The last ice age
The Toba supereruption was a
eruption that is believed to have occurred some time between 69,000
and 77,000 years ago at
It is recognized as one of the Earth's largest known eruptions. The related catastrophe hypothesis holds that this event plunged the planet into a 6-to-10-year volcanic winter and possibly an additional 1,000-year cooling episode. This change in temperature is hypothesized to have resulted in the world's human population being reduced to 10,000 or even a mere 1,000 breeding pairs, creating a bottleneck in human evolution.
The Toba event is the most closely studied supereruption. In
The Toba eruption or Toba event occurred at what is now Lake Toba about 73,000±4,000 years ago. The Toba eruption was the latest of the three major eruptions which occurred at Toba in the last 1 million years. The last eruption had an estimated Volcanic Explosivity Index of 8 (described as "mega-colossal"),
It was two orders of magnitude greater in erupted mass than the largest volcanic eruption in historic times, in 1815 at Mount Tambora in Indonesia, which caused the 1816 "Year Without a Summer" in the northern hemisphere.
Although the Toba eruption took place in Indonesia, it deposited an ash layer approximately 15 centimetres thick over the entirety of South Asia. A blanket of volcanic ash was also deposited over the Indian Ocean, and the Arabian and South China Sea. Deep-sea cores retrieved from the South China Sea extended the known distribution of the eruption and suggest that the 2,800 km3 calculation of the eruption magnitude is a minimum value or even an underestimate.
The apparent coincidence of the eruption with the onset of the last glacial period attracted the scientists' interest. Michael L. Rampino and Stephen Self argued that the eruption caused a "brief, dramatic cooling or 'volcanic winter'", which resulted in a global mean surface temperature drop of 3–5 °C and accelerated the glacial transition from warm to cold temperatures of the last glacial cycle. Some scienteis t believe that an ice age was already underway and that the superuption accelated the process.
According to the supporters of the genetic bottleneck theory, between 50,000 and 100,000 years ago, human population suffered a severe population decrease—only 3,000 to 10,000 individuals survived—followed eventually by rapid population increase, innovation, progress and migration.
Several geneticists, including Lynn Jorde and Henry Harpending have proposed that the human race was reduced to approximately five to ten thousand people. Genetic evidence suggests that all humans alive today, despite apparent variety, are descended from a very small population, perhaps between 1,000 to 10,000 breeding pairs about 70,000 years ago. Note that this is an estimate of ancestors, not of total human population. Isolated human populations that eventually died out without descendants may have also existed in numbers that cannot be estimated by geneticists.
Ambrose and Rampino proposed in the late 1990s that a genetic bottleneck could have been caused by the climate effects of the Toba eruption. The supporters of the Toba catastrophe theory suggest that the eruption resulted in a global ecological disaster with extreme phenomena, such as worldwide vegetation destruction, and severe drought in the tropical rainforest belt and in monsoonal regions. Τhis massive environmental change created population bottlenecks in species that existed at the time, including hominids; this in turn accelerated differentiation of the reduced human population. Therefore, Toba may have caused modern races to differentiate abruptly only 70,000 years ago, rather than gradually over one million years
Richard Dawkins has postulated that human
mitochondrial DNA (inherited only from one's mother) and
Y chromosome DNA (from one's father) show
coalescence at around 140,000 and 60,000 years ago respectively. In
other words, all living humans'
female line ancestry trace back to a single female (Mitochondrial
Eve) at around 140,000 years ago. Via the
male line, all humans can trace their ancestry back to a single male
Adam) at 60,000 to 90,000 years ago.
This is consistent with the Toba catastrophe theory which suggests that a bottleneck of the human population occurred c. 70,000 years ago, proposing that the human population was reduced to c. 15,000 individuals when the Toba supervolcano in Indonesia erupted and triggered a major environmental change, including a volcanic winter. The theory is based on geological evidences of sudden climate change at that time, and on coalescence evidences of some genes (including mitochondrial DNA, Y-chromosome and some nuclear genes) and the relatively low level of genetic variation among present-day humans.
Recent analyses of mitochondrial DNA have set the estimate for the major migration from Africa from 60,000–70,000 years ago, around 10–20,000 years earlier than previously thought, and in line with dating of the Toba eruption to around 66,000–76,000 years ago. During the subsequent tens of thousands of years, the descendants of these migrants populated Australia, East Asia, Europe, and the Americas.
|Lake Toba is the resulting crater lake|