According to the Toba catastrophe theory, the consequences of a massive volcanic eruption severely reduced the human population. This may have occurred around 70,000–75,000 years ago when the Toba caldera in Indonesia underwent an eruption of category 8 (or "mega-colossal") on the Volcanic Explosivity Index. This released energy equivalent to about , fifty times greater than the 1980 eruption of Mount St. Helens, and twenty times greater than the largest man-made explosion, the October 30, 1961 detonation of the Soviet Union's Tsar Bomba thermonuclear device. According to Ambrose, the Toba explosion reduced the average global temperature by 5 degrees Celsius (9 degrees Fahrenheit) for several years and may have triggered an ice age.
Ambrose proposes that this massive environmental change created population bottlenecks in the species that existed at the time; this in turn accelerated differentiation of the isolated human populations, eventually leading to the extinction of all the other human species except for the two branches that became Neanderthals (H. neanderthalensis) and modern humans (H. sapiens).
Some geological evidence and computed models support the plausibility of the Toba catastrophe theory. The Greenland ice core data displays an abrupt change around this time, but in the corresponding Antarctic data the change is not easily discernible. Ashes from this eruption of Lake Toba, located near the equator, should have spread all over the world.
Genetic evidence suggests that all humans alive today, despite their apparent variety, are descended from a very small population, perhaps between 1,000 and 10,000 breeding pairs about 70,000 years ago.
Using the average rates of genetic mutation, some geneticists have estimated that this population lived at a time coinciding with the Toba event. These estimates do not contradict the consensus estimates that Y-chromosomal Adam lived some 60,000 years ago, and that Mitochondrial Eve is estimated to have lived 140,000 years ago, because Toba is not conjectured to be an extreme bottleneck event, where the population is reduced to a small number of breeding pairs.
Gene analysis of some genes shows divergence anywhere from 60,000 to 2 million years ago, but this does not contradict the Toba theory, once again because Toba is not conjectured to be an extreme bottleneck event. The complete picture of gene lineages (including present-day levels of human genetic variation) allows the theory of a Toba-induced human population bottleneck.
Recent work by archaeologist Michael Petraglia suggests that in fact modern humans survived relatively unscathed in at least one settlement in India.