New research suggests that interbreeding between early humans and Neanderthals was not the rarity scientists had once thought it was, but rather a more regular occurrence over several thousands of years.
In a study published Thursday in the journal Nature Ecology and Evolution, a pair of researchers from Temple University explained how the interbreeding started about 75,000 years ago -- not long after early humans moved out of Africa and into Europe and certain parts of Asia. There, the early humans first encountered Neanderthals. As noted by History, earlier studies had suggested that most modern humans have about 2 percent Neanderthal DNA as a result of interbreeding between the two species.
While a number of papers had suggested in recent years that modern humans, with the exception of those whose ancestors never left Africa, got their Neanderthal DNA from occasional encounters with the extinct hominid species, the researchers behind the new study stated otherwise. Said research team suggested that the early humans who populated Eurasia after leaving Africa interbred with Neanderthals at "multiple points in time" over a span of 35,000 years.
As quoted by the Daily Mail, study co-author Joshua Schraiber explained that there might have been "much more" interbreeding between early humans and Neanderthals in Eurasia over that timeframe.
"Some of the fantastical aspects come from a lack of clear definition of species in this case. It is always very hard to know if an extinct group constituted a different species or not," Schraiber continued.
"My guess is that any time two different human groups lived in the same place at the same time for a while, they probably had some sort of breeding contact."Given the theory that East Asians have about 12 to 20 percent more Neanderthal DNA than Europeans, Schraiber and co-author Fernando Villanea performed computer simulations to determine whether there might have been a substantially greater number of interbreeding episodes between early humans and Neanderthals.
The researchers' AI-based techniques revealed that modern humans have varying percentages of Neanderthal DNA due to the frequent interbreeding that took place between Neanderthals, East Asians, and Europeans -- which Schraiber described as proof of a "more complex" series of interactions involving our ancestors.As further proof of the frequent interbreeding theorized by the Temple researchers, the early human mandible known as Oase 1 was mentioned as a more recent example. According to Phys.org, this fossil was first discovered in 2002 in Romania -- and was believed to have had a Neanderthal ancestor removed by about four to six generations.
"It had recent Neanderthal ancestry. These fossils are about 37,000 to 38,000 years old -- so at least some interbreeding must have been going on as recently as then," Schraiber said.
The new study, however, comes with its share of limitations. According to GenomeWeb, the Temple researchers' model took the assumption that Neanderthal ancestry in present-day humans is neutral -- as opposed to being deleterious, or harmful -- and did not take into account the possibility that Neanderthal ancestry in today's East Asians might have actually been associated with the Denisovans, a recently discovered species of ancient hominids.
Despite those limitations, Max Planck Institute for Evolutionary Anthropology scientist Fabrizio Mafessoni explained in a commentary on the study that the findings mesh with the "emerging view of complex and frequent interactions" between various hominid groups.