Human DNA Shows We Had Plentiful Sex With Ancient Denisovan Hominins Thousands of Years Ago

homo heidelbergensis skull burgos
The cranium of an adult Homo heidelbergensis, an extinct species of the genus Homo, dubbed 'Skull 4' is displayed at the Museum of Human Evolution (MEH) in Burgos on May 11, 2016. Scientists have found another species of human may have genetically intermingled twice with our own. CESAR MANSO/AFP/Getty Images

If you think your love life is crazy, wait until you hear about what early humans got up to. New research shows that thousands of years ago, our ancestors were having children with another hominin known as the Denisovans—according to new data published Thursday in Cell.

The comingling came in two distinct waves, thousands of years ago. These two periods help explain an odd pattern. People from countries in the Pacific Ocean, including Papua New Guinea, can chalk about 5 percent of their DNA up to Denisovan sources. Chinese and Japanese people also carry some Denisovan DNA, too, though not as much. But among East Asian populations, researchers could discern, that genetic material had to have come from two different populations that were distantly, but not closely, related. The two waves of sexual relations explains why the East Asian gene pool appears to have two separate sources of Denisovan DNA.

These hominins were first discovered in 2008; the first paper about them was published in 2010. But although we have very little to go on—just four fossils from one woman discovered in a Siberian cave, The Atlantic reported—it's not hard to find DNA that might have come from that time our ancestors were shacking up with other, related species.

Neanderthal bones and teeth California
Helene Rougier, anthropologist at California State University Northridge, in the United States, displays some bones and teeth from five Neanderthal individuals found in Belgium. Neanderthals are just one ancient hominin whose DNA can be found in modern humans; another species called the Denisovans has made two separate genetic contributions, according to new research published March 15. EMMANUEL DUNAND/AFP/Getty Images

The thousands of years since those waves is not enough time to scrub the evidence of ancient dalliances from our DNA. Scientists can look at odd genes that tend to be inherited together to spot where the Neanderthal or the Denisovan might be in our DNA, a phenomenon called linkage disequilibrium. There's actually a statistic to measure linkage disequilibrium called S*; Sharon Browning at the University of Washington and her colleagues riffed on S* for this recent paper.

Genetic remnants of other ancient populations can provide useful insights to other areas of science. Browning and her colleagues found two new regions that appear to be from another hominin on chromosome 3 that both are involved in our immune systems. These particular regions code for genes that express two receptors, CCR9 and CXCR6, which both interact with an immune protein called a chemokine.

There's a good chance all of humanity's affairs haven't yet been exposed. About 25 percent of the DNA that Browning classified is likely coming from a hominin other than a Neanderthal or a Denisovan, New Scientist reported. Which hominin those might be is still unclear.