origin of humans; Neanderthal interbreeding over more time (Origins)

by David Turell , Friday, July 12, 2024, 19:32 (132 days ago) @ David Turell

Latest analysis more than 100 thousand years:

https://www.sciencemagazinedigital.org/sciencemagazine/library/item/12_july_2024/420649...

"A paper this week in Science concludes Neanderthals inherited as much as 10% of their genome from modern humans, including several genes involved in brain development. The Neanderthal-eye view allowed the researchers to date when the two groups mingled, finding they made babies together remarkably early: more than 200,000 years ago, not long after Homo sapiens coalesced as a species. The dalliances were repeated 105,000 to 120,000 years ago, and 45,000 to 60,000 years ago, the ancient Neanderthal DNA suggests. “It argues mating was more common than previously thought,” says Princeton University geneticist Joshua Akey, who led the study.

"This new picture further blurs the boundaries between Neanderthals and modern humans. And it identifies features of the Neanderthal genome suggesting our big-brained, heavy-browed relatives were pitifully rare, which could help explain why they went extinct. “It’s alarming to see how small the Neanderthal populations were—this is a very powerful result,” says paleogeneticist Maanasa Raghavan of the University of Chicago.

"Once seen as a separate species, Neanderthals have enjoyed a complete makeover in the 14 years since researchers first sequenced DNA in their fossils and found they interbred with modern humans. Most living people outside of Africa have inherited about 1% to 2% of their DNA from Neanderthals, perhaps from a prolonged period of mixing 45,000 to 60,000 years ago in Europe or the Middle East.

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"By analyzing the length and other features of the diverse segments of modern human DNA, Akey and Li could calculate when and how often these ancient hookups happened. The smaller the stretches of DNA, the earlier Neanderthals got them, because inherited segments get shorter over generations.

"The first mating episode the team spotted was very ancient—200,000 to 250,000 years ago, around the time anatomically modern humans first show up in the fossil record in Africa. The team speculates that perhaps some early modern humans crossed the Sahara Desert when the climate was humid, on the trail of antelope, ostrich, and other game, and eventually wandered to the Middle East, where they met the Neanderthals.

"A few tantalizing fossils—a purported modern human skull from Greece and a jaw from Israel—support the idea that modern humans did leave Africa that early. Their lineages died out—but not before they left their mark on the Neanderthals’ genome.

"Akey’s team dated another round of Neanderthal-modern mating to about 105,000 to 120,000 years ago. Researchers speculate those encounters, too, could have happened in the Middle East, because modern humans and Neanderthals are known to have lived in nearby caves at that time. One team even thinks they have found hybrid offspring: The remains of a strange-looking Neanderthal with modern tools dating to 120,00 to 130,000 years ago from the Nesher Ramla quarry in central Israel.

"The third bout of mixing is the familiar one, about 50,000 to 60,000 years ago, likely in the Middle East or Europe, where Neanderthals and modern humans overlapped for thousands of years."

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"If some of the diverse DNA in Neanderthals came from modern humans, then our cousins had even less genetic diversity than previously thought—and therefore even smaller populations. The study implies Neanderthal numbers dwindled between 250,000 years and 40,000 years ago, and that by the end of their time on the planet, their breeding population was less than 3000, compared with at least 10,000 breeding modern humans, Akey says. Stringer says Neanderthal numbers may have been so small because they mostly lived in northern regions where they were vulnerable to climate change and glaciation.

"People with Neanderthal genes didn’t abruptly vanish, Akey says—their offspring just acquired more and more modern human DNA. “They were overwhelmed by waves of modern humans extending out of Africa,” he says. “The modern human population eventually absorbed the Neanderthals.”

"Even before then, the findings suggest, our ancestors and Neanderthals had more in common than we ever knew. “I think this paper closes the loop in terms of us thinking about ‘us’ versus ‘them,’” Raghavan says."

Comment: By studying Neanderthal DNA for the presence of human DNA the picture of interbreeding becomes clearer and more time expansive.