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The Species Problem

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Science  28 Jan 2011:
Vol. 331, Issue 6016, pp. 394
DOI: 10.1126/science.331.6016.394

Our ancestors are now thought to have mated with at least two kinds of archaic humans at two different times and places. Were they engaging in interspecies sex, or does the fact that they were able to produce offspring mean they were all members of the same species?

Our ancestors had sex with at least two kinds of archaic humans at two different times and places—and those liaisons produced surviving children, according to the latest ancient DNA research (see main text, p. 392). But were the participants in these prehistoric encounters members of separate species? Doesn't a species, by definition, breed only with others of that species?

These are the questions paleogeneticist Svante Pääbo dodged twice last year. His team published two papers proposing that both Neandertals and mysterious humans from Denisova Cave in Siberia interbred with ancient modern humans. But the researchers avoided the thorny question of species designation and simply referred to Neandertals, Denisovans, and modern humans as “populations.” “I think discussion of what is a species and what is a subspecies is a sterile academic endeavor,” says Pääbo, who works at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

The question of how to define a species has divided researchers for centuries. Darwin's words in On the Origin of Species still hold: “No one definition has satisfied all naturalists.” However, many scientists use the biological species concept proposed by Ernst Mayr: “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.”

The draft versions of the Neandertal and Denisovan nuclear genomes show low levels of interbreeding between each of them and modern humans. Apply Mayr's definition strictly, and all three must be considered Homo sapiens. “They mated with each other. We'll call them the same species,” says molecular anthropologist John Hawks of the University of Wisconsin, Madison.

But that's a minority view among paleoanthropologists. Many consider Neandertals a species separate from modern humans because the anatomical and developmental differences are “an order of magnitude higher than anything we can observe between extant human populations,” says Jean-Jacques Hublin, a co-author of Pääbo's at Max Planck. In the real world, he says, Mayr's concept doesn't hold up: “There are about 330 closely related species of mammals that interbreed, and at least a third of them can produce fertile hybrids.”

There's also no agreed-upon yardstick for how much morphologic or genetic difference separates species. That's why Pääbo's team avoided the species question a second time with respect to the Denisovans. These hominins are known only from a scrap of bone, a single tooth, and their DNA. They are genetically closest to Neandertals. The genetic distance between Denisovans and Neandertals, in fact, is only 9% larger than that between a living Frenchman and a living San Bushman in Africa, both of whom belong to H. sapiens. But so far Neandertals seem to have low genetic diversity, based on the DNA of six Neandertals from Russia to Spain. To Pääbo's team, that makes the difference from the Denisovans significant.

Also, the Denisovan tooth doesn't look much like that of a Neandertal. So the team considers them a distinct population but declined to name a new species. “Why take a stand on it when it will only lead to discussions and no one will have the final word?” asks Pääbo.

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