Expert: John Shea - 8/10/2011

Question
I hope this is an appropriate question for your expertise. I have been hearing about the lack of variation in the human genotype resulting from a worldwide event--probably volcanic-- that happened 75K years ago. It was alleged in a documentary I saw that there is more variation in the DNA of a single troupe of chimps than there is on the entire scope of the human genome existing today.

So then my question is, why does this bottleneck occur apparently only for the human genotype? Wouldn't whatever catastrophe killed off humans also kill other mammalian species just as well?

Answer
Hi Eleanor,
It is on the periphery of my expertise, but I'll give it a try.
Chimpanzees live only in the forests of equatorial Africa and are largely vegetarian/insectivores. As such, they might not have been as severely affected by cooler climates as humans, who were living all over Africa and south Asia and who were more dependent on large game.
This is a good question, and if I were one of those arguing in favor of a "bottleneck', I would expect to find evidence for it in the genetic record for large Asian carnivores, as well as among big carnivores in Europe, like the Neanderthals, too.
For what it is worth, I don't think Toba caused continent-wide wipeouts, but rather localized population extinctions and population displacements.  From an archaeological standpoint, the two look very similar to one another.  I keep a skeptical, yet open mind about this bottleneck business.  Where I work in the Middle East, there is a clear change in the archaeological record at exactly 75 Ka, but where I work in eastern Africa, there is no such inflection point in the archaeological record. Different regions, different effects.
Cheers,
John Shea