Genetically, Only a Small Percentage of Humans is Uniquely HumanPerhaps the variation in the amount that the individual has, helps explain why SOME people behave less human.
Only a tiny fraction of our DNA is uniquely human
The result underscores how big of a hand interbreeding among ancient hominids had in shaping us
By Tina Hesman Saey
July 16, 2021 at 2:07 pm
The genetic tweaks that make humans uniquely human may come in small parcels interspersed with DNA inherited from extinct ancestors and cousins.
Only 1.5 percent to 7 percent of the collective human genetic instruction book, or genome, contains uniquely human DNA, researchers report July 16 in Science Advances.
That humans-only DNA, scattered throughout the genome, tends to contain genes involved in brain development and function, hinting that brain evolution was important in making humans human. But the researchers don’t yet know exactly what the genes do and how the exclusively human tweaks to DNA near those genes may have affected brain evolution.
“I don’t know if we’ll ever be able to say what makes us uniquely human,” says Emilia Huerta-Sanchez, a population geneticist at Brown University in Providence, R.I., who was not involved in the study. “We don’t know whether that makes us think in a specific way or have specific behaviors.” And Neandertals and Denisovans, both extinct human cousins, may have thought much like humans do (SN: 2/22/18).
The results don’t mean that individual people are mostly Neandertal or Denisovan, or some other mix of ancient hominid. On average, people in sub-Saharan Africa inherited 0.096 percent to 0.46 percent of their DNA from ancient interbreeding between their human ancestors and Neandertals, the researchers found (SN: 4/7/21). Non-Africans inherited more DNA from Neandertals: about 0.73 percent to 1.3 percent. And some people inherited a fraction of their DNA from Denisovans as well.
Using a new computational method, researchers at the University of California, Santa Cruz examined every spot of DNA in the genomes of 279 people. The team compiled results from those individual genomes into a collective picture of the human genome. For each spot, the team determined whether the DNA came from Denisovans, Neandertals or was inherited from a common ancestor of humans and those long-lost relatives.
Although each person may carry about 1 percent Neandertal DNA, “if you look at a couple hundred people, they mostly won’t have their bit of Neandertal DNA in the same place,” says Kelley Harris, a population geneticist at the University of Washington in Seattle who wasn’t involved in the work. “So if you add up all the regions where someone has a bit of Neandertal DNA, that pretty soon covers most of the genome.”
In this case, about 50 percent of the collective genome contains regions where one or more people inherited DNA from Neandertals or Denisovans, the researchers discovered. Most of the rest of the genome has been passed down from the most recent common ancestor of humans and their extinct cousins. After whittling away the ancient heirloom DNA, the team looked for regions where all people have human-specific tweaks to DNA that no other species have. That got the estimate of uniquely human DNA down to anywhere between 1.5 percent and 7 percent of the genome.
The finding underscores just how much interbreeding with other hominid species affected the human genome, says coauthor Nathan Schaefer, a computational biologist now at the University of California, San Francisco. The researchers confirmed previous findings from other groups that humans bred with Neandertals and Denisovans, but also with other extinct, unknown hominids (SN: 2/12/20)....
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