DNA puts Neandertals on edge of human ancestry.

In the ongoing battle over their role in human evolution, Neandertals have taken another hit. An unprecedented amount of genetic material removed from Stone Age fossils indicates that the heavy-boned, beetle-browed Neandertals made, at most, a small genetic contribution to our prehistoric ancestors.

A team led by David Serre of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, compared mitochondrial DNA sequences extracted from fossils of four Neandertals unearthed in Belgium, Croatia, and France with those of five early modern humans found either in the Czech Republic or France. The specimens range in age from around 30,000 to 60,000 years old.

When combined with prior evidence of mitochondrial-DNA differences between Neandertals and Stone Age Homo sapiens (SN: 5/17/03, p. 307), "the data are more supportive of the out-of-Africa hypothesis of human evolution," Serre says. In that scenario, people evolved around 200,000 years ago in Africa and then moved into Asia and Europe, replacing Neandertals and other Homo species.

The new findings buttress the out-of-Africa theory, agrees Christopher Stringer of the Natural History Museum in London, England.

Ancient bones don't often yield mitochondrial DNA, which is located outside the cell nucleus and inherited from the mother, Serre notes. His team successfully obtained only nine genetic samples from 24 Neandertal and 40 early modern-human remains.

Nucleotide sequences of mitochondrial DNA extracted from the four Neandertal specimens resembled comparable genetic sequences previously taken from four other Neandertal fossils, the scientists report in the March Public Library of Science Biology. None of the five mitochondrial DNA samples taken from early modern humans contained sequences corresponding to those of Neandertals.

A statistical analysis of the genetic finds indicates that Neandertals made no more than a 25 percent contribution to early modern-human mitochondrial DNA, Serre says. To make that estimate, the scientists assumed that Stone Age H. sapiens consisted of only 10,000 individuals capable of breeding.

It's more likely that the spread of modern-humans out of Africa stimulated population growth beyond that number, in which case Neandertals' influence on people's genetic makeup would have been even less, Serre holds.

Serre's results suggest either that Neandertals had no genetic effect on ancient humans or that a small Neandertal population interbred with a large H. sapiens population, proposes John H. Relethford of the State University of New York at Oneonta.

Milford Wolpoff of the University of Michigan in Ann Arbor disagrees. Wolpoff, who argues that H. sapiens evolved in Africa, Asia, and Europe over the past 2 million years, says that recent research indicates that mitochondrial DNA is changed by natural selection rather than by random mutation, as Serre's team assumes. Natural selection would obliterate the material's value as a molecular clock, useful as a means to reconstruct human evolution, Wolpoff says.

Serre isn't convinced that mitochondrial DNA responds to natural selection. Like Wolpoff, though, he eagerly awaits results from evolutionary studies of DNA from the cell's nucleus.