From the Wikepedia article on the Mt. Toba supervolcano eruption:
According to the Toba catastrophe theory, 70,000 to 75,000 years ago a supervolcanic event at Lake Toba, on Sumatra, reduced the world's human population to 10,000 or even a mere 1,000 breeding pairs, creating a bottleneck in human evolution. The theory was proposed in 1998 by Stanley H. Ambrose of the University of Illinois at Urbana-Champaign.
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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 to 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.
Values genetically incorporated into prehuman and hominoid brains, as a general adaptation of many branching pre-human species, would have been made extremely uniform as a result of the population bottleneck.
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