Toba Catastrophe Theory is the idea that a population bottleneck in humanity's past, which is inferred from gene analysis, was caused by a supervolcano eruption 75,000 years ago on Lake Toba is what is now Indonesia. The Toba Catastrophe Theory was first proposed in 1998 by Stanley H. Ambrose of the University of Illinois at Urbana-Champaign. According to gene analysis, human genetic diversity is actually quite low in comparison to similar species, and all humans alive today are descended from a population of 1,000 - 10,000 breeding pairs that lived 50,000 - 150,000 years before the present. This is called a population bottleneck.
The Toba Catastrophe Theory is supported by geologic evidence (ice cores from Greenland) that show a substantial change in global climate around the time. Gene analysis of human hair lice even supports the idea. Anecdotal evidence from 1816, the so-called "Year Without a Winter" — caused by the colossal eruption of Mt. Tambora, also in Indonesia — shows that human populations suffer tremendous casualties during volcanic winters. The eruption of Mt. Toba released an estimated 2800 cubic km (670 cubic miles) of pyroclastic material, an 8 on the Volcanic Explosivity Index ("mega-colossal"), about 30 times greater than the greatest volcanic eruption in the last two thousand years.
According to Dr. Ambrose, the temperature worldwide dropped by about 5 °C (11 °F) in the immediate aftermath of the eruption. This was due to dust high in the atmosphere obscuring the Sun, and the effect would have lasted for six years. Ambrose further claims that the explosion of Mt. Toba was the cause of the termination of the last interglacial period — which, perhaps not coincidentally, ended around the same time as the eruption. The most severe decrease in temperature would have occurred for the first thousand years after the eruption, which is when the bottleneck is supposed to have occurred. A ninteen-thousand year glacial period, the Würm glaciation, followed.
Toba Catastrophe Theory also explains the large apparent variation of human beings in spite of our relatively low genetic diversity. Ambrose believes humans survived the bottleneck in several isolated, non-interbreeding pockets. Tropical refugia in Africa would be the few places where humans survived. The total number of breeding pairs on Earth would have never exceeded 10,000 for a millennium-long period. After the 1000 years, continued migrations would have quickly brought the breeding populations back into contact, preventing them from diverting into distinct species.
The Toba Catastrophe Theory has been treated with a mixed response by the scientific community. In general, it seems as if evidence is building up on its side. Because there would have been limited artifacts left behind by such small populations, our only hope for more knowledge are genetic and climactic studies.