In the past, when people traveled less and marriages were likely to be between neighbors, races tended to develop and remain in geographic localities.
Every individual has two parents, four grandparents, eight great-grandparents, and so on.
At first glance, it appears that each individual had billions of ancestors. The only problem is, the world did not then contain that many people. The early ancestors of all humans, if known, would appear as the progenitors of many different lines of descent. This is called in-breeding. In-breeding maintains a degree of separation among subpopulations and also a degree of similarity within related populations.