A discovery in 2010 that early humans and Neanderthals interbred It was a scientific bombshell—the revelation of a genetic legacy that has since played a major role in modern human life, influencing biological rhythms, immune system function, and even the way some people feel pain.
Scientists, however, have found that it is surprisingly difficult to piece together gene flow in the opposite direction: how mixing between the two groups may have influenced Neanderthals which went extinct about 40,000 years ago. With the help of new techniques, a new study is shedding light on this question.
The analysis, published on July 12 in the journal Scienceshowed that the two groups exchanged DNA at several points over the past 250,000 years, shedding light on how Neanderthals disappeared and potentially rewriting the history of how and when our Homo sapiens ancestors left Africa.
“To date, most genetic data suggest that modern humans evolved in Africa 250,000 years ago, remained there for the next 200,000 years, and then decided to disperse out of Africa 50,000 years ago and settle 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 doesn’t leave ancestry for current populations. What I think is kind of cool about this [artigo] is that it provides genetic insights into these out-of-Africa dispersals that we couldn’t see before,” Akey said.
The findings suggest that early human history was complex, and modern humans probably interbred with Neanderthals — and with other types of archaic humans, including the enigmatic Denisovans — much more frequently than previously known since our emergence as a species, around 250,000 to 300,000 years ago.
Several 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 time elapsed between when two groups exchanged DNA — like the ticking of a molecular clock.
The study found that humans left Africa, encountered 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 about 50,000 or 60,000 years ago.
The most recent episode is widely recognized and was first identified in 2010, when the first Neanderthal genome was sequenced by Nobel Prize-winning geneticist Svante Pääbo. However, the new research has shown that the first two waves differed significantly from the third — a sweeping migration that eventually led to modern humans taking up residence in every corner of the globe.
Scientists have found that the percentage of Homo sapiens DNA in the Neanderthal genome may have reached 10% more than 200,000 years ago and has been decreasing over time; on average, it was 2.5% to 3.7%.
A similar study published last year identified genetic traces of an encounter between the two groups about 250,000 years ago, but the contribution of Homo sapiens DNA to Neanderthals about 100,000 years ago is a new discovery, said Laurits Skov, a geneticist and postdoctoral researcher at the University of California, Berkeley, who was not involved in the study.
“What seems certain, however, is that human and Neanderthal history are much more intertwined than we previously thought,” he said in an email.
Genetic detective
During the previous two waves of interbreeding, the Neanderthal population absorbed human genes and the descendants remained within Neanderthal groups, according to the new study.
These early mating episodes, the result of small groups of pioneering Homo sapiens migrating — but not establishing a solid foothold — out of Africa, left little trace in the genetics of modern human populations but had a major impact on the Neanderthal genome, Akey explained.
“I think the simplest explanation is that it reflects changes in population size over time,” he added.
“Early modern humans were just moving out of Africa, and Neanderthal populations were large enough that they could essentially absorb these early dispersals of humans and their genes into the Neanderthal population,” Akey explained.
However, when Homo sapiens left Africa about 60,000 years ago in a long-lasting migration around the world, the descendants resulting from the encounters between Homo sapiens and Neanderthals grew up within modern human populations and their genetic signature remained in human genes, influencing our lives to this day, he added.
In the study, the team used machine learning techniques to decode and sequence genomes from the remains of three Neanderthals, which dated back to 50,000 to 80,000 years ago and were found in three different locations: Vindija, Croatia, and the Denisova and Chagyrskaya caves in the Altai Mountains.
The researchers then compared this data with the genomes of 2,000 modern-day humans.
“We developed a framework to determine whether gene flow from humans to Neanderthals occurred, estimate how much modern human sequence there is in Neanderthal genomes, and identify the specific places in the Neanderthal genome that carry…modern human sequences,” Akey said.
Mystery of the disappearance of the Neanderthals
There are a handful of Homo sapiens fossils that may reflect the species’ early, less successful journeys from Africa to the Middle East and Europe, said Chris Stringer, research leader in human evolution at the Natural History Museum in London, who was not involved in the study.
These relics include a Homo sapiens fossil found in the Apidima Cave in southern Greece that dates back 210,000 years, and remains found at the Israeli sites of Skhūl and Qafzeh. The Homo sapiens fossils found in Israel had “primitive traits,” such as larger brow ridges, flatter skulls, and variable chins.
“I interpreted these features as being retained from more primitive non-Neanderthal ancestors, but they could be signs of gene flow from Neanderthals, and perhaps such features should be examined again now in light of this new work,” Stringer said.
A population dynamics identified in this research may be one of the main reasons why Neanderthals disappeared 40,000 years ago Akey noted. The researchers’ analysis suggests that the Neanderthal population size at the time was 20 percent smaller than previously thought.
“Human populations were larger and, like waves crashing on a beach, they eventually eroded the Neanderthals,” with Neanderthal genetics likely absorbed into the human population in the last wave of interbreeding, Akey said.
“Extinction is complicated, so I think I would hesitate to say that’s the only explanation… but I think the absorption of Neanderthals into human populations probably explains a significant portion of why Neanderthals disappeared,” he added.
Stringer said he agreed that the last phase of interbreeding may have contributed to the extinction of the Neanderthals, with the Neanderthal population becoming even smaller and less diverse as Neanderthal DNA entered the human gene pool.
“I think this is an important point,” Stringer said. “Factoring in the increase in Neanderthal genetic diversity from interbreeding with sapiens also significantly reduces their effective population size, adding further evidence that the last Neanderthals may have already been an endangered species even without competition from an expanding Homo sapiens population.”
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Source: CNN Brasil
Charles Grill is a tech-savvy writer with over 3 years of experience in the field. He writes on a variety of technology-related topics and has a strong focus on the latest advancements in the industry. He is connected with several online news websites and is currently contributing to a technology-focused platform.
