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The 2010 discovery that early humans and Neanderthals had once interbred was a scientific bombshell – revealing a since-acquired genetic inheritance that played a role in modern humans, affecting circadian rhythms, immune system function and the way some feel pain.
Surprisingly, however, it has been difficult for scientists to put together a gene flow in the opposite direction: how can combinations between the two groups have shaped Neanderthals, who became extinct around 40,000 years ago. With the help of new techniques, a new study is painting a clearer picture.
The analysis, published on July 12 in the journal Science, showed that the two groups exchanged DNA at multiple points over the past 250,000 years, shedding light on how Neanderthals diverged and could rewrite the story of how and when our ancestors Homo sapiens left Africa.
“So far, most of the genetic data suggests that modern humans arose in Africa 250,000 years ago, remained buried for 200,000 years ago, and then decided to spread out of Africa 50,000 years ago and spread to people in the rest of the world,” said Joshua Akey, a professor at Princeton University’s Lewis-Sigler Institute and senior author of the study.
“But genetics is essentially blind to anything that is not left by an ancestor in today’s populations. What I think (is) amazing about this (paper) is that it provides genetic insights into these dispersals out of Africa that we haven’t been able to see before,” Akey said.
The findings suggest that early human history was very complex, and that modern humans probably interacted with Neanderthals – and other types of ancient humans, including the enigmatic Denisovans – much more often than recognized. previously since we emerged as a species around 250,000 to 300,000 years ago.
Multiple mating episodes
By comparing DNA sequences in databases, scientists can reconstruct relationships between different populations, or species, and because genetic changes occur at a constant rate over a generation, geneticists can calculate the past when two groups of DNA exchange — like ticks. on a molecular clock.
The study found that humans left Africa, turned and interbred with Neanderthals in three waves: One about 200,000 to 250,000 years ago, not long after the first Homo sapiens fossils appeared in Africa; another 100,000 years ago; and the last around 50,000 to 60,000 years ago.
The latest episode is widely recognized and was first identified in 2010 when Nobel Prize-winning geneticist Svante Pääbo sequenced the first Neanderthal genome. However, the new research has shown that there was a significant difference between the first two waves and the third, a massive migration that eventually led to modern humans living in all corners of the world.
The scientists discovered that the percentage of Homo sapiens DNA in the Neanderthal genome could be as high as 10% more than 200,000 years ago and it decreased over time; on average, it was 2.5% to 3.7%.
A similar study published last year identified genetic traces of contact between the two groups around 250,000 years ago but the contribution of Homo sapiens DNA to Neanderthals around 100,000 years ago is a novel finding, said Laurits Skov, a geneticist and postdoctoral researcher at the University. of California Berkeley, who were not involved in the study.
“Human history and Neanderthal history are definitely much more intertwined than we previously thought,” he said via email.
Genetic detective work
During the two earlier waves of interbreeding, the Neanderthal population took on human genes and the offspring remained within Neanderthal groups, according to the new study.
These early mating episodes, the result of small pioneering Homo sapiens groups migrating — but not establishing a strong foothold — out of Africa, left little record in the gene pool of today’s human population but they had a major impact on the Neanderthal genome. , said Akey.
“I think the simplest explanation is that this reflects changes in population sizes over time,” he said.
“First, (early) modern humans were emerging out of Africa, and Neanderthal populations were large enough that they were able to essentially absorb these early dispersals of humans and their genes into the Neanderthal population,” Akey explained.
However, when Homo sapiens left Africa around 60,000 years ago on a permanent worldwide migration, the offspring resulting from Homo sapiens-Neanderthal encounters grew up within new human populations- modern and its genetic signature remained within the human gene pool, influencing our lives today, he added.
In the study, the team used machine learning techniques to decode and sequence genomes from the remains of three Neanderthals, which went from 50,000 to 80,000 years ago and were found in three different locations: Vindija, Croatia, and Denisova and Chagyrskaya caves in the Altai. Mountains. The researchers then compared that data to the genomes of 2,000 people today.
“We developed a framework to determine whether gene flow occurred from human to Neanderthal, to estimate how many modern human sequences are in the Neanderthal genome, and the specific places in the Neanderthal genome that are carrying … modern human sequences recognize modernity,” said Akey.
The mystery of the disappearance of Neanderthals
There are a handful of Homo sapiens fossils that may represent early, less successful journeys of the species out of Africa into the Middle East and Europe, said Chris Stringer, head of research in human evolution at the Museum. Natural History in London, which was not. t participate in the study.
Among these remains is a Homo sapiens fossil found in Apidima Cave in southern Greece 210,000 years ago and still found at the Israeli sites of Skhūl and Qafzeh. The fossils found in Israel had “primitive characteristics”, such as larger eyebrows, a smoother skull and a variable chin.
“I interpreted (the) traits (as) retained from more primitive non-Neanderthal ancestors, but they could be signs of gene flow from Neanderthals, and maybe those traits should be looked at again now, in the light of this new. work,” Stringer said.
The population dynamics identified in this research could be a major reason why Neanderthals disappeared 40,000 years ago, Akey noted. The researchers’ analysis suggests that the Neanderthal population size at the time was 20% smaller than previously thought.
“Human populations were larger, and like waves crashing on the beach, the Neanderthals eventually eroded away,” and the Neanderthal gene pool was likely absorbed into the human population in the last wave of interbreeding, Akey said.
“The extinction is complicated, so I think I would hesitate to say that it’s the only explanation … but I think the absorption of Neanderthals into human populations explains a significant amount of why the Neanderthals disappeared,” he said.
Stringer said he agreed that the final stage of interbreeding may have contributed to the Neanderthal extinction, with the Neanderthal population becoming less and less diverse as Neanderthal DNA entered the larger human gene pool.
“I think that’s an important point,” Stringer said. “The increase in Neanderthal genetic diversity from interbreeding with sapiens also significantly reduces the effective population size, adding further evidence that the late Neanderthals may have already been an endangered species even without competition from a population of Homo sapiens increase.”
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