Modern humans migrated to Eurasia 75,000 years ago, where they encountered and interbred with Neanderthals. A new study published in the journal Current Biology shows that at this time Neanderthals were already carrying human DNA from a much older encounter with modern humans. A new study shows that an ancient lineage of modern humans migrated to Eurasia over 250,000 years ago where they interbred with Neanderthals. Over time, these humans died out, leaving a population with predominantly Neanderthal ancestry ( https://www.cell.com/current-biology/fulltext/S0960-9822(23)01315-5 )
“We found this reflection of ancient interbreeding where genes flowed from ancient modern humans into Neanderthals,” says Alexander Platt, a senior research scientist in the Perelman School of Medicine and one of the study’s first authors. “This group of individuals left Africa between 250,000 and 270,000 years ago. They were sort of the cousins to all humans alive today, and they were much more like us than Neanderthals.”
Because most Neanderthal-human interbreeding is thought to have occurred in Eurasia, not in Africa, Neanderthal ancestry is expected to be limited in sub-Saharan Africa; however, a recent study made the puzzling observation that several sub-Saharan populations contain chunks of DNA that resemble Neanderthal DNA. The study was unable to determine how this Neanderthal-like DNA entered these populations, whether it originated from modern humans who had migrated from Africa, interbred with Neanderthals in Eurasia, and then returned, or whether it was the result of an earlier encounter between Neanderthals and humans. Because the study relied on a limited number of genomes from the 1,000 Genomes Project, all of which share a relatively recent common ancestry in Central and Western Africa, it was also unclear whether Neanderthal-like DNA is widespread among sub-Saharan populations.
To better understand how widespread these Neanderthal-like DNA regions are across sub-Saharan Africa and to elucidate their origins, Tishkoff’s team leveraged a genetically diverse set of genomes of 180 individuals from 12 different populations in Cameroon, Botswana, Tanzania, and Ethiopia. For each genome, the researchers identified regions of Neanderthal-like DNA and looked for evidence of Neanderthal ancestry.
Then, they compared the modern human genomes to a genome belonging to a Neanderthal who lived approximately 120,000 years ago. For this comparison, the team developed a novel statistical method that allowed them to determine the origins of the Neanderthal-like DNA in these modern sub-Saharan populations, whether they were regions that Neanderthals inherited from modern humans or regions that modern humans inherited from Neanderthals and then brought back to Africa.
They found that all of the sub-Saharan populations contained Neanderthal-like DNA, indicating that this phenomenon is widespread. In most cases, this Neanderthal-like DNA originated from an ancient lineage of modern humans that passed their DNA on to Neanderthals when they migrated from Africa to Eurasia around 250,000 years ago. As a result of this modern human-Neanderthal interbreeding, approximately 6% of the Neanderthal genome was inherited from modern humans.
In some specific sub-Saharan populations, the researchers also found evidence of Neanderthal ancestry that was introduced to these populations when humans bearing Neanderthal genes migrated back into Africa. Neanderthal ancestry in these sub-Saharan populations ranged from 0 to 1.5%, and the highest levels were observed in the Amhara from Ethiopia and Fulani from Cameroon.
To try to understand whether carrying modern human DNA was helpful or harmful when introduced into the Neanderthal genome, the researchers also investigated where these chunks of modern human DNA were located. To try to understand whether carrying modern human DNA was helpful or harmful, the researchers also investigated where these chunks of modern human DNA were located within the Neanderthal genome. They found that most of the modern human DNA was in noncoding regions of the Neanderthal genome, indicating that modern human gene variants were being preferentially lost from coding sections of the genome, which suggests that having modern human genes in a Neanderthal background is detrimental to fitness.
AR #81
The Fate of the Watchers
by Andrew Collins
