Conny Waters – AncientPages.com – A recent study has uncovered that modern humans descended from at least two ancestral populations, which initially diverged and later reconnected before spreading globally.
Researchers from the University of Cambridge utilized advanced complete genome sequence analysis to discover evidence of a genetic mixing event between these ancient groups. This divergence occurred approximately 1.5 million years ago, and around 300,000 years ago, the groups reunited.
One group contributed about 80% to the genetic composition of modern humans, while the other contributed roughly 20%. For the past twenty years, it was widely believed in human evolutionary genetics that Homo sapiens emerged in Africa between 200,000 and 300,000 years ago from a single lineage. However, these new findings published in Nature Genetics indicate a more intricate evolutionary history.
“The question of where we come from is one that has fascinated humans for centuries,” said first author Dr Trevor Cousins from Cambridge’s Department of Genetics. “For a long time, it’s been assumed that we evolved from a single continuous ancestral lineage, but the exact details of our origins are uncertain.”
“Our research shows clear signs that our evolutionary origins are more complex, involving different groups that developed separately for more than a million years, then came back to form the modern human species,” said co-author Professor Richard Durbin, also from the Department of Genetics.
Earlier research has established that Neanderthals and Denisovans, two extinct relatives of modern humans, interbred with Homo sapiens approximately 50,000 years ago. However, new findings indicate that a significant genetic exchange occurred much earlier—around 300,000 years ago. This ancient mixing event contributed up to ten times more genetic material than the Neanderthal DNA found in non-African modern humans today.
The researchers employed a novel approach by analyzing modern human DNA instead of extracting it from ancient remains. This method allowed them to detect ancestral populations that might not have left any physical evidence. The study utilized data from the 1000 Genomes Project, which includes DNA sequences from diverse populations across Africa, Asia, Europe, and the Americas.
To facilitate their analysis, the team developed a computational algorithm named cobraa. This tool models how ancient human populations diverged and later reconnected. After validating cobraa with simulated data, they applied it to real genetic information from the 1000 Genomes Project.
In addition to identifying these two ancestral groups, the researchers uncovered notable changes that occurred after these populations initially separated.
“Immediately after the two ancestral populations split, we see a severe bottleneck in one of them—suggesting it shrank to a very small size before slowly growing over a period of one million years,” said co-author Professor Aylwyn Scally, also from the Department of Genetics. “This population would later contribute about 80% of the genetic material of modern humans, and also seems to have been the ancestral population from which Neanderthals and Denisovans diverged.”
The study revealed that genes inherited from the second population were frequently positioned away from areas of the genome associated with gene functions. This observation suggests a potential incompatibility with the predominant genetic background. Such findings point to a process called purifying selection, where natural selection gradually eliminates harmful mutations, thereby maintaining genetic integrity over time.
“However, some of the genes from the population which contributed a minority of our genetic material, particularly those related to brain function and neural processing, may have played a crucial role in human evolution,” said Cousins.
Beyond human ancestry, the researchers say their method could help to transform how scientists study the evolution of other species. In addition to their analysis of human evolutionary history, they applied the cobraa model to genetic data from bats, dolphins, chimpanzees, and gorillas, finding evidence of ancestral population structure in some but not all of these.
“What’s becoming clear is that the idea of species evolving in clean, distinct lineages is too simplistic,” said Cousins. “Interbreeding and genetic exchange have likely played a major role in the emergence of new species repeatedly across the animal kingdom.”
Who were our mysterious human ancestors? Fossil evidence points to species like Homo erectus and Homo heidelbergensis, who inhabited both Africa and other regions during this era, as potential candidates for these ancestral populations. However, further research and additional evidence are necessary to determine which genetic ancestors align with specific fossil groups.
Moving forward, the team aims to refine their model to incorporate more gradual genetic exchanges between populations instead of distinct splits and reunions. They also intend to investigate how their findings connect with other anthropological discoveries, such as fossil evidence from Africa indicating that early humans might have been much more diverse than previously believed.
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“The fact that we can reconstruct events from hundreds of thousands or millions of years ago just by looking at DNA today is astonishing,” said Scally. “And it tells us that our history is far richer and more complex than we imagined.”
The study was published in the journal Nature Genetics
Written by Conny Waters – AncientPages.com Staff Writer
