Did our species’ evolution fork like a river millions of years ago only to flow back together and produce modern humanity? Join EarthSky’s Dave Adalian at 12:15 p.m. CDT (17:15 UTC) this Wednesday, April 2, as he talks with human evolutionary genetics researcher Dr Aylwyn Scally.
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- Early human ancestors split into two groups 1.5 million years ago and later merged 300,000 years ago to form modern humans.
- Researchers used DNA from living individuals, rather than fossils, to trace human evolution and developed a new tool called cobraa to model population changes.
- Genetic evidence suggests that one group contributed 80% of modern humans’ DNA, while the other contributed 20%, with some genes possibly influencing brain function.
Early modern humans had surprising origins
For two decades, scientists have thought that early modern humans (Homo sapiens) descended from a single lineage. But new genetic research suggests that our origins are far more complex. Scientists now say that a population of human ancestors split into two groups about 1.5 million years ago. Then, about 300,000 years ago in Africa, those two populations came back together, merging to form modern humans. One population contributed 80% of its genetic makeup to us, while the other population contributed just 20%.
These findings were published in the journal Nature Genetics on March 18, 2025.
Trevor Cousins, at the University of Cambridge, is the paper’s lead author. He said in a statement:
The question of where we come from is one that has fascinated humans for centuries. 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.
Richard Durbin, a paper co-author, added:
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.
New developments in the human evolution story
According to new analysis of human DNA data, a population of early humans split in two about 1.5 million years ago. And the researchers were able to identify changes that occurred in these populations after that split.
Aylwyn Scally, a co-author of the paper, said:
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.
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.
It’s also worth noting that about 50,000 years ago, modern humans interbred with other human species including Neanderthals and Denisovans. Today, modern humans outside Africa carry a small percentage of DNA – roughly 2% – from these extinct humans.
The scientists also discovered that genes inherited from the population that contributed 20% were located away from parts of the genome (an organism’s complete set of genetic information) associated with gene functions. This suggests that much of the second population’s genes may have been less compatible with the other population. And over time, natural selection removed those genes.
But that’s not to say the second population didn’t have a genetic impact. Cousins explained:
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.
Tracing early human evolution in modern-day DNA
You might assume that researchers extracted DNA from ancient human fossils for this study. But instead, they took a different approach in tracking down our human evolution trail.
The DNA came from living healthy individuals, collected between 2008 and 2015 by the 1000 Genomes Project. It’s a program that created a catalog of human genetic variation. Ethnic groups originating from Africa, Asia, Europe, and the Americas contributed their DNA to the project.
For this study, the scientists used fully sequenced genomes (data showing the order of all or most genetic material in DNA) from the 1000 Genomes Project. They also developed a new tool, based on a powerful statistical method, that they called cobraa. It modeled how ancient human populations could split and later merge.
In their analysis of the sequenced DNA data, the researchers were able to infer which sections of our genetic code came from long-gone ancestral populations. And results from their cobraa tool revealed the new evidence of genetic mixing between two populations in our deep past.
Scally commented:
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.
Who were these human ancestors?
The genomic analysis does not identify the two human ancestor groups. But scientists know from the fossil record that around one million years ago, there were two human species living in Africa: Homo erectus and Homo heidelbergensis. Could they have been one or both of the ancestral populations identified by the scientists? We don’t know for sure, and more research is needed to investigate this intriguing possibility.
Bottom line: A population of human ancestors split into two groups about 1.5 million years ago. Then, about 300,000 years ago, they merged to form modern humans.
Source: A structured coalescent model reveals deep ancestral structure shared by all modern humans
Read more: A meeting of 2 ancient human species in fossil footprints
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