According to a study conducted by researchers at the University of Cambridge, modern humans descended not from a single lineage but from two distinct populations of ancestors that first diverged and then reunited. Published in Nature Genetics, the research offers a more complex theory of human evolution than the commonly accepted one, based on extensive genomic analysis.
"The question of where we come from is one that has fascinated humans for centuries," said Trevor Cousins, paper author and evolutionary biologist at the University of Cambridge, according to Newsweek.
For decades, the prevailing opinion in the field of evolutionary genetics has been that Homo sapiens emerged from Africa between 200,000 and 300,000 years ago from a single lineage. Cousins said, "For a long time, it has been assumed that we evolved from a single continuous ancestral lineage, but the exact details of our origins are uncertain."
According to the study, modern humans are the result of genetic mixing between two ancient populations that diverged approximately 1.5 million years ago, with one group contributing 80% and the other 20% to the genetic makeup of modern humans. These two ancestral populations drifted apart and later reconnected long before modern humans spread across the globe. Approximately 300,000 years ago, these two populations regrouped.
"Our research shows clear signs that our evolutionary origins are more complex, involving different groups that developed separately for over a million years and then came back together to form the modern human species," said Richard Durbin, co-author and computational biologist at the University of Cambridge.
Instead of extracting genetic material from ancient bones, the research team relied on the analysis of modern human DNA from the 1000 Genomes Project, a global initiative that contains DNA sequences from populations in Africa, Asia, Europe, and America. They developed a computational algorithm called cobraa that models how ancient human populations separated and later merged. They tested the cobraa algorithm with simulated data and applied it to real human genetic data from the 1000 Genomes Project. This approach allowed the team to infer the presence of ancestral populations that would have otherwise left no physical traces.
The study not only found evidence of these two ancestral populations but also detected surprising changes that occurred after the two populations initially separated, including changes in one of the two ancestral populations. "Immediately after the two ancestral populations separated, we observed a severe bottleneck in one of them, suggesting it was reduced to a very small size before slowly growing over a period of a million years," Cousins pointed out.
"This population would later contribute around 80% of the genetic material of modern humans, and it also seems to have been the ancestral population from which Neanderthals and Denisovans diverged," said Aylwyn Scally, co-author of the paper and computational genomicist at the University of Cambridge.
Unlike Neanderthal DNA, which accounts for about 2% of the genome of modern non-African humans, this ancient mixing contributed up to ten times that amount and is found in all modern humans. "Some of the genes from the population that contributed a minority of our genetic material, particularly those related to brain function and neuronal processing, may have played a crucial role in human evolution," Cousins said, according to DW.
The researchers discovered that genes inherited from the second population were often located far from regions of the genome linked to gene functions, indicating a process known as purifying selection, where natural selection eliminates harmful mutations over time. This suggests that the inherited genes from the second population may have been less compatible with the majority genetic background.
Fossil evidence suggests that species like Homo erectus and Homo heidelbergensis lived in both Africa and other regions during this period, making them possible candidates for these ancestral populations. Carles Lalueza-Fox, a paleogeneticist at the Institute of Evolutionary Biology in Barcelona, stated that the study by Durbin contains "interesting results because they go against the widespread thinking that our species has a diffuse, pan-African origin,". "A derivative of the study would be to look with new eyes at the African fossil record of the last million years to see if it is possible to identify these two ancestral populations or species," he added, according to ABC Color.
Alongside studying humans, the researchers also used cobraa to explore the genetic history of other species, including bats, chimpanzees, dolphins, and gorillas. They found evidence of ancestral population structures in some (but not all) of these groups. The research team argues that their method could help transform how scientists study the evolution of other species. With their initial study complete, the researchers hope to refine their cobraa model to enable it to reveal more gradual genetic exchanges between different populations.
"The fact that we can reconstruct events from hundreds of thousands or millions of years ago, just by looking at DNA today, is astonishing—and it tells us that our history is far richer and more complex than we imagined," Scally concluded.
The article was written with the assistance of a news analysis system.
