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.
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).