Expert: John Shea - 7/26/2010

Question
QUESTION: After the Toba volcanic eruption, wouldn’t the remaining 10,000 people in one geographic area mean that only one race on earth would have populated? If not, why?

ANSWER: That is exactly what this Toba "bottleneck" theory implies (humans repopulate from one isolated survivor population).  If the hypothesis is correct, the location of the survivor population ought to be the region in which there is the greatest genetic diversity among living humans (as a result of the longest evolution in place).  The current lead candidates for such regions are East Africa and South Africa.
This being said, there are increasing doubts about the reality of this bottleneck event from genetics.  Some recent studies suggest the possibility that expanding Homo sapiens populations may have interbred with survivors on non-sapiens populations, like the Neandertals.  (Doubt it, personally, but it is one of several ways to interpret the Neandertal DNA data.)
Sincerely
John Shea

---------- FOLLOW-UP ----------

QUESTION: John,
Thank you very much for the quick reply.

Maybe you answered this in the second paragraph, but if the Toba bottleneck theory is true, what explains the physical differences in the historic three great races, Caucasoid, Mongoloid and Negroid?

Thanks again,
Marc

Answer
Hi Marc
The simple answer for a recent origin of some "racial" features is a combination of natural selection (for things having to do with insulation from light, heat-such as skin color and body shape) and sexual selection for other qualities (e.g., eye color, hair texture), both acting on small populations.  When you have small populations, you can
get rapid morphological differentiation due to what is called "founder effect" (genetics term).
This all being said, these "races" vary as much among themselves as they do between one another.  Few real anthropologists use these terms any longer.
Cheers,
John Shea