It’s impossible to pinpoint the moment when a paradigm shifts. But when it comes to views of the origin of Homo sapiens, last month may be as good a time as any: That’s when two papers independently suggested that early H. sapiens interbred with now-extinct forms of humans in Africa, so that some living Africans carry genes from archaic people, just as all Europeans and Asians recently have been shown to do. The new data imply that there were at least three fruitful encounters between H. sapiens and archaic species. And one of the new papers, on African fossils, was co-authored by paleoanthropologist Chris Stringer of the Natural History Museum in London, once the leading advocate of a conflicting hypothesis that H. sapiens simply replaced, rather than mated with, archaic peoples in Africa, Asia, and Europe. Says Stringer now: “Africa doesn’t have a simple story of modern humans appearing and everything else disappearing.”
That marks a turning point in views of modern human origins, says geneticist Michael Hammer of the University of Arizona in Tucson, lead author of another paper on African genetics. “There is a paradigm shift,” he says. “The burden of proof is now on those who want to claim complete replacement. Two years ago, it was on those who wanted to claim [mixing].” After sequencing the genome of Neandertals and that of mysterious hominins from Denisova Cave in Siberia, researchers concluded last year that our ancestors interbred with both Neandertals and Denisovans. One meeting may have been in western Asia with Neandertals, and a separate intermingling with Denisovans may have taken place in Asia (Science, 23 September, 1689).
Now, a team led by Hammer has found that some hunter-gatherers in Africa also carry unusual segments of DNA that the researchers propose are archaic, as reported last month in the Proceedings of the National Academy of Sciences (PNAS). The team screened 61 regions of DNA from three relatively isolated groups in sub-Saharan Africa, examining noncoding DNA that was less likely to be influenced by natural selection. They found three regions whose pattern of alleles varied widely among individuals—a sign of great antiquity. But the variants were linked to each other across extended regions of the chromosome, which suggested they were inherited recently, because such associations among chromosomal regions break down over time. So the overall pattern suggests that ancient segments of DNA recently entered the H. sapiens genome, as would be the case if the DNA was inherited from an archaic human.
The team screened for these three variants in an expanded sample of 500 Africans and found them in pygmy and nonpygmy populations from Zaire, Cameroon, and the Democratic Republic of the Congo. By using computational and statistical methods to simulate the distribution of the variants, Hammer’s team concluded that modern humans mated with archaic humans perhaps as recently as 35,000 years ago in central Africa. Hammer stresses that it was a small amount of interbreeding, “or we would have discovered this long ago.”
Just what archaic human did the ancestors of these Africans mate with? Geneticists can’t say because they have no ancient DNA from African fossils. And there are no fossils of Neandertals or Denisovans on the continent. But fossil hunters have long puzzled over a few strange African bones that carry both archaic and modern features. In a PLoS ONE paper published a week after Hammer’s, a team led by Stringer and Katerina Harvati of the Eberhard Karls University of Tübingen in Germany took a new look at the cranium of a modern human found in 1965 at Nigeria’s Iwo Eleru rock shelter. Stringer had studied these bones for his Ph.D. thesis and had always wondered about them, for they didn’t fit the theory of replacement: They had a thick, archaic brow ridge, but the radiocarbon dating of the time had put them at only about 13,000 years old. With respect to replacement, “it was an anomaly,” Stringer says. “We didn’t expect to have archaic people hanging around so long after the appearance of modern humans.”
Harvati analyzed the skull quantitatively and found that in addition to the thick brow ridge and robust jaw, the cranium is elongated from front to back, transitional between the shape seen in archaic humans like Neandertals and the globular H. sapiens skulls. New uranium-series dating on a piece of the bone, done by Rainer Grun of the Australian National University in Canberra, confirmed it is only 11,700 to 16,300 years old.
The Iwo Eleru bones “could be a relic of ancient people” who went extinct, Stringer says, or a descendant of mating between modern and archaic humans. The specimens haven’t yielded any ancient DNA. But researchers are already studying archaic features in other H. sapiens fossils. “I think there’s more to come,” Hammer says. Indeed, “if interbreeding happened outside of Africa,” as the complete genomes of Neandertals and Denisovans suggest, “it is quite likely it also happened within Africa,” says population geneticist Laurent Excoffier of the University of Bern in Switzerland.But he says Hammer’s team needs to do more modeling to see if conditions such as population bottlen ecks could produce the same genetic patterns without interbreeding. Geneticist Sarah Tishkoff of the University of Pennsylvania would like whole-genome analyses, which Hammer says are in the works. Still, Tishkoff says, “it looks increasingly possible that there’s been some low level of admixture” in Africa as well as Europe and Asia.
SOURCE : SCIENCE MAGAZINE VOL 334