Ancient humans may not have interbred with Neanderthals, after all

If you hail from outside of Africa, there's a decent chance that you share as much as 4% of your DNA with a long-extinct lineage of Neanderthals. Many scientists agree that this small percentage of shared genetic information is evidence that humans and Neanderthals interbred with one another in Europe tens of thousands of years ago.

But what if the consensus is wrong? What if those genetic remnants were inherited not from Neanderthals, but from a common ancestor, an evolutionary forebear to both humans and Neanderthals? A newly published study by researchers at the University of Cambridge suggests it's possible.

In a study recounted in the latest issue of Proceedings of the National Academy of Sciences, a team of researchers led by evolutionary biologist Andrea Manica used computer models to simulate the last 500,000 years of population dynamics in Europe and Africa, under the assumption that there were two major migrations out of Africa. In the first migration, the common ancestor to modern humans and Neanderthals spread throughout Africa and Europe. In the process, their vast geographic distribution gave rise to genetic heterogeneity, not only between the continents, but within them, as well.

The model then posits that between 300 and 350 thousand years ago, the European populations became separated from the African ones. Those populations in the European range evolved into Neanderthals, the African populations into anatomically modern humans. Crucially, however, genetic heterogeneity within each continent was maintained. As a result, modern human populations in northern Africa may have retained chunks of their ancestral DNA that those in the southern reaches of the continent did not, genetic information that they shared with the recently evolved Neanderthals of Europe. When the second migration from Africa took place between 60,000 and 70,000 years ago, these northernmost African populations would have carried these genetic remnants into Europe with them; while those populations lacking the ancestral code remained in Africa.

"Based on common ancestry and geographic differences among populations within each continent, we would predict out of Africa populations to be more similar to Neanderthals than their African counterparts," explains Manica in a press release. "But this pattern was attributed to hybridisation... 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 hybridisation."

In other words, Manica and his team contend that the ancestral genetic traces that we find in today's Europeans and Asians are NOT necessarily dependent on Neanderthals and modern humans interbreeding tens of thousands of years ago. But here's the thing: just because Neanderthals and humans didn't have to interbreed, doesn't mean they didn't do it; as Manica himself points out, "it's difficult to conclusively prove [that hybridization] never happened." What's important to take away from this study, therefore, is that there is more than one plausible explanation for the genetic traces that we see in today's modern humans, and nowhere is it written that they must be mutually exclusive.

The best analysis of this study's impact that I've read thus far comes from University of Washington geneticist Joshua Akey, in an interview with MSNBC's Alan Boyle. "To me, I don't think it's a case of either-or," explains Akey, "I think that both things can be going on... introgression [interspecies breeding] is a sexier mechanism, but even if it turns out to be a case of archaic population structure, that still tells us something about our past that we didn't know before." He continues:

Although I do think that both ancestral population structure and introgression are not mutually exclusive events, [recent research shows] pretty compelling evidence that introgression of Neanderthal lineages into anatomically modern humans occurred. Thus, the real debate moving forward will be about the relative contributions of these two processes.

The researchers' findings are published in the latest issue of PNAS.