A new genetic analysis reveals a ‘lost chapter’ in the history of our species

In a newly published study in ‘Nature Genetics’, The researchers provide evidence that modern humans, we are the result of a genetic mixing event between two old populations, which diverged approximately 1.5 million years ago and that, more than one million years later, about 300,000 years ago, they met again. Analysis reveal that one of these groups accounts for 80% of our genetic composition, while the other contributes with the remaining 20%.

More than one lineage

During the last two decades, evolutionary genetics seemed to have made it very clear that Homo sapiens It appeared for the first time in Africa between 200,000 and 300,000 years ago, and that descended from a single lineage. But the new study points to a much more complex story.

“The question of where we come has fascinated humans for centuries,” says Trevor Cousins, the first author of the article. For a long time, it has been assumed that we evolve from a single continuous ancestral lineage, but the exact details of our origins remain uncertain.

“Our research,” explains the co -author Richard Durbin, shows clear signs that our evolutionary origins are more complex, and involve different groups that developed separately for more than one million years, and then met again to form the modern human species. “

While previous investigations had already shown that other human species, Neanderthals and Denisovanos, crossed with Homo sapiens About 50,000 years ago, this new research suggests that long before those interactions, about 300,000 years ago, a much more important genetic mixture took place. The Neanderthal DNA, in fact, barely represents about 2% of the genome of modern non -African humans (who stayed in the black continent and that, therefore, never found any Neanderthal), while the old newly discovered mixing event contributed to dozens of times that amount, and is also found in all members of our species.

Modern human DNA

To reach these conclusions, Cambridge scientists were based on the analysis of modern human DNA (the only one that allows complete genomes), instead of extracting fragmentary genetic material from ancient bones, which allowed them to infer the presence of ancestral populations that otherwise would not have left physical trace. The data used in the study comes from the 1000 Genomas project, a global initiative that sequenced the DNA of populations of Africa, Asia, Europe and America.

For analysis, the team developed a computational algorithm called ‘Cobraa’ capable of modeling the successive meetings and disagreements of former human populations. First they tested the algorithm with simulated data and then applied it to real human genetic data of the 1000 genomes project.

In this way, the team managed to identify these two ancestral populations, but also discovered some surprising changes that happened in the long interval in which both remained separate.

Surprising changes

“Immediately after the two ancestral populations separated,” says the Aylwyn Scally co -author, we see a severe bottleneck in one of them, which suggests that it was reduced to a very small size before slowly growing again for a period of one million years. This population would later contribute approximately 80% of the genetic material of modern humans, and also seems to have been the ancestral population of which the Neanderthals and denisovanos diverged ».

Which does not mean that the other population was not important. “In fact,” says Cousins ​​- some of the genes that come to us from the population that contributed to the slightest part of our genetic material (20%), particularly those related to brain function and neuronal processing, may have played a crucial role in human evolution. “

The study also found that the genes inherited from the second population were often located far from the genome regions that are linked to the functions of the genes, which suggests that they could have been less compatible with the majority genetic background. This hints a process known as purifying selection, where natural selection eliminates, over time, harmful mutations.

Also in animals

Beyond human ancestry, the authors of the study believe that their method could also help transform the way scientists study the evolution of other species. Therefore, in addition to their analysis of human evolutionary history, Cousins ​​and their colleagues applied the ‘Cobraa’ model to genetic data of bats, dolphins, chimpanzees and gorillas, and managed to find evidence of the ancestral population structure in some, although not in all of them.

“What is increasingly clear,” says Cousins, “is that the idea that species evolve in clean and different lineages is too simplistic. Miscegenation and genetic exchange have probably played an important and repeated role in the emergence of new species throughout the animal kingdom ».

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

Looking ahead, the team hopes to refine its model to take into account more gradual and subtle genetic exchanges between populations, and not just abrupt separations and meetings. They also plan to explore how their findings are related to other discoveries in anthropology, such as the growing African fossil evidence that suggests that the first humans could have been much more diverse than what was thought so far.

“The fact that we can rebuild events of hundreds of thousands or millions of years simply looking at the current DNA,” says Scally, “is amazing. And he tells us that our story is much richer and more complex than we had been able to imagine.