[teragram (OP)]

It seems that human DNA (except native African) contains a small percentage of Neanderthal DNA, the suggestion being that humans mated with that species in our early history. Could this be attributable instead  to the presence of  Neanderthal DNA being the result of both species having the same ancestors?
 

[evan_au]

No.
Two major reasons:
1) A child of a Neanderthal and Human would have a complete Neanderthal chromosome paired with a complete Human chromosome. As generations pass by, the large pieces of Human and Neanderthal DNA are broken up into smaller and smaller pieces due to crossover during production of egg and sperm.
2) Humans and Neanderthals share almost all their genes - but both the Human and Neanderthal genomes have unique changes that differ from each other (and from their most recent common ancestor - but that must be inferred, since we haven't sequenced their common ancestor). So by looking a the DNA of a modern Human, you can identify certain snippetts as having uniquely Neanderthal sequences, while other people have a uniquely Human variant of that same stretch of DNA. (In fact, one person could have both a Human and Neanderthal version of the same DNA segment).

See: https://en.wikipedia.org/wiki/Interbreeding_between_archaic_and_modern_humans

[chris]

Both species definitely share ancestors, which is why our DNA is so similar in the first place, but Neanderthals and anatomically modern humans diverged from one another 200-300,000 years ago. Current thinking is that the last ancestor we both share is Homo erectus, which was spawned about 1.8 million years ago. Neanderthals disappeared about 30-40,000 years ago.

Because they are relatively recently extinct, Neanderthal remains are a good prospect for DNA recovery, and about 10 years ago scientists completed the Neanderthal genome project; this is the Neanderthal equivalent of the Human Genome Project that decoded all 3 billion letters of the human DNA code.

Armed with this sequence, and because we are so closely related, it's possible to align the Neanderthal DNA makeup with the modern human genome and to highlight areas that are shared and areas that are distinct.

Areas of the genome that are shared by both were inherited from an ancestor to both Neanderthals and us and will be universally present. Similarly, sequences that are specific to one species or the other - having evolved after the two species split - will be exclusively present in each species.

But there will also be regions of the genome that are variably shared in human and Neanderthal populations, indicating gene mixing. This is the example you highlight of some human populations carrying Neanderthal sequences but not others. It is also the evidence that tells palaeontologists and geneticists that the two species interbred, which is how these gene sequences came to be variably shared.

[Georgia]

This interview might be of interest to the OP!
https://www.thenakedscientists.com/articles/interviews/unlocking-your-ancestry-genetic-test [nofollow]

[Halc]

It seems that human DNA (except native African) contains a small percentage of Neanderthal DNA

It is actually only a very small group in an isolated tribe in Africa that has remained free of Neanderthal DNA.
If the Germans wanted a pure race, that tribe has a far better claim to it than do the Aryans.  Fair skin and hair (especially red hair), blue eyes, and all that are traits from the Neanderthals, who evolved earlier on to colder climates.