Modern humans have been around for about 350,000 years. In that time, we have continued to evolve and our DNA has changed – but, only a small per centage of our genome may be unique to us.
Nathan Schaefer at the University of California, San Francisco, and his colleagues created a tool called the Speedy Ancestral Recombination Graph Estimator (SARGE), which allowed them to estimate the ancestry of individuals.
More specifically, it helped identify which bits of the modern human genome aren’t shared with other hominins – meaning they weren’t present in the ancient ancestors we shared with Neanderthals and Denisovans, and also haven’t been introduced to the human gene pool through interbreeding with these ancient humans.
“Instead of building a tree across the genome that shows how a bunch of genomes are related on average genome-wide, we wanted to know what the ancestry of individuals looks like at specific sites in the genome,” says Schaefer. “We basically wanted to be able to show how everyone is related at every single variable position in the genome.”
The team analysed one Denisovan, two Neanderthal, and 279 modern human genomes to distinguish what parts of the genome separate modern humans from archaic hominins. They found that only 1.5 to 7 per cent of the modern human genome is unique to us.
The figure may seem low but that is partly because we inherited plenty of DNA from the ancient ancestral species that ultimately gave rise to modern humans and the Neanderthals and Denisovans.
What’s more, modern humans then interbred with Neanderthals and Denisovans, picking up even more DNA that isn’t unique to our lineage.
“It’s true that individual humans have a very low per cent of their genome that might have been from Neanderthal or Denisovan ancestry – non-Africans can have between 1.5 to 2.1 per cent of their genome that originated from Neanderthal ancestry,” says Schaefer.
But we know that the exact form taken by that small amount of Neanderthal DNA varies from individual to individual – meaning two people can both have 2 per cent Neanderthal DNA but share little Neanderthal DNA in common. These differences add up, says Shaefer. Some estimates suggest about 40 per cent of the Neanderthal genome can be pieced together by combining genetic information from a wide variety of living people.
The mutations that contribute to uniquely human features are contained within a small part of the genome and seem to mainly affect genes related to brain development.
“Knowing how those variants affect human mental capacities would help us understand the cognitive differences between humans and Neanderthals,” says Montgomery Slatkin at the University of California, Berkeley.
Journal reference: Science Advances, DOI: 10.1126/sciadv.abc0776
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