A new timeline of the crossing of Neanderthals and ancient humans

Hidden in the genetic codes of many people is a mystery that has long intrigued scientists: a tiny slice of Neanderthal DNA that persists tens of thousands of years after the species became extinct. Most non-African peoples can attribute about 1-2% of their DNA to their Neanderthal ancestors.

But the details of this evolutionary history remain unclear. How often did ancient humans and Neanderthals interbreed? When exactly did this happen? Why did Neanderthals disappear and why did modern humans survive? What does this Neanderthal DNA do for us now?

Two research groups separately analyzed collections of ancient genomes and came to the same conclusions on some of these fundamental questions. Published studies in Nature journals And Science Thursday suggest that ancient humans and Neanderthals interbred during a limited period of time when humans left Africa and migrated to new continents.

The wave of interbreeding took place around 43,500 to 50,500 years ago, according to the results. Then, over the next 100 generations, most of the Neanderthal DNA was eliminated – but not all. Today, the DNA that remains is linked to characteristics such as skin pigmentation, immune response and metabolism.

According to the new findings, the interbreeding event occurred more recently than some previous estimates suggested, which in turn changes and narrows the possible time range in which humans spread to places like China and present-day Australia.

It also clarifies the importance of fossilized human remains discovered outside of Africa, such as in Europe, dating back more than 50,000 years. New studies indicate that these populations died out and became evolutionary dead ends.

“The history of humanity is not just a story of success. In fact, we went extinct several times,” said Johannes Krause, author of the Nature article and professor at the Max Planck Institute for Evolutionary Anthropology in Germany. “We have identified several lineages now that did not contribute to later people.”

The results also demonstrate how proficient anthropologists have become at reconstructing ancient DNA and analyzing it to draw conclusions about the course of human history.

“It’s just really cool that we can look at these past events and really reconstruct what our paths look like,” said the scientific paper’s author, Priya Moorjani, assistant professor of molecular and cellular biology at the University of California. Berkeley. “Fifty thousand years ago is a long time ago, but being able to have genetic data from these samples really helps us paint a more and more detailed picture.”

The two research groups took different approaches to their work.

Moorjani’s group built a catalog of genomic information from 59 ancient individuals – who lived between 2,000 and 45,000 years ago – and 275 modern-day people. Next, the researchers analyzed changes over time in the distribution and length of Neanderthal DNA in these genomes.

They determined that Neanderthal gene flow into humans occurred around 47,000 years ago and lasted no more than 7,000 years. These findings are consistent with archaeological evidence that suggests Neanderthals and humans overlapped geographically as humans left Africa. Many scientists suspect that the two species interbred in the Middle East, but this is unconfirmed.

After interbreeding, natural selection retained some Neanderthal traits while abandoning many others.

“The majority of selection, positive and negative, on Neanderthal ancestry occurred very quickly after gene flow, over about 100 generations,” said Leonardo Iasi, co-author of the scientific paper, a postdoctoral researcher at the department of evolutionary genetics at New York University. Max Planck Institute for Evolutionary Anthropology.

The other new study, in Nature, was based on an analysis of six genomes from remains discovered at Ranis, a cave site in present-day Germany that lies beneath a medieval castle. The remains date back around 45,000 years and DNA suggests that two of the six individuals were a mother and daughter.

“They now constitute the oldest nuclear genomes we have in modern humans,” Krause said.

The researchers also analyzed DNA from an individual found in a cave in what is now Czechia, about 220 km from Ranis. Both sites date from around the same time. The results indicated that two of the individuals found at Ranis were closely related to the one at Czechia Cave – five or six degrees of family relationship. They concluded that the individuals found at both sites were most likely part of a small, isolated population. of perhaps only 200. Genetically speaking, they did not survive – the population became extinct.

“They present a genetic lineage that has no descendants — that actually later became extinct,” Krause said.

However, the DNA of these individuals shares the same traces of Neanderthal influence as the remains analyzed by the other team of researchers. This reinforces the idea of ​​a single event of “mixing” – or crossbreeding.

“It’s always nice to have two independent studies that work with independent data using independent methods and come up with essentially the same answer. That breeds a lot of confidence,” said Joshua Akey, a professor at the Lewis-Sigler Institute for Integrative Genomics at Princeton University, who was not part of either research group.

Chris Stringer, professor and head of human evolution research at the Natural History Museum in London, said identifying the interbreeding event helps align other key elements of the evolutionary timeline human. The findings limit “the timing of the arrival of populations in areas such as China and Australasia (Australia, New Zealand and Papua New Guinea) that gave rise to current populations in these areas to less than 50,000 years, because their genomes share the same genome type. same interbreeding event,” he said.

The studies also clarify the period when humans interbred with Denisovans, another extinct species, Stringer added – which took place after the introduction of Neanderthal DNA.

Akey said questions remain, however. It’s not clear how often humans and Neanderthals mated. And there’s even more to learn about the traits humans obtained from Neanderthal and Denisovan DNA. Plus, there is the mysterious disappearance of the Neanderthals 39,000 years ago.

Akey said he believes mating between humans and Neanderthals could have led to the latter’s extinction.

“I tend to think that mating was quite common,” Akey said. “And that there was enough mating that it contributed to the disappearance of Neanderthals by incorporating them into human populations. But this remains speculative.