Neanderthals and modern humans shared an ancestor that lived about 660,000 years ago, according to scientists who have pieced together the first complete sequence of maternal DNA from humanity's closest cousins.
The DNA evidence also verified that the two species did not interbreed during the 10,000 to 20,000 years they coexisted in Europe and western Asia after humans migrated there from Africa. The last of the Neanderthals died out about 30,000 years ago, though some scientists speculate that at least a few of their genes live on in humans.
"Neanderthals made no lasting contribution to the modern human [maternal] DNA gene pool," a team of German, American, Croatian and Finnish researchers wrote in Friday's edition of the journal Cell.
The team focused on mitochondrial DNA, a relatively short string of 16,565 As, Ts, Cs and Gs that spell out 13 genes for controlling the energy sources of cells. Unlike nuclear DNA, which is unique for every person, mitochondrial DNA is passed virtually unchanged from mother to child.
Members of the research group are engaged in a two-year effort to decode the roughly 3 billion letters of nuclear DNA contained in a 38,000-year-old Neanderthal bone fragment discovered in a Croatian cave.
In the process, they collected enough maternal DNA to sequence that genome with a high degree of certainty, said lead author Ed Green, a postdoctoral scientist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
Comparison of the Neanderthal sequence to 10 human sequences suggests that the species diverged 520,000 to 800,000 years ago -- earlier than the 400,000 years scientists had previously estimated using fossil finds.
Scientists have sequenced maternal DNA from thousands of people around the world to study the history of human migration out of Africa. All of them are distinct from the Neanderthal version, Green said.
Most scientists accept the view that there aren't any Neanderthal genes in the human genome, but evolutionary geneticist Jeff Wall of UC San Francisco said that only "large amounts of high-quality Neanderthal nuclear DNA sequences" will resolve the issue once and for all.