For decades, human evolution researchers have debated whether Neandertals and modern humans interbred. Most scientists have come down on the side that any romances between these hominid cousins must have been fleeting at best. But a new study suggests that a few of these passing dalliances might have had a major impact on the evolution of the Homo sapiens brain. If so, Neandertals, although long extinct, may have left humanity a lasting genetic gift.
Some anthropologists have argued that a handful of hominid skeletons show features of both Neandertals and modern humans (Science, 11 February 2005, p. 841). But so far sequencing of Neandertal ancient DNA has turned up no signs of such interbreeding (Science, 11 July 1997, p. 176). As a result, most researchers have considered the two species genetically separate.
Now, University of Chicago geneticist Bruce Lahn and his colleagues report evidence that at least one gene might have bridged the evolutionary divide. Lahn's team analyzed the origins of the gene microcephalin, thought to be involved in regulating brain growth. Last year, the team reported in Science that a particular variant of the gene, now present in 70% of the world's population, arose about 37,000 years ago and quickly spread around the globe. Apparently the variant, known as haplogroup D, was favored by natural selection, although no one is sure of its function (Science, 9 September 2005, p. 1662).
In the new study, published online this week in the Proceedings of the National Academy of Sciences, Lahn and coworkers analyzed microcephalin genes from 89 people from around the world. They found that haplogroup D differed in so many bases from other versions of microcephalin that it must have arisen very early, probably a little more than 1 million years ago, according to statistical tests. Yet it appeared in modern humans only 37,000 years ago.
The Lahn group concluded that the most likely scenario was interbreeding between prehistoric modern humans and a now extinct hominid that carried haplogroup D--most likely Neandertals. The haplogroup was probably beneficial enough to spread quickly in modern human populations, says Lahn. But he's not sure what advantage it offered. Because most researchers agree that Neandertals were not as cognitively advanced as modern humans, Lahn and his coauthors suggest that the haplogroup might have made Homo sapiens better able to adapt to the Eurasian environments that Neandertals had occupied long before modern newcomers arrived.
Ancient DNA pioneer Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, says that this new work is "the most compelling case to date for a genetic contribution of Neandertals to modern humans." Indeed, Pääbo says, he will now search for the haplogroup D variant of microcephalin in his own studies of the Neandertal genome.