A new paper reveals that modern humans co-existed and interbred with archaic human Denisovans, not only with Neanderthals—another type of extinct human species—at least twice between 200,000 and 50,000 years ago.

Researchers at the University of Washington unexpectedly discovered two distinct episodes of Denisovan genetic intermixing with modern humans, while testing a new DNA method for comparing whole genomes between the two species.

To reach their findings, scientists studied more than 5,600 genome sequences from individuals from Europe, Asia, America, and Oceania, comparing them to the Denisovan genome.

According to a paper newly published in the journal Cell, researchers found that the genomes of the two groups of modern humans showing Denisovan ancestry—from Oceania and East Asia—are uniquely different, indicating that two separate episodes of Denisovan admixture existed.

Denisovans are a distinct branch of the Homo or human family tree whose existence was first known in 2010 when scientists found a bone fragment and molars in Denisova Cave in Siberia. The discovered bone was analysed, revealing a complete genome for a previously unknown type of human. But other than their DNA, little is known about the Denisovans.

It was known prior to this study that ancestry in Asia came 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 South Asians and Papuans,” said study lead author Sharon Browning, who is a professor of biostatistics at the University of Washington.