[jcranmer]

Humans have been living in Africa for as long as humans have existed--let's call that a round 300,000 years. The populations around where humanity first existed therefore reflects accumulated genetic diversity for all 300,000 years.

Let's say 100,000 years ago, a small group of humans left Africa for the other places of the world. That small group of humans represent a very small fraction of genetic diversity--it's effectively genetically identical. Assuming no more admixture over the millennia, the out-of-Africa humans will get more genetically diverse. But so are the original Africans, who also have the genetic diversity they started with--they're getting diverse no less quickly than the out-of-Africans, and since they started more diverse, the entire out-of-Africa can't ever catch up.

Real genetics is of course more complicated than this simple picture, but the basic principle holds that you find more diversity the closer you get to the origin.

[alex_young]

Also - Africa is huge. It's the second largest continent after Asia, and has 1.4 billion people living there.

[twobitshifter]

And it’s a north south orientation which leads to more diversity in species and perhaps people too (my hunch)