I do not know exactly how much Neanderhtal/Denisova DNA non-subsaharan peoples carry around today, but I have heard figures of less than two percent in Europe, up to three percent in East Asia and more than four percent in the highlands of New Guinea. Here is one picture outlining how much it may be:
According to the out of Africa theory, which I am not questioning, a small group of people, not necessarily very different from the population in Africa at the time, left Africa a few tens of thousands years ago, and shortly after they departed from Africa crossbred with a group of people that had left the african population maybe some 300 000 years earlier, the Neanderthals.
Now if we did not know that the Neanderthals had ever existed and crossbred with the non-subsaharans we would think that all the differences between human populations would be due to "Mutation, migration (gene flow), genetic drift, and natural selection" which are the four mechanisms usually mentioned and find that the subsaharans and non-subsaharans must have separated at an earlier time, when in reality much (how much?) of the difference might be due to recent crossbreding with "non-human Neanderthals".
- Can we say something about how much of the genetic difference between different ethnic groups that might be due to differential amounts of crossbreeding with Neanderthals?
2.How does the knowledge of crossbreeding with Neanderthals affect time estimates of when people left Africa and the most common recent ancestor?
Here is one diagram by Cavalli-Sforza, that some people do not like, showing genetic differences by fixation index. Maybe one could construct a diagram which would be much different, if we somehow could disregard all the Neanderthal gene contributions and get some kind of non-crossbreeding genetic distance estimate...