What makes modern humans who we are and distinct from other extinct species of hominids that might share just a small fraction of our genetic materials? New work suggests that only between 1.5 to 7 percent of our genome is unique to modern humans and not shared with relatives such as Neanderthals and Denisovans.
As reported in Science Advances, the part of the modern human genome that is uniquely ours appears to be related to neural development and function. It also appears that this shift didn’t happen in a single go. Our species didn’t simply wake up one day as modern humans in the current scientific meaning of the term. Multiple waves of human-specific mutation occurred over hundreds of thousands of years.
It is not easy to determine which genes belong uniquely to modern humans. We have limited genetic information about the other members of the homo genus and there’s the problem that some of our ancestors interbred with Neanderthals and Denisovans. So people originating in certain areas of the planet might have more or less of their genome.
The researchers developed an algorithm to let them estimate which genetic material came from the species from which both us and Neanderthals originated and which came from intermixing with them. The Speedy Ancestral Recombination Graph Estimator (SARGE) used 279 modern human genomes, two Neanderthal genomes, and one Denisovan genome.
The analysis from SARGE identified two major waves of mutation in humans one happening around 600,000 years ago and one about 200,00 years ago. It also identified at least one wave of intermixing between the ancestors of all non-Africans and Neanderthals, as well as regions across Eurasia and Oceania that had intermixing between humans and both Neanderthals and Denisovans.
The work shows that the parts of the genome that appear to be uniquely ours are mutations involved in neural development and function, as well as RNA splicing. And the importance and consequences of this for our species could be investigated further in the lab.
“Using new tools for genome editing and brain organoid models for neural function, these mutations are obvious and important targets for experimental studies to determine what was selected in our human ancestors after divergence from our most closely related, extinct relatives,” the authors wrote in the paper.
The team hopes that in the future more DNA-bearing fossils of extinct archaic humans could lead to more refined analysis, providing more clues about our past and how we came to be.