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Toba catastrophe theory
From Wikipedia, the free encyclopedia
According to the Toba catastrophe theory, 70 to 75 thousand years ago, a super volcanic event at Lake Toba reduced the human population to 10,000 or even 1,000 breeding pairs, creating a bottleneck in human evolution. The theory was proposed by Stanley H. Ambrose of the University of Illinois at Urbana-Champaign.[1][2]
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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 severely reduced the human population. This may have occurred when around 70–75,000 years ago 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 one gigaton of TNT, three thousand times greater than the 1980 eruption of Mount St. Helens. According to Ambrose, this reduced the average global temperature by 5 degrees Celsius for several years and may possibly have triggered an ice age.
Ambrose postulates that this massive environmental change created population bottlenecks in the various 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 and modern humans.
Some geological evidences 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 between 1,000 and 10,000 individuals.[3][4]
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 Mitochondrial Eve is estimated to have lived 140,000 years ago, since Toba is not conjectured to be an extremal bottleneck event with only one breeding pair surviving.
However, other most recent common ancestors traced via large set of different genes lived anywhere from 2 million to 60,000 years ago. The complete picture of gene lineages do not support the theory of a human population bottleneck.[5]
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.[citation needed]
Migration routes to Asia created population centers in Uzbekistan, Afghanistan, and India.
Possibly substantial 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.
- ^ Stanley H. Ambrose (1998). "Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans". Journal of Human Evolution 34 (6): 623–651. DOI:10.1006/jhev.1998.0219.
- ^ Ambrose, Stanley H. (2005). Volcanic Winter, and Differentiation of Modern Humans. Bradshaw Foundation. Retrieved on April 8, 2006.
- ^ When humans faced extinction. BBC (2003-06-09). Retrieved on January 5, 2007.
- ^ Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans by Stanley H. Ambrose
- ^ Dawkins, R (2004) The Ancestor's Tale, A Pilgrimage to the Dawn of Life. Weidenfeld & Nicolson. ISBN 0-7528-7321-0