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)