A few years ago, DNA evidence suggested that humans and their close evolutionary relatives might have rolled around in the rocks, so to speak. When they sequenced the Neanderthal genome, they found that one to four percent of our genes today were derived from Neandertals.
But the reality of just how often humans and Neanderthals got it on is murky. New research suggests that perhaps it didn’t happen that often after all. Or perhaps it did. Two papers, each claiming quite different results, were released recently. A paper in PNAS suggested that our ancestors never mated with Neanderthals, while a paper scheduled to be released by PLoS ONE argues strongly that they did.
The argument against interbreeding goes like this, as summarized by Ars Technica:
The paper raises the possibility, recognized in the original Neanderthal genome work, that the pre-modern African population was structured. So, the group that gave rise to Neanderthals could have a genetic signature that was rare elsewhere in Africa. And, if that same population gave rise to the modern humans that left Africa, it could leave them with that same genetic signature. Thus, Neanderthals and non-Africans would end up looking more similar than we’d otherwise expect.
When the authors of that paper built a model assuming that the population in Africa had structure – that is, different groups of people lived in different places and didn’t interact – they were able to predict an outcome consistent with the current human genome without any interbreeding.
Of course, there are always the weird ones in the population. The paper suggests that perhaps some humans did breed with some Neanderthals. But those offspring were almost never viable – which would explain why we find Neanderthal DNA in our main genome but not in our mitochontrial DNA. Ed Yong for Discover Magazine explains:
Currat and Excoffier suggest that either modern humans and Neanderthals didn’t have sex very often, or their hybrids weren’t very fit. They favour the first idea. According to their model, it would only have taken between 197 and 430 liaisons between ancient humans and Neanderthals to fill 1-3 percent of modern Eurasian genomes with Neanderthal DNA. Considering that the two groups probably interacted for 10,000 years or so, it would have been enough for one human to sleep with one Neanderthal every 23 to 50 years.
It’s hard to tell exactly who did what to whom, because scientists are working the fragile, and difficult to extract, genetic record. They have to guess how Neanderthals and humans might have actually interacted and met each other. Anthropologist Chris Stringer suggests that humans and Neanderthals probably met not all at once, but in waves. Yong says:
In the first waves, small groups of modern humans would have met large groups of Neanderthals. In later waves, the situation was reversed. The encounters between the two groups would have been very different across many thousands of years. Stringer asks, “The remaining question is whether those earliest modern waves survived to contribute their genes (including Neanderthal ones) to the succeeding waves, or was the slate essentially wiped clean each time?”
Essentially, researchers will need more data and a better understanding of how we lived and moved to really know just how often we hooked up with our hairy relatives.
More from Smithsonian.com:
Humans and Neanderthals Interbred
New Study Suggests Humans, Not Climate, Killed Off Neanderthals