One of the more interesting things to come out of all the cheaper, more robust DNA sequencing technology has been our deeper understanding of human history. Now that we can get a pretty good read from 30,000 year old Neanderthal DNA, we can tell that humans and Neanderthals had babies together. The key to figuring this out was the fact that some human ancestors had more opportunity to hook up with Neanderthals than did others.
See, around one million or so years ago, there wasn’t a lot of difference between a human and a Neanderthal. We were one big happy species living in Africa. Then, maybe 800,000-900,000 years ago, two of these groups became separated from one another. Each group built up changes in their DNA so that they gradually became two separate species. One group stayed in Africa and became our ancestors while the other group, who became Neanderthals, left Africa through the Middle East and spread across Europe and Asia.
Now at this point these two groups still shared a lot of the same DNA. After all, they started from the same pool of DNA in the relatively recent past. If they all came back together and started having kids together, we wouldn’t be able to easily tell there had been interbreeding. We would see a lot of Neanderthal DNA in human DNA, but it would be very tricky to distinguish DNA that we originally shared from recently reintroduced Neanderthal DNA.
The reason we can more easily see that there was interbreeding is because of another group of Africans that decided to leave Africa via the Middle East. These folks left somewhere between 120,000 and 60,000 years ago and stumbled across the Neanderthals already out there in the world. This group of Africans eventually supplanted the Neanderthals and went on to become Europeans, Asians, Native Americans and everyone else out there (except for the Africans who stayed behind).
A couple of years back, scientists were able to get a good read of a few Neanderthals’ DNA. They then compared this DNA to lots of other people’s DNA. What they found was that Africans and Neanderthals shared less DNA than did Neanderthals and everyone else. They interpret this result to mean that Africans and Neanderthals never had the chance to breed while the ancestors of the rest of humanity and Neanderthals did.
What this all means is that we can see the Neanderthal in some of us because only some of our ancestors had kids with Neanderthals. If this weren’t the case, then we wouldn’t be able to easily tell that interbreeding had happened.
Of course now that we have good evidence for this, people are trying to link weird, mostly European traits with Neanderthals. For example, there is a whole lot of chatter out on the web that Rh negative blood comes from Neanderthals. While this may be true, there isn’t any real evidence to support it. That part of Neanderthal DNA has yet to be sequenced. (Although we do know that at least some of them had O type blood which isn’t surprising since it probably arose at least one million years ago when we were all still one big happy species.)
Another idea is that red hair came from Neanderthals. Again, maybe, but there isn’t any evidence yet to support it. At least one Neanderthal did have red hair but he got it from a DNA difference not seen in people. In other words, there is evidence to support that Neanderthals had red hair too but not that we got it from them.
What our ancestors apparently got from Neanderthals (besides a good time) was immunity genes to help them survive. When our ancestors burst out of Africa, they encountered germs they had never seen before (think Native Americans and smallpox). The Neanderthals had adapted to these germs and so were more resistant to them. The human-Neanderthal hybrids (Humanderthals?) would have survived better than the humans meaning they were more likely to survive. The rest is the history of people living outside of Africa.