Two years ago, researchers made the startling announcement that genetic similarities between modern humans and Neanderthals suggest the two species interbred. But common ancestry could also account for the shared DNA, according to new research.

Since 2010, a number of studies have suggested that Neanderthals, previously considered our close evolutionary cousins, are actually the ancestors of most humans alive today. After successfully sequencing the extinct hominins’ genome, scientists at the Max Planck Institute for Evolutionary Anthropology announced that Neanderthal gene fragments make up 1 to 4 percent of the DNA in people of European and Asian origin. Researchers theorized that modern humans venturing out of Africa interbred with Neanderthals, who had preceded Homo sapiens along those migration routes tens of thousands of years earlier. As a result, according to this theory, Neanderthal DNA is virtually absent from the genomes of today’s Africans.

The case for human-Neanderthal interbreeding, known as the “hybridization” hypothesis, has contributed to an image makeover enjoyed by the stocky hunters in recent years. Once, science and popular culture viewed Neanderthals as intellectually inferior to Homo sapiens and lacking in anything resembling technology or a symbolic culture. Now, however, experts believe Neanderthals crafted complex tools, buried their dead, spoke a language and expressed themselves artistically.

Published today in the journal Proceedings of the National Academy of Sciences, a new paper offers an alternative to the hybridization theory, raising doubts about whether modern humans and Neanderthals interbred. Led by Andrea Manica of the University of Cambridge, its authors suggest that a common ancestor shared by both species could explain the genetic similarities. Between 350,000 and 300,000 years ago, they write, human ancestors living in Africa separated into two isolated populations. One group developed into Neanderthals, while the other evolved into modern humans. Among modern humans, some—those who would go on to settle Europe and Asia—remained genetically closer to their Neanderthal cousins.

Manica and his colleagues assessed their model by examining the genetic makeup of living human populations, hoping to determine how much variation can occur within a single continent. “Our work shows clearly that the patterns currently seen in the Neanderthal genome are not exceptional, and are in line with our expectations of what we would see without hybridization,” Manica explained in a statement. “So, if any hybridization happened—it’s difficult to conclusively prove it never happened—then it would have been minimal and much less than what people are claiming now.” If interbreeding took place, in other words, it was not widespread enough to account for modern humans’ Neanderthal DNA.