Shock Discovery - Humans First Interbred with Neanderthals 250,000 Years Ago

A groundbreaking study has shattered the conventional timeline of human-Neanderthal interactions. The prevailing belief was that Homo sapiens ancestors first mingled with Neanderthals in Eurasia after a massive exodus from Africa about 75,000 years ago. However, the startling revelations from this research expose a far more ancient narrative, one that traces back over 250,000 years, suggesting that early human DNA flowed into Neanderthals long before our anticipated migrations.

Early Species Interbreeding

A team of geneticists led by researchers from the University of Pennsylvania’s Perelman School of Medicine has revealed that interbreeding between Neanderthals and modern humans in Eurasia started much earlier than had previously been believed.

The conventional view is that interbreeding only occurred sometime after humans migrated to Eurasia from Africa in large numbers beginning about 75,000 years ago. But this new study found that Neanderthals were already carrying sections of human DNA in their genomes by this time, from encounters with their human cousins that likely took place more than 250,000 years ago.

As the geneticists explain in an article appearing in the journal Current Biology, these interactions must have taken place in Eurasia, since there is no record of Neanderthals ever having lived in sub-Saharan Africa, which is where the human DNA detected in the Neanderthal genome came from.

“We found this reflection of ancient interbreeding where genes flowed from ancient modern humans into Neanderthals,” study co-author Alexander Platt, a researcher at the Perelman School of Medicine, said in a University of Pennsylvania press release.

“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.”

Before this study, Neanderthal DNA was assumed to have remained in a pristine state until humans arrived in Eurasia during their most recent mass migration. But the new research found that about six percent of what had supposedly been pure DNA in the Neanderthal genome had been inherited from early modern humans. Because it was absorbed by Neanderthals approximately 250,000 years ago, its actual origin had been obscured—up until now.

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There is no way to tell how many of our Homo sapiens ancestors completed this epic trek to the north and west.  But it was obviously a large enough group that they were able to leave a clear sign of their presence in the DNA of their Neanderthals cousins. These humans ultimately did not survive, however, which is why no human skeletal remains from a quarter of a million years ago have ever been found in Europe or Asia.

Tracing the Complex History of Neanderthal and Modern Human Interactions

The team of genetic researchers, which included scientists from multiple African universities, reached their stunning conclusion following their analysis of the fully sequenced genomes of modern indigenous people living in sub-Saharan Africa.

Another recent study looking at these same groups found traces of what was presumed to be Neanderthal DNA in several different individuals, which raised the question of how exactly it got there. The earlier study was conducted under the sponsorship of the ongoing 1,000 Genomes Project, which has been cataloging genetic diversity across the planet for the past 15 years.

Intrigued by this anomalous finding, the genetic researchers involved in the new study obtained samples from the genomes of 180 people from 12 different populations living in Ethiopia, Tanzania, Botswana and Cameroon, which are all located in sub-Saharan Africa. This was a far more comprehensive sampling that what was used by the 1,000 Genomes Project researchers, and it was necessary to collect this much data to determine just how common the alleged Neanderthal DNA was in this part of the world.

DNA was sampled from 12 populations of sub-Saharan Africa. (Steve Evans from Citizen of the World/CC BY 2.0)

When the 1,000 Genomes Project researchers discovered the link between the Neanderthal genome and that of sub-Saharan Africans, the first assumption was that the Neanderthal-like genes had reached the ancestors of these humans directly, through interbreeding. This would have meant that either Neanderthals and modern humans interbred in Africa long ago, or that ancestors of the modern humans who’d migrated to Eurasia 75,000 years ago had returned to Africa at some point and passed genes they’d inherited from Neanderthals back into the indigenous population.

While either of these scenarios would have explained the findings, the team of African and American researchers involved in the new study considered a different possibility. They wondered if perhaps the Neanderthal-like DNA found in sub-Saharan Africans has actually originated in modern humans long ago, only to be passed on to Neanderthals later on.  

They quickly found evidence in favor of this theory when they finished their study of modern sub-Saharan African indigenous DNA. The data they obtained showed that each group they studied was carrying this Neanderthal-like DNA, which strongly suggested this genetic material had originated in the region and had not been introduced from elsewhere.

Evidence that confirmed this suggestion emerged from the researchers’ analysis of the DNA of a Neanderthal specimen that lived approximately 120,000 years ago. They found that this individual possessed genetic material that matched what had been found in the sub-Saharan Africans, and that this material was contained in sections of the genome known to have been inherited from Homo sapiens. This was significant, because it showed that human DNA had been transferred to the Neanderthal genome well before 75,000 years ago.

Through a more refined analysis of the Neanderthal DNA, the researchers eventually discovered that about six percent of this Neanderthal’s total genetic inheritance had come from ancient modern humans who’d arrived in Eurasia at least 250,000 years ago. These humans had obviously come from sub-Saharan Africa, since the genes they passed on to Neanderthals remain ubiquitous among the indigenous people of the region still today.

“This study highlights the importance of including ethnically and geographically diverse populations in human genetics and genomic studies,” said University of Pennsylvania Professor of Genetics and Biology Sarah Tishkoff, who served as team leader on this breakthrough research project.

Interestingly, the researchers did find proof of authentic Neanderthal DNA in some sub-Saharan indigenous populations. This would have been inherited from human ancestors who’d traveled from Eurasia to Africa carrying Neanderthal genetic materials with them, reversing the path of their ancestors who’d traveled in the opposite direction 250,000 and 75,000 years ago.

The Neanderthal Lives on Inside Us—But Not for Much Longer

While Homo sapiens and their Neanderthal cousins may have been passing their DNA back and forth for approximately 200,000 years, it seems this cross-species genetic mixing was not so useful. In both species the genetic material inherited from the other was ultimately confined to noncoding sections of the genome, which put it on a pathway that guaranteed it would be eliminated by evolution over time.

“A Neanderthal allele [DNA sequence] might work great in Neanderthals, but you plop it into a modern human genome and it causes problems,” explained Professor Platt.

“Both modern humans and Neanderthals slowly rid themselves of the alleles of the other group,” he continued. “In the almost 500,000 years between the ancestors of Neanderthals splitting off from the ancestors of modern humans and these other modern humans being reintroduced to Neanderthal populations, we had become such different organisms that, although we were still able to interbreed quite readily, the hybrids didn’t work so well, which means we were very far along the path to becoming distinct species.”

Approximately 20 percent of the Neanderthal genetic heritage has been preserved inside the human genome, as a result of past interbreeding. But this is only a temporary situation, and there will come a time when the last remaining traces of Neanderthal DNA will completely disappear from the human gene pool. At this point the extinction of our Neanderthal cousins will be complete, leaving us with nothing but artifacts and skeletal remains to remember them by.

Top image: Neanderthal man, now shown to have DNA from modern human relatives 250,000 years ago. Source: iridescentstreet/Adobe Stock

By Nathan Falde