Residents of the Pacific islands of Melanesia share fragments of genetic code with two early human species: Denisovans, whose remains were found in Siberia, and Neanderthals, first discovered in Europe, according to a report published this week in the journal Science.
Denisovans were probably dark-skinned, unlike the pale Neandertals. The picture shows a Neanderthal man. Image credit: Mauro Cutrona.
In the past, ancestors of many modern human populations interbred with other hominin species that have since become extinct, such as Neanderthals and Denisovans.
Mapping the gene flow of surviving genetic sequences from these species, as well as other species of hominin, helps shed light on how past interbreeding has affected human evolution.
While previous studies have documented Neanderthal gene flow in modern humans, much less is known about the characteristics of Denisovan DNA that persist in humans today.
To gain more insights, study first author Benjamin Vernot from the University of Washington, Seattle, and his colleagues from Papua New Guinea, Germany, Italy, and the United States, analyzed the genomes of 1,523 individuals from around the world, including 35 individuals from 11 locations in the Bismarck Archipelago of Northern Island Melanesia.
Their results showed that while all non-African populations surveyed inherited roughly 1.5-4% of their genomes from Neanderthals, Melanesians were the only population that also had significant Denisovan genetic ancestry, representing between 1.9% and 3.4% of their genome.
“I think that Neanderthals and Denisovans liked to wander. Studies like this can help us track where they wandered,” Vernot said.
“Denisovans are the only species of archaic humans about whom we know less from fossil evidence and more from where their genes show up in modern humans,” added study senior author Dr. Joshua Akey, also from the University of Washington.
The team also mapped out the genetic flow of Neanderthal and Denisovan sequences, finding that Neanderthal admixture, or gene flow, occurred at least three distinct times in modern human history.
In contrast, Denisovan admixture likely only occurred once.
Further analysis revealed that certain regions of the modern human genome are particularly depleted of these archaic lineages, including those that play a role in the developing cortex and adult striatum.
“This type of study gives perspective on human expansion across Eurasia, and possibly what sort of conditions those humans encountered on their way,” Vernot said. “The work demonstrates how we can learn about human history, and our archaic relatives, by studying ancient and modern DNA.”
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Benjamin Vernot et al. Excavating Neandertal and Denisovan DNA from the genomes of Melanesian individuals. Science, published online March 17, 2016; doi: 10.1126/science.aad9416
