Ancient DNA

My brother loaned me a copy of WHO WE ARE AND HOW WE GOT HERE  by David Reich, a scientist in the field of ancient DNA. It's a slow read, but here is some of what I retained.

The new ability to read ancient human DNA is a breakthrough comparable to the discovery of carbon dating. Suddenly many questions have answers.

About 70,000 years ago there were at least four or five human races, which then mixed together, dominated by one race, the modern human, which is what we all are today, with traces in our DNA from the other early human races. The size of these DNA traces is tiny, much smaller than reported by the commercial companies that provide us with personal data. 

Over the last 50,000 years there have been constant migrations and mixings, and that process continues today. Our European ancestors, for example, did not even reach Europe until about 5,000 years ago. The Japanese are 80% from Korea and 20% Ainu. There is a small group of humans in  the Amazon basin whose ancestors settled there before the Indians arrived from Asia 15,000 years ago  This group came from southwest Pacific islands and/or Australia.

People today share many genetic features; the DNA differences are too slight to justify sorting people into races. 

Small differences do exist between this group and that group, which can have medical consequences. It is true that natural selection in different environments creates subgroups with slightly different abilities. Perhaps Jamaica produces exceptionally fast runners. That does not mean that the average Jamaican is a fast runner. Jamaica may also produce exceptionally slow runners, who go unnoticed. But differences between groups will always exist. And then the groups migrate and mix together again in Los Angeles.