For those who are unfamiliar, the Toba eruption was a super volcanic event that occured roughly 73,000 years ago and was thought to have caused a sharp decline in human populations all over the world. So sharp was this decline, as theory goes, that it reduced the total worldwide human population to a mere 10,000. An extremely small bottleneck.

Over the past few years, scientists have begun using this theory to "explain" why there is "so little genetic varation between human populations".

In summary, we have not been able to find any evidence to support the hypothesis that the Toba super-eruption of 73.5 Ka caused a bottleneck in the human population. The direct effects of the eruption were fairly localised, and at the time probably had a negligible effect on any human population in Asia, let alone Africa. Genetic evidence indicates that the Pleistocene human population bottleneck was not hour-glass shaped, but rather an up-side down bottle with a long neck. Modern humans at that time were adaptable, mobile, and technologically well-equipped, and it is likely that they could have dealt with the short-term environmental effects of the Toba event. Finally, we have found no evidence for associated animal decline or extinction, even in environmentally-sensitive species. We conclude that it is unlikely that the Toba super-eruption caused a human, animal or plant populationbottleneck.

Source (http://www.dienekes.com/blog/archives/000345.html)