Two teams using different methods both conclude that humans and Neanderthals had children together starting roughly 50,000 years ago

Neanderthals and humans interbred more recently than scientists thought

Two teams using different methods both conclude that humans and Neanderthals had children together starting roughly 50,000 years ago

The oldest human genomes ever sequenced are helping to illuminate some extremely ancient baby-making.

The Neanderthal DNA found in all people with ancestors outside Africa entered the family tree much more recently than previously thought, according to two analyses that together examine DNA from people who lived across Eurasia over the past 45,000 years. One study¹ finds that modern humans swapped genes with our sister species in a roughly 7,000-year period starting around 50,500 years ago; the other² finds that the mixing took place between 45,000 and 49,000 years ago.

The data also suggest that some genetic variants from Neanderthals were helpful to modern humans encountering new climates and diseases outside of Africa. The findings are published today by separate teams, in papers in Science¹ and Nature².

That both papers reinforce the idea that the ancestors of all people outside Africa got their Neanderthal ancestry in a single swoop is “striking”, says Alexander Platt, a geneticist at the University of Pennsylvania in Philadelphia, who was not involved with either study. “It’s eye-opening”, he says, that this model of human evolution is correct.

Dating details

Neanderthals and modern humans shared the planet for thousands of years. Whether our two species mingled was hotly debated for decades — until research revealed that Neanderthal DNA makes up a small percentage of the genomes of all humans currently living, other than people whose ancestry comes solely from sub-Saharan Africa.

Since then, the details of the human–Neanderthal meet cute have remained “one of the main questions” in human evolutionary biology, says Benjamin Peter, a population geneticist at the Max Plank Institute for Evolutionary Anthropology in Leipzig, Germany, and a co-author of the Science study.

Neanderthals, who lived in what’s now Europe and western Asia, and modern humans, who evolved in Africa, probably met and mingled throughout our shared time on Earth. Previous research estimated that any gene flow between them did not leave a mark until more recently ― as early as 65,000 years ago.

The authors of the Nature paper reassessed the timing of that interbreeding by examining the DNA of a male Homo sapiens found near Ranis, Germany, and that of a female Homo sapiens whose remains were discovered in a cave at a site called Zlatý kůň in the Czech Republic. The researchers’ analysis showed that both people lived roughly 45,000 years ago, making them the oldest Homo sapiens genomes ever sequenced.

Relatively recent mixing

The team then sought to understand the history of the Neanderthal DNA embedded in these two individuals’ genomes. The researchers found that it came from a single wave of Homo sapiens and Neanderthal baby-making between 45,000 and 49,000 years ago, almost certainly in the Middle East, the scientists said at a press briefing. The authors found no evidence that the ancestors of the Zlatý kůň woman, the Ranis man and other people found at Ranis intermingled with Neanderthals in Europe after that event.

The date range is much later than previous estimates, says Pontus Skoglund, an evolutionary geneticist at the Francis Crick Institute in London. But it wasn’t the last surprise. DNA showed that the Zlatý kůň woman and the Ranis people were all part of the same extended family, even though their remains were found 230 kilometres apart.

These people don’t seem to have any descendants who are alive today — but they are the kin of many humans. Every living person with non-African heritage has inherited similar genetic variants from Neanderthals. This tells researchers that the ancestors of all people with heritage outside sub-Saharan Africa met Neanderthals shortly after leaving the continent, and mixed with them before spreading around the planet. The people at Ranis and Zlatý kůň carried the same stretches of Neanderthal DNA as many modern humans— making them a long-lost branch of a shared family tree.

Neanderthal bonus

The authors of the Science paper took another approach: they analysed existing genomic data from 275 contemporary humans and from 59 Homo sapiens who lived between 2,200 and 45,000 years ago. By comparing the sequences, the researchers inferred that Neanderthal contributions to the human genome started around 50,500 years ago and lasted for another 6,800 years. That start time is very close to the one described in the Nature paper, even though the two teams used different methods.

The authors of the Science paper also examined stretches of modern-human genomes that are completely lacking in Neanderthal DNA, indicating that either genetic drift — which results in the disappearance of random areas of a genome — or natural selection deleted the Neanderthals’ genetic contribution. Other areas of many modern-human genomes are rich in Neanderthal ancestry, including sections with genes relating to skin colour and immunity.

Neanderthals had hundreds of thousands of years to adapt to the world beyond Africa. Some of their genetic variants might have proved useful to modern humans moving out of the continent — a point reinforced by the fact that selection for some segments of Neanderthal DNA seems to have happened in the first 100 generations after the two species intermixed.

This level of detail shows how “this time period was really complex” for early humans, says Priya Moorjani, a population geneticist at the University of California, Berkeley, and a co-author of the Science paper. People were meeting, breeding and moving to new places. And it hints that — at least in the eyes of some of our ancestors — Neanderthals “weren’t that different from us”, she says.

doi: https://doi.org/10.1038/d41586-024-04065-y

This story originally appeared on: Nature - Author:Freda Kreier