Scientists have uncovered evidence that modern humans emerged from two long-separated ancestral groups, not just one.
This genetic reunion reshaped our species, introducing key traits that may have influenced brain function. Unlike Neanderthal interbreeding, this ancient event contributed a massive portion of our DNA.
A New Perspective on Human Ancestry
Using advanced analysis based on full genome sequences, researchers from the University of Cambridge have found evidence that modern humans are the result of a genetic mixing event between two ancient populations that diverged around 1.5 million years ago. About 300,000 years ago, these groups came back together, with one group contributing 80% of the genetic makeup of modern humans and the other contributing 20%.
“Our history is far richer and more complex than we imagined.”
Aylwyn Scally
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, these latest results, reported in the journal Nature Genetics, suggest a more complex story.
A More Complex Origin Story
“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.
Evidence of Ancient Genetic Merging
While earlier research has already shown that Neanderthals and Denisovans – two now-extinct human relatives – interbred with Homo sapiens around 50,000 years ago, this 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.
A Breakthrough in Analyzing Ancient DNA
The team’s method relied on analysing 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 data used in the study is from the 1000 Genomes Project, a global initiative that sequenced DNA from populations across Africa, Asia, Europe, and the Americas.
The team 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.
Drastic Population Shifts Over Time
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 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 Role of Brain-Related Genes
“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.
The study 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.
Beyond Humans: A New Evolutionary Framework
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.”
Identifying Our Mysterious Ancestors
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.
What’s Next for Human Evolution Studies?
Looking ahead, the team hopes 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,” said Scally. “And it tells us that our history is far richer and more complex than we imagined.”
Reference: “A structured coalescent model reveals deep ancestral structure shared by all modern humans” by Trevor Cousins, Aylwyn Scally and Richard Durbin, 18 March 2025, Nature Genetics.
DOI: 10.1038/s41588-025-02117-1
The research was supported by Wellcome. Aylwyn Scally is a Fellow of Darwin College, Cambridge. Trevor Cousins is a member of Darwin College, Cambridge.
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4 Comments
Very interesting! I hope all the scientists keep doing what they’re doing! 🖤
Obviously, hominims were quite happy to screw around with whatever came along. Why should this be surprising? Religious puratinism is only a couple of thousand or so years old, at best.
This is model dependent, which their own reference from 2023 with a similar structure but different timing shows. And some of their analysis of other species may make less sense, such as the Nigeria–Cameroon chimpanzee having recent or ongoing gene flow. A similar but super-archaic model show the similar timing and gene flows though. [Bergström, A., Stringer, C., Hajdinjak, M. et al. Origins of modern human ancestry. Nature 590, 229–237 (2021).]
That said, the match with known coexisting fossil species is intriguing regardless of model, if there is a match on the genetic level (but we have no genomes from them). Even more intriguing, the chromosome 2 fusion has been putatively pushed back to about 1 Myrs, which could explain the bottleneck in the major ancestral population. [Poszewiecka, B., Gogolewski, K., Stankiewicz, P. et al. Revised time estimation of the ancestral human chromosome 2 fusion. BMC Genomics 23 (Suppl 6), 616 (2022).]
Wonder if the group representing 20% of our DNA and involving our modern brain function as well as our neural processing might have evolved with the ‘help’ of some third party intervention? Wonder if the other population, the 80% of our current DNA that almost went extinct as a result of a ‘bottleneck’, was not ‘supposed’ to survive.