As I said, scientists have looked at Neanderthal mitochondrial DNA (mtDNA) because there is a lot of it in fossils. But to really figure out if our ancestors mixed with Neanderthals, we probably need to look at nuclear DNA.
And two groups have done just that. These groups looked at up to one million letters of DNA of Neanderthals (out of 6 billion or so).
One group concluded that there was no mixing. The other group concluded that there might have been. To really resolve this, more Neanderthal DNA will need to be sequenced.
Is it possible to read more? To understand this, let's look at how ancient DNA is sequenced.
A few years ago looking at ancient nuclear DNA was impossible. DNA that has been sitting around for 40,000 years takes a bit of a beating.
The DNA tends to end up in little chemically damaged pieces that are hard to deal with by conventional technologies. And there is a whole lot of other DNA contaminating the sample from all of the beasts that have lived there for all of those years.
Two recent technological advances have made looking at this DNA possible. The first is the ability to get lots of DNA from very little.
Now, we have been able to do this for awhile using a technique called PCR. PCR uses special proteins to let you make lots of the DNA you're interested in from very little starting DNA (click here
to learn more about PCR).
But while PCR is great, it still needs more DNA then we can get from Neanderthal DNA. In fact, one estimate I saw said that regular old PCR would need pounds of Neanderthal fossils to get enough DNA. There just aren't enough high quality fossils to make this happen.
New methods have become available that allow scientists to get DNA from smaller, more fragmented samples. One example is from a company called 454 Life Sciences (click here
to learn more). The 454 method takes all of the pieces of DNA in a sample and let's you read it.
So now you have lots of DNA information, some of which is Neanderthal. And some of which is bacterial or fungal or who knows what else.
How do you separate out the Neanderthal DNA? With computers and fancy computer tools (this process is called metagenomics).
What scientists do is use the DNA information about known animals to pick out the DNA they're interested in. For example, scientists recently sequenced a mammoth's DNA.
They were able to pick out mammoth DNA by looking for DNA that was similar to an elephant's. And to rule out DNA that looked more bacterial or that looked like human contamination. They also could rule out any DNA that was too intact (after thousands of years, the DNA tends to be in very small chunks).
This is the sort of thing that has been done with Neanderthals. And the results are so cool!
It looks like we split from Neanderthals around 500,000 years ago, just as the mtDNA suggested. Humans and Neanderthal are around 99.5% the same. The similarity between any two random people is 99.9%. And between a chimp and a human is 98.7%.
But did Neanderthal and Cro-Magnon mix? One group that looked at ~60,000 bases concluded there probably wasn't any mixing. The second group looked at one million bases and concluded there may have been.
Interestingly, the second group's data was consistent with male Neanderthals interbreeding with female Cro-Magnon. This is just the sort of liaison that would be invisible in mtDNA.
So we don't know the answer yet because there is still too little of the DNA sequenced to get a definitive answer. A lot more sequencing will be needed to nail this down.
But there may be light at the end of the tunnel. As more and more DNA is sequenced from many different fossils, the truth should be revealed. Did we wipe them out? Or do Europeans have a bit of Neanderthal in them.