – called ‘Xach’itee’aanenh t’eede gay’, or ‘sunrise girl-child’ by indigenous Alaskans – dated at 11.5 ka, has yielded a precise genome (Moreno-Mayar, J. and 17 others 2018. Terminal Pleistocene Alaskan genome reveals first founding population of Native Americans. Nature; doi:10.1038/nature25173).  The baby girl’s DNA shows that the group to which she belonged was ancestral to contemporary and fossilised ancient Native Americans. She was probably a member of a founding population of ‘Beringians’. At the end of the last glacial epoch (11.5 ka) a separate branch of Native Americans was already established in unglaciated North America further south. That group had split into two further groups sometime between 17.5 to 14.6 ka, who became ancestors of most of the indigenous people of the Americas. The ‘Beringian’ people were therefore probably stranded in the far north by the difficulties of crossing the vast North American ice sheet. Probing deeper into time, using demographic modelling, suggests that the founding population of all Native Americans, including the ‘Beringians’, split from East Asians around 36 ka ago. Gene flow among them and with East Asians persisted until about 25 thousand years ago, with some admixture with ancient northern Eurasians up to 20 ka. It seems that the ‘Beringians’, of whom little ‘sunrise girl-child’ was a late member, became isolated genetically between 22-18 ka.

The ancestral mixture of both East Asian and northern Eurasians that led to the founders of the whole panoply of geographically isolated Native Americans is remarkable. It shows just how far human groups moved and mingled during the run-up to the Last Glacial Maximum, which made the far north just about uninhabitable – or so it has been assumed. For a small ethnically mixed group to survive such conditions for so long suggests considerable ingenuity in living off the land.

http://www.bbc.co.uk/news/science-environment-42555577

Pre-sapiens hominins reached North America?

In 1991-2 palaeontologists excavated a site near San Diego, California where broken bones had been found. These turned out to be the disarticulated remains of an extinct mastodon. One feature of the site was the association of several large cobbles with bones of large limbs that seemed to have been smashed either to extract marrow or as source of tool-making material. The cobbles showed clear signs or pounding, such as loss of flakes – one flake could be fitted exactly to a scar in a cobble – pitted surfaces and small radiating fractures. The damage to one cobble suggested that it had been used as an anvil, the others being hammer stones.  Broken pieces of rock identical to the hammer stones were found among the heap of bones. No other artefacts were found, and the bones show no sign of marks left by cutting meat from them with stone tools. The breakage patterns of the bones included spiral fractures that experimental hammering of large elephant and cow bones suggest form when bone is fresh. Other clear signs of deliberate breakage are impact notches and small bone flakes. Two detached, almost spherical heads of mastodon femora suggest that marrow was the target for the hammering and confirmed the breakage was deliberate.

Mastodon.
Artist’s impression of American mastodon. (credit: Wikipedia)

Since the sediment stratum in which the remains occurred consists of fine sands and silt, typical of a low-energy river system, the chances that the cobbles had been washed into association with the mastodon are very small. The interpretation of the site is that it was the result of opportunistic exploitation of a partial carcase of a young adult mastodon by humans. In the early 1990s attempts were made to date the bones using the radiocarbon method, but failed due to insufficient preserved collagen. That the site may have been much older than the period of known occupation of North America by ancestors of native people (post 14.5 ka) emerged from attempts at optically stimulated luminescence dating of sand grains that can suggest the age of burial. These suggested burial by at least 60 to 70 ka ago. It was only when the uranium-series disequilibrium method was used on bone fragments that full significance of the site emerged. The results indicated that they had been buried at 130.7±9.4 ka (Holen, S.R. and 10 others 2017. A 130,000-year-old archaeological site in southern California, USA. Nature, v.  544, p. 479—493; doi:10.1038/nature22065 – full paper and supplements available free)

Not only is the date almost ten times that of the earliest widely accepted signs of Homo sapiens in the Americas, the earliest anatomically modern humans known to have left Africa are around the same age, but restricted to the Levant. The earliest evidence that modern humans had reached East Asia and Australasia through their eastward migration out of Africa is no more than 60 ka. The date from southern California is around the start of the interglacial (Eemian) before the one in which we live now. It may well have been possible then, as ~14 ka ago, to walk across the Bering Straits due to low sea level, or even by using coast-hugging boats – hominins had reached islands in the Mediterranean and the Indonesian peninsula certainly by 100 ka, and probably earlier. But whoever exploited the Californian mastodon marrow must have been cold-adapted to achieve such a migration. While the authors speculate about ‘archaic’ H. sapiens the best candidates would have been hominins known to have been present in East Asia: H. erectus, Neaderthals and the elusive Denisovans.

Surely there will be reluctance to accept such a suggestion without further evidence, such as tools and, of course, hominin skeletal remains. But these long-delayed findings seem destined to open up a new horizon for American palaeoanthropology, at least in California.

You can find more information on hominin migration here.

https://www.newscientist.com/article/2129042-first-americans-may-have-been-neanderthals-130000-years-ago/

Yukon colonised during Last Glacial Maximum

For many years anthropologists were certain that the Americas remained outside the human realm until the great icecap of North America had begun to melt decisively. This view stemmed partly from the only conceived route being across the exposed floor of the Bering Sea when sea-level had fallen to leave it as a landmass known as Beringia. The other literal stumbling block had been the glacial blockage of the only lowland corridor from Alaska to the Great Plains which roughly follows the Alberta – British Columbia border in Canada. There is abundant evidence that the corridor did not become ice-free until about 13 ka, an important fact that for a long while bolstered the Clovis-First hypothesis, from the eponymous and highly distinctive stone tools that date back to just after that time. After a long, sturdy rearguard action by its devotees that view was transcended by finds of earlier tools with dates as old as 15.5 ka that extend close to the southernmost tip of South America. Studies of Y-chromosome DNA from living First Nations men that suggested that all early Americans stemmed from 4 separate colonising populations who may have entered via Beringia by different routes, including along the Pacific coast. A possible common ancestor of all native Americans has turned up from the mitochondrial and Y-chromosome DNA of a fossil skeleton from near Lake Baikal in Siberia who lived about 24 ka ago. But yet another twist has emerged from the Yukon Territory of Northern Canada.

Beringia Land Bridge. Animated gif of its prog...
Beringia Land Bridge. Animation of its development from 21.000 BC to modern times.(Photo credit: Wikipedia)

Since 1987 it has been known that animal bones with clear signs of butchery occurred in the Bluefish Cave on the Yukon – Alaska border. Dating of the bones by the 14C method seemed to support human occupation there during the Last Glacial Maximum; highly controversial at the time, in the absence of any other sites of that age in the whole Americas. The material has now been re-examined and dated by a more advanced radiocarbon method (Bourgeon, L. et al. 2017. Earliest human presence in North America dated to the Last Glacial Maximum: new radiocarbon dates from Bluefish Caves, Canada. PLoS ONE, v. 12; doi:10.1371/journal.pone.0169486). This work has confirmed the earlier view since the ages of bones range from 24 to 12 ka. But the discovery of what seems long-term occupation under the most arduous glacial conditions is not the only outcome of the research. One hypothesis for the genetic diversity among living indigenous people of the Americas is that their forebears, the first people of the Americas, may have been from genetically isolated populations stranded on Beringia, yet surviving eventually to migrate southward once climate warmed. The ‘Beringian standstill hypothesis’ suggest that the small population underwent genetic drift for about eight thousand years, their descendants inheriting the genetic diversity produced by this process. Bluefish Cave is probably where some of those pioneers waited-out the Ice Age