GabyIn evolutionary genetics over the past two decades, the prevailing view has been that Homo sapiens first emerged in Africa about 200,000 to 300,000 years ago and descended from a single ancestral lineage.
However, a new study from the University of Cambridge proposes that modern humans are the product of two populations: Homo heidelbergensis and Homo erectus. Those populations diverged about 1.5 million years ago and mixed roughly 300,000 years ago in about an 80:20 ratio.
“The question of where we come from has intrigued science for centuries. For a long time it was believed we descended from a single continuous ancestral line, but the exact details were unknown,” said Dr. Trevor Kazins, co-author of the study.
“A new study shows our evolutionary origins are more complex, involving groups that evolved separately for over a million years and then came back together to form the modern human species,” added Professor Richard Durbin of the university.
How did the scientists come to that conclusion?
Previous studies have shown that Neanderthals and Denisovans interbred with Homo sapiens about 50,000 years ago. However, the authors of the new study suggest that long before those interactions — around 300,000 years ago — there was a much larger genetic mixing event.
Neanderthal DNA makes up about 2 percent of the genomes of non-African modern humans. By contrast, this earlier mixing event contributed roughly ten times more genetic material, and its signal appears in all modern humans.
The team’s method didn’t rely on extracting DNA from ancient bones; instead, they analyzed modern human genomes. From that, the scientists concluded there were ancestral populations that might not have left physical traces.
The authors developed a computational algorithm called cobraa that models how ancient human populations split and later rejoined, writes Sci.news.
Researchers tested the algorithm using simulated data and applied it to real genetic information from the ‘1000 Genomes’ project, a global initiative that sequenced DNA from populations in Africa, Asia, Europe, and the Americas.
Thus, the team managed to identify two key ancestral populations. However, it also discovered some striking changes that occurred after those two populations initially diverged.
“Immediately after the two ancestral populations split, we see a severe bottleneck in one of them,” said Professor Elwyn Scully of Cambridge University. “That suggests it shrank to very small sizes and then slowly expanded over a million years. This population later contributed about 80 percent of the genetic material in modern humans and was likely the ancestral population from which Neanderthals and Denisovans descended.”
Scientists also found that genes inherited from the second population were often located far from regions of the genome associated with known gene functions. That pattern suggests those genes may have been less compatible with the majority genetic background and were gradually removed by purifying selection, the process that eliminates harmful mutations over time.
As the team noted, fossil material indicates that species such as Homo erectus and Homo heidelbergensis lived in Africa and in other regions during that period. This potentially makes them the ancestral populations the study identified. However, further research is needed to confirm the study’s results.
“The fact that we can now reconstruct events that occurred hundreds of thousands or millions of years ago simply by studying DNA is astonishing. It shows that our history is much richer and more complex than we had imagined,” said Professor Scully.
The results of the study were published in the journal Nature Genetics.
