A team at the University of Washington in Seattle stumbled across the evidence of a third interbreeding event while sequencing DNA from the genomes of 5,600 living humans, trying to find evidence of ancient DNA, according to Newser. The team's research was detailed in a recent publication in the journal Cell.
However, scientists already knew that humans and Denisovans had mated — the original discovery of the Denisovan genome was found to match 5 percent of the DNA of certain Southeast Asian and Melanesian populations. Lead author Sharon Browning said their find, which is a much smaller percentage of the genome in question, a mere 0.2 percent, indicates that the group sharing that Denisovan DNA acquired it separately from when the other group of humans did.
Browning speculated that it might show "a large group of East Asians meeting a small number of Denisovans," as the former spread across the continent, reported The Atlantic.
However, there's still other ancient DNA unaccounted for, meaning that humans probably mated with other as-yet undiscovered species of hominids, too.
Denisovans form a distinct branch of the Homo family tree, although they share a common ancestor with Neanderthals and modern humans, who most likely lived in Africa between 550,000 and 765,000 years ago, according to Discovery Magazine. As with Neanderthals, there is significant dispute about whether or not Denisovans form a distinct species of hominid or should be categorized as a subspecies of Homo sapiens, given that Neanderthals shared 99.7 percent of their DNA with modern humans, according to Science Daily.
Denisovans were "like an Eastern cousin" of Neanderthals, says geneticist Matthias Meyer of the Max Planck Institute in Leipzig, where the Denisovan genome was first identified.
Although the three hominids diverged into their different (sub)species after migrating out of Africa, with Neanderthals heading toward western Eurasia and Denisovans toward central and eastern Eurasia, they never became so genetically distinct as to block mating between them — mating that likely happened extensively within the last 100,000 years. All three subspecies had effectively identical cognitive capabilities and engaged in similarly complex tool-building and hunting activities, although extremely little evidence of Denisovan culture has been discovered thus far, according to David Reich, a Harvard geneticist who led data analysis of the Denisovan genome.
That is probably because of the scant evidence of Denisovans themselves, of whom only four bones are known to exist, according to The Atlantic. Their place of discovery, Denisova Cave in Siberia's Altai Mountains, was found in 2010. The bones belong to the pinky finger of a young Denisovan girl.