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Sharing a common ancestor, rather than sharing a bed, may be a better explanation for the genetic traits shared by humans and Neanderthals, a U.K. study found.

Genetic similarities between the two species are unlikely to be the result of human-Neanderthal sex, known as hybridization, during their 15,000-year co-existence in Europe, researchers from the University of Cambridge wrote in a paper published today in the journal Proceedings of the National Academy of Sciences.

People living outside Africa share as much as 4 percent of their DNA with Neanderthals, a cave-dwelling species with muscular short arms and legs and a brain slightly larger than ours. The Cambridge researchers examined demographic patterns suggesting that humans were far from intimate with the species they displaced in Europe almost 40,000 years ago. That conflicts with recent studies that found inter-species mating probably occurred.

“The levels of hybridization people have spoken about is too high,” said Andrea Manica, a researcher from Cambridge University who authored the study, in a telephone interview. “If any hybridization happened, then it would have been minimal.”

Neanderthals were driven into extinction by humans who were more efficient at finding food and multiplied at a faster rate, evidence shows.

Common Ancestry

A previous study in 2010 suggested that interspecies liaisons near the Middle East resulted in Neanderthal genes first entering humans 70,000 years ago. Modern non-Africans share more with Neanderthals than Africans, supporting the claim that the mixing occurred when the first early humans left Africa to populate Europe and Asia.

The existence of a 500,000-year-old shared ancestor that predates the origin of Neanderthals provides a better explanation for the genetic mix, the Cambridge scientists said. Diversity within this ancestral species meant that northern Africans were more genetically similar to their European counterparts than southern Africans through geographic proximity, Manica said. This likeness persisted over time to account for the overlap with the Neanderthal genome we see in modern people today.

“Differences between populations can be explained by common ancestry,” said Manica, whose research was funded by the Biotechnology and Biological Sciences Research Council and the Leverhulme Trust. “Northern Africans would be more similar to Europeans and ancient similarity stayed because there wasn’t enough mixing between northern and southern Africans.”

Population Diversity

Population diversity, known as substructure, can’t explain data on the shared genes, said David Reich, a professor of genetics at Harvard Medical School in Boston and author of the 2010 study.

“We have ruled out the possibility that ancient substructure can explain all the evidence of greater relatedness of Neanderthals to non-Africans than to Africans,” Reich said in an e-mail.

Hybridization between Neanderthals and humans can never be disproved entirely, according to Manica, who questioned what inter-species breeding may mean for the human genome today.

“The further we bring down the level of hybridization, the more unlikely it is it would have shaped the ways modern humans evolved,” Manica said.