Confirmation of Human Neanderthal Interbreeding

 by Stephen Luntz

Photo credit: ICHTO. Come on, isn't it clear why humans interbred with Neanderthals like this one

A new study claims to have settled one of the great debates of human evolution, concluding that modern humans interbred with Neanderthals.

It is known that people whose recent ancestry is in Africa have fewer genes in common with Neanderthals than those whose origins lie in Europe or Asia.

However, there are two explanations for this. The more obvious one is that humans and Neanderthals occasionally got frisky, and that the genetic relationship was close enough that some of the offspring survived to pass on their genes to us. Call it the Clan of the Cave Bear scenario.

The alternative is that Neanderthals split off from a sub-population of our common ancestors, and that this sub-population also provided most or all of the people who subsequently left Africa. 

The debate between these two has gone back and forth. Like most matters in the field of early human origins it has become rather heated at times. Previous methods of genome analysis have struggled to distinguish between the two.

Now a paper in Genetics may have found a way through the issue. “Our approach can distinguish between two subtly different scenarios that could explain the genetic similarities shared by Neanderthals and modern humans from Europe and Asia," said co-author Dr Konrad Lohse of the University of Edinburgh. 

The technique was originally developed for comparing related insect species and subspecies, as well as isolated island populations of pigs in South East Asia."Because the method makes maximum use of the information contained in individual genomes, it is particularly exciting for revealing the history of species that are rare or extinct," said Lohse.

Lohse compared four genomes, those of an African, someone from outside Africa, a chimpanzee and Neanderthal fossil. He split the genome into short blocks and created estimates of the likelihood of the two scenarios for each given the similarities of the two modern humans compared to the other species. The method proved robust for different block lengths. The paper states, “Our analysis allows us to conclusively reject a model of ancestral structure in Africa and instead reveals strong support for Neanderthal admixture in Eurasia.”

The study also found a higher proportion of Neanderthal genes in non-African humans than previous research (3.4-7.3%, rather than 1.5-2.1%). However, Lohse says different methods will produce somewhat different results on this question.

Lohse estimates the time for the divergence between humans and Neanderthals as 329-349 thousand years ago, and between those who left Africa and those who stayed behind at 122-141 thousand years. His work is consistent with recent studies suggesting there was also interbreeding amongst humans and Neanderthals with Denisovans and another mystery relative as well, although many estimates of the dates for leaving Africa are substantially later.

Genetics Editor-in-chief Mark Johnston praised the work for not only putting the controversial question to rest, but opening a path to understanding other species' evolutionary history. 

Settling the question is important for our understanding of our own history. It also arguably tells us something philosophically significant about or place in nature and our capacity to relate to those different from ourselves.