Unveiling a Lost Chapter in Human Evolution: A Groundbreaking Discovery–Alost chapter in human evolution has been revealed after an analysis of modern DNA found that we come from not one but two ancestral populations—ones that drifted apart and later reconnected long before modern humans spread across the globe.
According to researchers at the University of Cambridge, England, the two ancient populations split 1.5 million years ago before coming back together 1.2 million years later.
“The question of where we come from is one that has fascinated humans for centuries,” said paper author and evolutionary biologist Trevor Cousins in a statement.
“For a long time, it has been assumed that we evolved from a single continuous ancestral lineage, but the exact details of our origins are uncertain.”
While the exact details are unclear, he explained, the prevailing belief for the last 20 years has been that modern humans first appeared in Africa some 200,000–300,000 years ago, descended from a single continuous ancestral lineage.
His colleague, Professor Richard Durbin, 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.”
Based on the fossil record, we know that species like Homo erectus and Homo heidelbergensis lived in Africa (and beyond) during the period in question, making them possible candidates for these ancestral populations.
The genetic data used in the analysis was collected as part of the so-called 1000 Genomes Project, which sequenced the DNA of people from across the globe.
The researchers developed an algorithm—which they have dubbed “cobraa”—which can reveal both how human populations split apart and how they can later merge back together.
Cobraa also revealed some significant changes in one of the two ancestral populations that occurred after they split 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 paper co-author professor Aylwyn Scally.
“This population would later contribute about 80 percent of the genetic material of modern humans—and also seems to have been the ancestral population from which Neanderthals and Denisovans diverged.”
Both the Neanderthals and Denisovans interbred with Homo sapiens around 50,000 years ago. While Neanderthal DNA accounts for around 2 percent of the genome of non-African modern humans, the newly identified mixing event 250,000 years earlier contributed as much as 10 times the DNA—which is found in all modern humans.
Cousins added: “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.”
The team’s analysis also indicated that the genes inherited from this second population are often located away from the parts of the genome involved in gene functions, suggesting that they may have been less compatible with the majority genetic background.
A form of natural selection known as “negative selection,” the researchers explained, acts to remove harmful mutations over time—and could have weeded out incompatible elements.
Alongside studying humans, the researchers also used cobraa to explore the genetic history of other species—including bats, chimpanzees, dolphins and gorillas—finding evidence of ancestral population structures in some (but not all) of these groups.
“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 across the animal kingdom.”
With their initial study complete, the researchers are now hoping to refine their cobraa model to enable it to reveal more gradual genetic exchanges between different populations.
Scally concluded: “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.”
