A team of researchers from the University of Washington and Princeton University has found that the genomes of two groups of modern humans with Denisovan ancestry — individuals from Oceania and from East Asia — are uniquely different, indicating that there were two separate episodes of Denisovan admixture.
Ancient hunter-gatherers. Image credit: T.P.S. Dave.
“What was known already was that Oceanian individuals, notably Papuan individuals, have significant amounts of Denisovan ancestry,” explained study senior author Professor Sharon Browning, from the University of Washington School of Public Health.
“The genomes of modern Papuan individuals contain approximately 5% Denisovan ancestry.”
“We also knew Denisovan ancestry is present to a lesser degree throughout Asia. The assumption was that the ancestry in Asia was achieved through migration, coming from Oceanian populations.”
“But in this new work with East Asians, we find a second set of Denisovan ancestry that we do not find in the South Asians and Papuans. This Denisovan ancestry in East Asians seems to be something they acquired themselves.”
After studying 5,639 whole-genome sequences from individuals from Europe, Asia, America, and Oceania and comparing them to the Denisovan genome, Professor Browning and co-authors determined that the Denisovan genome is more closely related to the modern East Asian population than to modern Papuans.
“We analyzed all of the genomes searching for sections of DNA that looked like they came from Denisovans,” Professor Browning said.
“When we compared pieces of DNA from the Papuans against the Denisovan genome, many sequences were similar enough to declare a match, but some of the DNA sequences in the East Asians, notably Han Chinese, Chinese Dai, and Japanese, were a much closer match with the Denisovan.”
Browning et al found evidence for two waves of Denisovan admixture, one from a population closely related to the Altai Denisovan individual, and one from a population more distantly related to the Altai Denisovan; the component closely related to the Altai Denisovan is primarily present in East Asians, whereas the component more distantly related to the Altai Denisovan forms the major part of the Denisovan ancestry in Papuans and South Asians. Image credit: Browning et al, doi: 10.1016/j.cell.2018.02.031.
“The assumption is that admixing with Denisovans occurred fairly quickly after humans moved out of Africa, around 50,000 years ago, but we do not know where in terms of location,” she said.
“Perhaps the ancestors of Oceanians admixed with a southern group of Denisovans while the ancestors of East Asians admixed with a northern group.”
Going forward, the team plans on studying more Asian populations and others throughout the world, including Native Americans and Africans.
“We want to look throughout the world to see if we can find evidence of interbreeding with other archaic humans,” Professor Browning said.
“There are signs that intermixing with archaic humans was occurring in Africa, but given the warmer climate no one has yet found African archaic human fossils with sufficient DNA for sequencing.”
The study was published online this week in the journal Cell.
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Sharon R. Browning et al. Analysis of Human Sequence Data Reveals Two Pulses of Archaic Denisovan Admixture. Cell, published online March 15, 2018; doi: 10.1016/j.cell.2018.02.031
