Genetic Research Uncovers a Lost Chapter of Human Evolution

Using sophisticated genome-wide analyses, researchers at the University of Cambridge have uncovered evidence that modern humans emerged from a genetic mixing event involving two ancient populations that diverged approximately 1.5 million years ago.

These groups reunited around 300,000 years ago, with one population contributing 80% of the genetic makeup of contemporary humans, while the other contributed 20%.

For the past two decades, the prevailing view in human evolutionary genetics has been that Homo sapiens first appeared in Africa around 200,000 to 300,000 years ago, descending from a single lineage. However, new findings published in Nature Genetics suggest a more intricate evolutionary history.

"The question of where we come from has fascinated humans for centuries. For a long time, it was assumed that we evolved from a single continuous ancestral lineage, but the details of our origins remain uncertain."

Dr. Trevor Cousins, Study First Author, Department of Genetics, University of Cambridge

"Our research provides clear evidence that our evolutionary origins are more complex, involving different groups that developed separately for over a million years before merging to form modern humans."

Professor Richard Durbin, Study Co-Author, Department of Genetics, University of Cambridge

While previous studies have established that Neanderthals and Denisovans—two now-extinct human relatives—interbred with Homo sapiens around 50,000 years ago, this new research suggests a much more significant genetic mixing event occurred far earlier, around 300,000 years ago.

Unlike Neanderthal DNA, which comprises roughly 2% of the genome of non-African modern humans, this earlier mixing event contributed up to ten times that amount and is present in all modern human populations.

A New Approach to Understanding Human Evolution

Instead of relying on ancient DNA extraction, the research team analyzed contemporary human genomes to infer the existence of ancestral populations that may not have left physical remains.

Their data came from the 1000 Genomes Project, a global initiative sequencing DNA from populations across Africa, Asia, Europe, and the Americas.

The team developed a computational algorithm called cobraa, which models how ancient human populations split and later merged. They validated the algorithm using simulated data before applying it to real human genetic data from the 1000 Genomes Project.

Key Findings and Evolutionary Implications

While identifying these two ancestral populations, researchers also observed significant genetic shifts following their initial divergence.

"Immediately after these populations split, one experienced a severe bottleneck, shrinking to a very small size before gradually expanding over a million years. This group would later contribute about 80% of the genetic material to modern humans and appears to have been the ancestral population from which Neanderthals and Denisovans emerged."

Aylwyn Scally, Study Co-Author and Professor, Department of Genetics, University of Cambridge

The study also found that genes inherited from the second ancestral population were often located in genome regions not associated with gene functions, suggesting they may have been less compatible with the dominant genetic background. This pattern indicates a process known as purifying selection, where natural selection gradually removes detrimental mutations over time.

"However, some genes from the minority-contributing population—especially those linked to brain function and neural processing—may have played a crucial role in human evolution."

Dr. Trevor Cousins, Study First Author, University of Cambridge

Broader Evolutionary Insights

Beyond human ancestry, the researchers believe their method could transform how scientists study the evolution of other species. They tested the cobraa model on genetic data from bats, dolphins, chimpanzees, and gorillas, uncovering evidence of ancestral population structures in some but not all species.

"What’s becoming clear is that the traditional idea of species evolving in distinct, linear paths is too simplistic. Interbreeding and genetic exchange have likely played a major role in the emergence of new species throughout the animal kingdom."

Dr. Trevor Cousins

Who Were These Mysterious Ancestors?

Fossil evidence suggests that species such as Homo erectus and Homo heidelbergensis inhabited Africa and other regions during this period, making them potential candidates for these ancestral populations. However, further research is needed to determine which fossil species correspond to the genetic ancestors identified in this study.

Looking ahead, the team aims to refine their model to account for more gradual genetic exchanges between populations rather than abrupt separations and reunions. They also plan to explore how their findings align with anthropological discoveries, such as fossil evidence indicating that early human populations in Africa were far more diverse than previously believed.

"The ability to reconstruct events from hundreds of thousands or even millions of years ago using only modern DNA is astonishing. Our history is far richer and more complex than we ever imagined."

Aylwyn Scally, Study Co-Author, University of Cambridge