For the last 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, and descended from a single lineage. However, according to new research from the University of Cambridge, modern humans are a result of two populations (potentially Homo heidelbergensis and Homo erectus) that diverged 1.5 million years ago and came together in an admixture event 300,000 years ago, in a ratio of 80:20%.

“The question of where we come from is one that has fascinated humans for centuries,” said University of Cambridge’s Dr. Trevor Cousins.

“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,” added University of Cambridge’s Professor Richard Durbin.

While earlier research has already shown that Neanderthals and Denisovans interbred with Homo sapiens around 50,000 years ago, the new research suggests that long before those interactions — around 300,000 years ago — a much more substantial genetic mixing took place.

Unlike Neanderthal DNA, which makes up roughly 2% of the genome of non-African modern humans, this ancient mixing event contributed as much as 10 times that amount and is found in all modern humans.

The team’s method relied on analyzing modern human DNA, rather than extracting genetic material from ancient bones, and enabled them to infer the presence of ancestral populations that may have otherwise left no physical trace.

The authors developed a computational algorithm called cobraa that models how ancient human populations split apart and later merged back together.

They tested the algorithm using simulated data and applied it to real human genetic data from the 1000 Genomes Project, a global initiative that sequenced DNA from populations across Africa, Asia, Europe, and the Americas.

While the researchers were able to identify these two ancestral populations, they also identified some striking changes that happened after the two populations initially broke apart.

“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 University of Cambridge’s Professor Aylwyn Scally.

“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.”

“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,” Dr. Cousins said.

This is an artist’s reconstruction of Homo erectus. Image credit: Yale University.

The scientists also found that genes inherited from the second population were often located away from regions of the genome linked to gene functions, suggesting that they may have been less compatible with the majority genetic background.

This hints at a process known as purifying selection, where natural selection removes harmful mutations over time.

So who were our mysterious human ancestors? Fossil evidence suggests that species such as Homo erectus and Homo heidelbergensis lived both in Africa and other regions during this period, making them potential candidates for these ancestral populations, although more research (and perhaps more evidence) will be needed to identify which genetic ancestors corresponded to which fossil group.

Looking ahead, the authors hope to refine their model to account for more gradual genetic exchanges between populations, rather than sharp splits and reunions.

They also plan to explore how their findings relate to other discoveries in anthropology, such as fossil evidence from Africa that suggests early humans may have been far more diverse than previously thought.

“The fact that we can reconstruct events from hundreds of thousands or millions of years ago just by looking at DNA today is astonishing,” Professor Scally said.

“And it tells us that our history is far richer and more complex than we imagined.”

The study was published today in the journal Nature Genetics.

_____

T. Cousins et al. A structured coalescent model reveals deep ancestral structure shared by all modern humans. Nat Genet, published online March 18, 2025; doi: 10.1038/s41588-025-02117-1