I remember seeing this thing on Discovery or some such about a genetic bottle-neck a LONG time ago, 70,000 years ago. Society and human populations has cycles too, according to natures whims.... It has a high probablility of having happened before and a VERY high possiblility of happening again.
From Wikipedia:
DNA evidence suggests that humans today are a legacy of a population bottleneck which occurred 70,000 years ago. This would have had the result of limiting the overall level of genetic diversity in the human species, possibly by a large amount. The evidence that all living humans are descended from fewer than ten thousand people alive at that time comes both from mitochondrial DNA coalescence, and the relatively small variations in the human Y chromosome.
One theory about this bottleneck is the Toba catastrophe theory, positing that the human population was reduced to a few thousand individuals when the Toba supervolcano in Indonesia erupted and triggered a massive environmental change.
In 2000, a Molecular Biology and Evolution paper suggested a transplanting model or a 'long bottleneck' to account for the limited genetic variation, rather than a catastrophic environmental change. (See "Population Bottlenecks and Pleistocene Human Evolution".)