Toba catastrophe theory

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Illustration of what the eruption might have looked like from approximately 50 miles (80 km) above Pulau Simeulue

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.^[1][2]

[edit] History

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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 species, including H. ergaster, H. erectus, H. neanderthalensis and possibly H. floresiensis.

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 1 teraton of TNT (4.2 ZJ), two thousand times greater than the 1980 eruption of Mount St. Helens^{[citation needed]}, and twenty thousand times greater than the largest human-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).

[edit] Evidence

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,^[3] 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.^[4][1]

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.^{[citation needed]}

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.^[5]

Recent work by archaeologist Michael Petraglia suggests that in fact modern humans survived relatively unscathed in at least one settlement in India.^[6][7]

[edit] Analysis of louse genes

Alan Rogers, a co-author of this study and professor of anthropology at the University of Utah, says: “The record of our past is written in our parasites.” Rogers and others have proposed the bottleneck may have occurred because of a mass die-off of early humans due to a globally catastrophic volcanic eruption. The analysis of louse genes confirmed that the population of Homo sapiens mushroomed after a small band of early humans left Africa sometime between 150,000 and 50,000 years ago.^[8]

[edit] Analysis of Helicobacter pylori genes

Recent research states that genetic diversity in Helicobacter pylori decreases with geographic distance from East Africa, the birthplace of modern humans. Using the genetic diversity data, the researchers have created simulations that indicate the bacteria seems to have spread from East Africa around 58,000 years ago. Their results indicate modern humans were already infected by H. pylori before their migrations out of Africa, and H. pylori remained associated with human hosts since that time.^[9]

[edit] Migration

According to this theory, humans once again fanned out from Africa after Toba when the climate and other factors permitted. They migrated first to Arabia and India and onwards to Indochina and Australia (Ambrose, 1998, p. 631), and later to the Middle East and what would become the Fertile Crescent following the end of the last glacial period (110,000–10,000 years ago).

[edit] See also

• Supervolcano
• Volcanic winter
• Wallace line
• Year Without a Summer
• Recent African origin of modern humans

[edit] References

[edit] External links

• Population Bottlenecks and Volcanic Winter
• 1998 article based on news release regarding Ambrose's paper
• Homepage of Professor Stanley H. Ambrose
• Toba Volcano, by George Weber
• Article in The Economist
• Journey of Mankind by The Bradshaw Foundation - includes discussion on Toba eruption, DNA and human migrations
• Geography Predicts Human Genetic Diversity ScienceDaily (Mar. 17, 2005) — By analyzing the relationship between the geographic location of current human populations in relation to East Africa and the genetic variability within these populations, researchers have found new evidence for an African origin of modern humans.
• Out Of Africa -- Bacteria, As Well: Homo Sapiens And H. Pylori Jointly Spread Across The Globe ScienceDaily (Feb. 16, 2007) — When man made his way out of Africa some 60,000 years ago to populate the world, he was not alone: He was accompanied by the bacterium Helicobacter pylori...; illus. migration map.