This is the sixth of a series of posts I will be doing on human evolution. I apologize for not being able to link to the prior ones. As always, please leave any questions you’d like to have answered.
In my previous post I introduced us to humanity’s cousins, the Neandertals, a species of hominid that lived until about 30,000 years ago. These were our closest relatives, but they went extinct. Honestly, we’ll probably never know why it was our species that survived and the Neandertals that did not.
I began writing these posts because of the publication of a major scientific breakthrough a few weeks ago: the Neandertal genome was sequenced. This will be remembered as one of the greatest achievements in biology; the task would have been impossible a mere five years ago. The DNA that was sequenced was 30,000 years old and was heavily degraded. Most of the DNA collected actually belonged to bacteria and other microbes. The scientists at the Max Plank Institute did something truly remarkable.
As I’m sure many of you know, this breakthrough led to quite a lot of interesting information about our own species. The most important thing was the identification of a few dozen genes that are present in our line, but not in Neandertals. These tell us what makes our species unique and, now that they are identified, can be researched to help further our understanding of human nature and, more importantly, human diseases.
But the thing most people have gotten excited about is the fact that there is clear evidence for interbreeding between Neandertals and humans about 80,000 years ago. At this time, the Neandertals were driven out of Europe into the Middle East by an ice age. At the same time, a group of humans had expanded into the Middle East from Africa. The fact that interbreeding occurred is not particularly surprising; 400,000 years of divergence is a relatively short amount of time on evolutionary scales.
The interesting, and somewhat inexplicable, finding is that these Neandertal genes are only found in non-Africans. It had been thought that the humans in the Middle East had returned to Africa before a group that may or may not have been related left about 60,000 years ago. Whether this model is overturned remains to be seen; it could be that there was just one migration out of Africa 80,000 years ago. Or the descendents of the first migration could have returned to Africa before leaving again.
For non-Africans, anywhere from 1 to 4% of our genome consists of genes from Neandertals. What does this mean? Does this make us less than completely human? Most scientists would respond that that is not an interesting question, which is a sentiment I agree with. But if you are troubled, bear in mind that human genetic diversity has roots far deeper in the past than the actual existence of our species; incidentally, it’s about 400,000 years, the same length as our divergence from Neandertals.
We don’t know why there was no gene flow from humans to Neandertals and we don’t know why there was no interbreeding when the two species came back into contact about 40,000 years ago in Europe. Unfortunately, these are questions we’ll probably never be able to answer; behavior doesn’t exactly fossilize. And while this is frustrating, the important point to remember is that we now know a lot more about human history than we did two months ago; each new discovery sheds more light on our species’ story.