“We find that only 1.5 to 7 percent of the modern human genome is uniquely human.”
Untangling what is ours and what came from our ancient kin is a difficult task. How do you tell which genetic variants are due to interbreeding (also called admixing) of Neanderthals and Homo sapiens for example, rather than variants that were passed onto both species from a common ancestor?
The team wanted to create a system that could identify both admixture events as well as this shared inheritance – called incomplete lineage sorting – that would help tell us which regions of our genome are unique to us.
They created an algorithm called SARGE – Speedy Ancestral Recombination Graph Estimator – so they could map how our genes have weaved through time and species, separating and joining back together at different points using something called ancestral recombination graphs.
They ran SARGE on 279 modern human genomes from Africa and elsewhere, two high-quality Neanderthal genomes, and one high-quality Denisovan genome.
“Using the resulting ancestral recombination graph, we map Neanderthal and Denisovan ancestry, incomplete lineage sorting, and the absence of both across modern human genomes,” the team wrote.
“We find evidence of at least one wave of Neanderthal admixture into the ancestors of all non-Africans.”
Along with the 1.5 to 7 percent of the genome that’s unique to modern humans, they also found “evidence of multiple bursts of adaptive changes specific to modern humans within the past 600,000 years involving genes related to brain development and function”.
The researchers explain that most of those genes that were uniquely ours were not genes with unknown functions, instead they were well known genes which coded for proteins used in the brain.
Obviously, this is not even close to the end of the story. For starters, between 1.5 and 7 percent is a pretty large range and the team think they can make it more specific with more genomes and more research.
There have also been plenty of other analyses looking at the percentage of DNA we take from our ancient cousins, so it’s unlikely this will be the last word on the matter.
Plus, SARGE isn’t able to tell the researchers why those bursts of adaptive changes happened when they did.
However, the team already has some ideas.
“It’s extremely tempting to speculate that one or more of these bursts had something to do the incredibly social behavior humans have – mediated in large part by our expert control of speech and language,” University of California, Santa Cruz paleogeneticist and one of the researchers, Richard Green, told Business Insider.