Mixed-species DNA casts new light on early humans


Fri, Aug 24, 2018 - Page 7

Denny was an inter-species lovechild.

Her mother was a Neanderthal, but her father was Denisovan, a distinct species of primitive human that also roamed the Eurasian continent 50,000 years ago, scientists reported in Nature on Wednesday.

Nicknamed by Oxford University scientists, Denisova 11 — her official name — was at least 13 when she died for reasons unknown.

“There was earlier evidence of interbreeding between different hominin, or early human, groups,” said lead author Vivian Slon, a researcher at the Max Planck Institute for Evolutionary Anthropology. “But this is the first time that we have found a direct, first-generation offspring.”

Denny’s surprising pedigree was unlocked from a bone fragment unearthed in 2012 by Russian archeologists at the Denisova Cave in the Altai Mountains of Siberia.

Analysis of the bone’s DNA left no doubt: The chromosomes were a 50-50 mix of Neanderthal and Denisovan, two distinct species of early humans that split apart between 400,000 to 500,000 years ago.

“I initially thought that they must have screwed up in the lab,” said senior author and Max Planck Institute professor Svante Paabo, who identified the first Denisovan a decade ago at the same site.

Worldwide, fewer than two dozen early human genomes from before 40,000 years ago — Neanderthal, Denisovan and Homo sapiens — have been sequenced, and the chances of stumbling on a half-and-half hybrid seemed vanishingly small. Or not.

“The very fact that we found this individual of mixed Neanderthal and Denisovan origins suggests that they interbred much more often than we thought,” said Slon.

A 40,000-year-old Homo sapiens with a Neanderthal ancestor a few generations back, recently found in Romania, also bolsters this notion, but the most compelling evidence that inter-species hanky-panky in Late Pleistocene Eurasia might not have been that rare lies in the genes of contemporary humans.

About 2 percent of DNA in non-Africans across the globe today originates with Neanderthals, earlier studies have shown.

Denisovan remnants are also widespread, although less evenly.

“We find traces of Denisovan DNA — less than 1 percent — everywhere in Asia and among native Americans,” Paabo said. “Aboriginal Australians and people in Papua New Guinea have about 5 percent.”

Taken together, these facts support a novel answer to the hotly debated question of why Neanderthals — who had successfully spread across parts of western and central Europe — disappeared about 40,000 years ago.

What if our species — arriving in waves from Africa — overwhelmed Neanderthals, and perhaps Denisovans, with affection rather than aggression?

“Part of the story of these groups is that they may simply have been absorbed by modern populations,” Paabo said.

Recent research showing that Neanderthals were not knuckle-dragging brutes makes this scenario all the more plausible. Far less is known about Denisovans, but they might have suffered a similar fate.

Paabo established their existence with an incomplete finger bone and two molars dated to about 80,000 years ago.

Among their genetic legacy to some modern humans is a variant of the gene EPAS1, which makes it easier for the body to access oxygen by regulating the production of hemoglobin, a 2014 study said.

Nearly 90 percent of Tibetans have this precious variant, compared with only 9 percent of Han Chinese, the dominant ethnic group in China.