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.