Friday, May 6, 2016

Lake Toba

70,000 years ago the volcano at Lake Toba in Sumatra, Indonesia--The Ring of Fire-- erupted. It is believed to be one of the most extraordinary events in human history.  It was 100 times greater than that of the largest volcanic eruption in recent history, the 1815 eruption of Mount Tambora in Indonesia, which caused the 1816 "Year Without a Summer" in the northern hemisphere.  The Toba eruption apparently coincided with the onset of the last glacial period. Michael L. Rampino and Stephen Self argue that the eruption caused a "brief, dramatic cooling or 'volcanic winter'", which resulted in a drop of the global mean surface temperature by 3–5 °C and accelerated the transition from warm to cold temperatures of the last glacial cycle. The eruption may have caused this 1,000-year period of cooler temperatures (stadial).  Assuming an emission of six billion tons of sulphur dioxide, computer simulations concluded that a maximum global cooling of approximately 15 °C occurred for three years after the eruption, and that this cooling would last for decades, devastating life.   
This is connected to the genetic "Bottleneck Theory." According to the bottleneck theory, between 50,000 and 100,000 years ago human populations sharply decreased to 3,000–10,000 surviving individuals. This is supported by genetic evidence suggesting that today's humans are descended from a very small population of between 1,000 and 10,000 breeding pairs that existed about 70,000 years ago. The theory goes that the Toba disaster altered climate, animal populations and plant life and may have been a factor in the decrease in human numbers.

Recent research shows the extent of climate change was much smaller than believed by proponents of the theory. In addition, coalescence times for Y-chromosomal and mitochondrial DNA have been revised to well above 100,000 years since 2011.
In 2000, a Molecular Biology and Evolution paper suggested a transplanting model or a 'long bottleneck' to account for the limited genetic variation, rather than a catastrophic environmental change.This would be consistent with suggestions that in sub-Saharan Africa numbers could have dropped at times as low as 2,000, for perhaps as long as 100,000 years, before numbers began to expand again in the Late Stone Age.

(Genetic drift can cause big losses of genetic variation for small populations. Population bottlenecks occur when a population's size is reduced for at least one generation. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population's genetic variation by a lot, even if the bottleneck doesn't last for very many generations. Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 — but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted. This thesis of Toba's impact has a lot of opponents who believe that the bottleneck period did occur, it just occurred hundred of thousands of years earlier.)

Comparisons of estimated cubic kilometers (cubic kilometers!) ejected:

Volcano                                Date                               Cubic kilometers of material ejected

Mt. St. Helens                    1980                                1 km3

Tambora                              1815                                80 km3

Indonesia?                           1257                                Theorized comparable to Tambora

Vesuvius                              79                                     3km3

Toba                                     70,000 B.C.                     2800km3!!!

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