Whether modern humans muscled out their Neandertal cousins or interbred them out of existence has been a nagging question for anthropologists. But a new genetic study bolsters the case for conquest by war rather than love. It suggests that Neandertal females rarely if ever mated with early humans.
Homo neandertalensis inhabited Europe, the Middle East, and Asia roughly 150,000 to 30,000 years ago, while Homo sapiens lived in those areas (and others) as far back as 200,000 years ago. Some researchers have proposed that interbreeding with early humans weakened the Neandertal line and eventually caused it to die out. If genetic mingling did occur, there should be some hints of Neandertal DNA in the human gene pool. Unfortunately, those hints are scarce, because any surviving DNA in fossils of Neandertals and early humans is usually severely degraded.
An international research team led by Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, gained access to remains of 24 Neandertals and 40 early humans--all roughly 40,000 years old--from Germany, Russia, and Croatia. The team determined that four Neandertal and five human remains were likely to contain intact mitochondrial DNA (mtDNA), which is hardier than the DNA in cell nuclei. When the team sequenced the mtDNA, they found that sequences from the Neandertal remains were similar to previously published sequences of four Neandertals. These sequences are not known in the gene pool of Neandertal's human contemporaries or humans of today.
The team concludes, based on statistical analyses, that Neandertals' contribution to the human gene pool of 30,000 years ago could not have been more than 25%, and was probably much less. The likelihood of Neandertal-human interbreeding is low to nonexistent, the authors argue in the March issue of Public Library of Science, Biology.
Although most evidence argues against Neandertals interbreeding with early humans, the sample size is still too low to be definitive, says molecular archaeologist Carney Matheson of Lakehead University in Ontario, Canada. "It's important to remember that mtDNA, passed on from mother to child, represents only half the story of parental lineage," he notes. The contribution of male Neandertals breeding with humans would not be detected in mtDNA. "Until our technology advances to the point to where we can recover nuclear DNA for analysis, the issue of interbreeding will remain open," says Matheson.