I watched a program on the science channel a while back and it was talking about a university that did a study of mitochondrial dna which is outside of the nucleus. Mitochondrial dna is inherited from your mom and passed down threw the females. It mutates at a regular rate which you can use to calculate certain things about the population. Anyway, they found that out of everyone they studied their mitochondrial dna was suprisingly similar.

They did some calculations and concluded around 70,000 years ago, the humans were nearly killed out and left only a few thousand alive. From these few came billions of humans meaning that genetic variations are less because of near extinction. They call it a population bottle neck big to small to big again.
They also went on connecting that time period to a super volcano that happend around that time. Well, evolution takes amillions of years and I dont think that we will notice any change in the species for quite some time.