Toba catastrophe theory

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Image:MountRedoubtEruption.jpg According to the Toba catastrophe theory, modern human evolution was affected by a recent, large volcanic event. The theory was proposed by Stanley H. Ambrose of the University of Illinois at Urbana-Champaign.<ref>{{cite journal

| author=Stanley H. Ambrose
| title=Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans
| journal=Journal of Human Evolution | year=1998 | volume=34 | issue=6 | pages= 623–651
| id=Template:DOI

}}</ref><ref>{{cite web

| author=Ambrose, Stanley H. | year=2005
| title=Volcanic Winter, and Differentiation of Modern Humans | work=Bradshaw Foundation
| url=http://www.bradshawfoundation.com/evolution/ | accessdate=2006-04-08

}}</ref>

Knowledge of human prehistory is largely theoretical, based in fossil, archeological, and genetic evidence. Within the last three to five million years, after human and other ape lineages diverged from the hominid stem-line, the human line produced a variety of human species. According to the Toba catastrophe theory, a massive volcanic eruption changed the course of human history by severely reducing the human population.

Around 70–75 thousand years ago the Toba caldera in Indonesia erupted with an energy release equivalent to about one gigaton of TNT, three thousand times greater than that of Mount St. Helens. According to Ambrose, this led to a decrease in average global temperatures by 3 to 3.5 degrees Celsius for several years. This massive environmental change is believed to have created population bottlenecks in the various human 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 branch that became modern humans.

Some geological evidence and computed models support the plausibility of the Toba catastrophe theory, and genetic evidence suggests that all humans alive today, despite their apparent variety, are descended from a very small population, perhaps around 10,000 individuals. Using the average rates of genetic mutation, some geneticists have estimated that this population lived at a time coinciding with the Toba event (see also Mitochondrial Eve and Y-chromosomal Adam).

According to this theory, humans once again fanned out from Africa after Toba when the climate and other factors permitted. They migrated first to Indochina and Australia, and later to the Fertile Crescent and the Middle East. Migration routes to Asia created population centers in Uzbekistan, Afghanistan, and India. Differences in skin color appeared as a result of varied melanin levels as local adaptations to varying ultraviolet intensities. Europe became populated by migrants from the Caspian Sea region when the last ice age ended and Europe became more hospitable.