But a key component of the theory is that early human groups were closely related.
As human groups got bigger, more social tolerance is what they had to have.
"In early human groups, women typically collected plants and turned them into food while men hunted.
Human groups that depended on Stone Age technologies also depended on mutual caring.
All human groups are equally old, being descended from the same ancestral population.
Dr. Reich and his colleagues probed their data for the oldest evidence of human groups genetically separating from one another.
And the existing genetic data testify that known DNA variations do not respect the boundaries of human groups.
Human groups, particularly those fuelled by religious fanaticism or the twentieth-century equivalent, blind nationalism, always tend toward exclusion.
"Race is a concept we invented to categorize the perceived biological, social and cultural differences between human groups".
More likely they were aggressive acts between competing human groups, Dr. Stringer said.
In any given habitat, natural forces operate on all of the living forms, including human groups.
