Ancestors of Neandertals and Denisovans left Africa for Eurasia around 700,000 years ago and then interbred with a Homo population that had exited Africa long before, according to a new genetic study. The finding reveals the oldest known case of interbreeding among members of the genus that includes people today, Homo sapiens.

Evidence of genetic exchanges between distinct hominid populations roughly 400,000 years before H. sapiens evolved highlights a role for interbreeding in Homo evolution long before ancient people occasionally mated with Neandertals and Denisovans.

The scenario begins with an early Homo species making its way into Eurasia roughly 1.9 million years ago, in what was probably the first Homo migration out of Africa, scientists report February 20 in Science Advances. Those now-extinct travelers may have been members of Homo erectus, a species that includes Eurasian fossils dating to about 1.8 million years ago (SN: 10/17/13), or Homo antecessor, a controversial species designation based on 1.2-million- to 1.1-million-year-old fossils found in Spain (SN: 3/26/08). Or they could have been part of another Homo population unknown from any fossils.

Then ancestors of Neandertals and Denisovans trekked out of Africa about 700,000 years ago, say the researchers, led by anthropologist and population geneticist Alan Rogers of the University of Utah in Salt Lake City. That timing would also have allowed for the evolution of Neandertals or their direct ancestors in what’s now northern Spain around 430,000 years ago (SN: 3/14/16). Some previous research had suggested that Neandertals originated roughly 300,000 years ago, raising questions about the evolutionary identity of older, Neandertal-like fossils in Spain.

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Rogers refers to ancestors of Neandertals and Denisovans as “neandersovans.” That genetically distinct population existed for a brief period of perhaps 15,000 years, Rogers estimates. Neandersovans’ numbers declined sharply after they left Africa around 700,000 years ago, he suspects. Survivors interbred with members of the Homo population that had long inhabited Eurasia, before largely replacing them and separating into eastern and western populations — Denisovans and Neandertals, respectively. Neandersovans inherited at least 2 percent of their DNA from the older Eurasian Homo population, Rogers calculates.

“It’s interesting that signals of interbreeding that far back can be seen in our genomes,” says UCLA geneticist Sriram Sankararaman. Further research needs to look for genetic links between members of that probable first Homo departure from Africa, identified in Rogers’ study, and a previously unknown Homo population that lived 1 million years ago or more and left a genetic mark on present-day West Africans, Sankararaman suggests (SN: 2/12/20). A genetic analysis by the UCLA researcher’s team identified the latter Homo group.

The new findings rest on a novel analysis of particular sets of gene variants found in people today, as well as in Neandertal and Denisovan fossils. Rogers previously determined that these gene forms had not undergone recent changes and thus could be traced back to ancient populations. A software program compared frequencies of the gene variants in DNA from three modern West African Yorubans, five French individuals, two English people, a Neandertal from Croatia’s Vindija Cave, a Neandertal from Siberia’s Denisova Cave and a Denisovan from the same Siberian site.

The researchers identified the best of eight simulations of how ancient interbreeding could have produced the shared genetic variants observed in both the modern and ancient individuals. Estimates of the rate at which genetic mutations accumulate enabled the scientists to gauge the timing of the ancient African departures.

While the newly proposed timing of interbreeding around 700,000 years ago seems reasonable, Rogers’ genetic data deserve closer scrutiny with alternative statistical techniques, says zoologist and evolutionary geneticist Peter Waddell of the Ronin Institute, a nonprofit research center in Montclair, N.J. Waddell previously found signs of a small amount of ancestry in Denisovan DNA from a much older Homo species, possibly H. erectus.

Rogers and his colleagues also suggest that a third major expansion out of Africa, involving H. sapiens, occurred around 50,000 years ago. As with the neandersovan expansion, the genetic evidence is consistent with H. sapiens arriving in Eurasia and then interbreeding with resident Neandertals and Denisovans before replacing those populations, the scientists say. Other fossil and ancient DNA studies, though, indicate that some H. sapiens reached Southeast Asian islands more than 60,000 years ago (SN: 8/9/17).