Two new studies have helped narrow down the time during which Neanderthals interbred with modern humans to a period starting about 50,500 years ago and lasting over seven millennia.
One of the studies, published in the journal Nature, presents the oldest known modern human genome ever sequenced of a small group of about seven early Europeans who had just recently mixed with Neandertals.
Another study, published in Science, assessed several present-day human genomes as well as 58 ancient genomes from modern human bones found around Eurasia.
It sought to determine the timing and duration of Neanderthal genes flowing into that of modern humans and their resulting evolutionary impact.
Archaeological and genetic evidence suggests both modern humans and Neanderthals lived side-by-side in Eurasia for between 6,000 and 7,000 years.
Interbreeding between the two populations left Eurasians with many genes inherited from their Neanderthal ancestors, which today make up between 1 and 2 per cent of our total genome, researchers said.
The new research estimates an average date for Neanderthal-Homo sapiens interbreeding of about 47,000 years ago, compared to previous estimates that ranged from 54,000 to 41,000 years ago.
It suggests the initial migration of modern humans from Africa into Eurasia was likely over around 43,500 years ago – more recent than previous estimates.
“To me it is striking that these are not really like species at the time but are groups that can interbreed with each other and have fertile offspring,” Priya Moorjani, senior author of the Science study, told The Independent.
“That means the differences that we imagine between these groups to be very big were actually very small genetically, and they could mix with each other for a long period and were living side by side.”
This shows Neanderthals and early modern humans “were far more similar than different”, she said.
These new findings present a “more complete picture” of the past and have direct implications on our understanding of the timing of when modern humans migrated out of Africa, according to scientists.
They suggest that the period of mixing between Neanderthals and modern humans was much more complex, extending over about 7,000 years rather than being intermittent.
“Different groups could have separated during the 6,000- to 7,000-year period and some groups may have continued mixing for a longer period” Benjamin Peter from the University of Rochester said.
The Nature study, led by researchers from the Max Planck Institute for Evolutionary Anthropology in Germany, corroborates these findings.
Its analysis of two newly sequenced genomes of Homo sapiens from about 45,000 years ago also found an average date of 47,000 years ago when interbreeding happened.
“The Max Planck group sequenced new ancient DNA samples that allowed them to date the Neanderthal gene flow directly. And they came up with a similar timing as us,” Manjusha Chintalapati, one of the authors of the Science study, said.
Researchers also probed how Neanderthal ancestry changed among the early modern humans over time.
They found some genes “present at high frequency” in the early humans which they think were likely inherited from Neanderthals and offered them beneficial traits.
Most of these “high-frequency” Neanderthal genes are related to immune function, skin pigmentation, and metabolism, scientists say.
Some of these genes from Neanderthals were found to have increased in frequency in Homo sapiens over time, implying that they may have been advantageous to human survival.
“Neanderthals were living outside Africa in harsh, Ice Age climates and were adapted to the climate and to the pathogens in these environments,” Leonardo Iasi, another author of the Science study, said.
“When modern humans left Africa and interbred with Neanderthals, some individuals inherited Neanderthal genes that presumably allowed them to adapt and thrive better in the environment.”
