Changes in the Earth’s orbit around the Sun hundreds of thousands of years ago allowed Neanderthals and Denisovans to mate with one another, new research has revealed. According to the study authors, these alterations in our cosmic trajectory facilitated major climate shifts on the ground, enabling our ancient relatives to expand their habitats and rendezvous together.
It’s well known that various human species got it on with one another in the distant past. The consequences of these ancient hook-ups can be seen in modern human genomes, with Neanderthal genes comprising around 2 percent of the DNA of non-African populations while people from Southeast Asia and Oceania share up to 5 percent of their genome with Denisovans.
And it wasn’t just our species that couldn’t resist a bit of hominin hanky-panky. In Siberia’s famous Denisova Cave, researchers unearthed the 90,000-year-old daughter of a Denisovan father and a Neanderthal mother.
To determine when, where, and how these two extinct human species may have done the dirty, the study authors looked at the age and location of 22 Denisovan artifacts and 773 Neanderthal remains. Combining these with genetic data and supercomputer simulations of the ancient climate, the researchers were able to map out the distribution patterns of the two hominid lineages over time.
“Compared to Neanderthals, Denisovans were present in hot and humid climates, which points to a comparatively wider niche space,” they write. “Whereas Neanderthals were more abundant in temperate forests, Denisovans were present in both boreal forest and tundra.”
"This means that their habitats of choice were separated geographically, with Neanderthals typically preferring southwestern Eurasia and Denisovans the northeast," explained study author Dr Jiaoyang Ruan in a statement.
However, during warm interglacial periods, when Earth’s orbit around the Sun was more elliptic and temperatures increased, changes in carbon dioxide levels brought about dramatic shifts in vegetation cover across the Northern Hemisphere. This resulted in the eastward expansion of temperate forests, creating dispersal corridors for Neanderthals into Denisovan territory.
"It is as if glacial-interglacial shifts in climate created the stage for a unique and long-lasting human love story, whose genetic traces are still visible today," said Ruan. Recreating this ancient romance, the researchers identified “contact hotspots in central Eurasia, the Caucasus, and the Tianshan and Changbai mountain ranges.”
“For instance, Denisovans and Neanderthals exhibited high contact probability in the Siberian Altai, mostly during interglacial periods,” they write. This matches up well with genetic data that indicates at least six separate episodes of interbreeding between the two species.
Of these, the researchers were able to trace five to central-southern Siberia during the interglacial period spanning 130,000 to 80,000 years ago. The sixth, they say, probably occurred slightly earlier than this in Eastern Europe.
Overall, the study authors say their findings highlight how “glacial-interglacial climate swings likely played an important role in favoring gene flow between archaic humans.”
The study is published in the journal Science.