Toba Catastrophe Theory - Genetic Bottleneck Theory

Genetic Bottleneck Theory

Ann Gibbons first suggested, in an article in the October 1993 edition of Science, that a bottleneck in human evolution about 50,000 years ago could be linked to the Toba eruption. Rampino and Self backed up this idea in a letter to the journal later that year. The bottleneck theory was then further developed by Ambrose in 1998 and Rampino & Ambrose in 2000, who invoked the Toba eruption to explain a severe culling of the human population.

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. Robock believes that, indeed, a 10-year volcanic winter triggered by YTT could have largely destroyed the food supplies of humans and therefore caused a significant reduction in population sizes.

Gene analysis of some genes shows divergence anywhere from 60,000 to 2 million years ago. This does not contradict the Toba theory, however, 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.

However, research by archaeologist Michael Petraglia's team cast doubt on Ambrose's theory. Petraglia and his team found stone tools in southern India, above and below a thick layer of ash from the Toba eruption. The tools from each layer were remarkably similar, and Petraglia says that this shows that the huge dust clouds from the eruption did not wipe out the local population of people:

A 2009 study by Martin A. J. Williams's team challenges Petraglia's findings. Williams analysed pollen from a marine core in the Bay of Bengal with stratified Toba ash, and argued that the eruption caused prolonged deforestation in South Asia. Ambrose, who is a co-author of the study, calls the evidence "unambiguous", and further argues that YTT may have forced our ancestors to adopt new survival strategies, which permitted them to replace Neanderthals and "other archaic human species". However, both Neanderthals in Europe and the small-brained Homo floresiensis in Southeastern Asia survived YTT by 50,000 and 60,000 years respectively.

Oppenheimer accepts that the arguments proposed by Rampino and Ambrose are plausible, but they are not yet compelling for two reasons: it is difficult to estimate the global and regional climatic impacts of the eruption, and, at the same time, we cannot conclude with any confidence that the eruption actually preceded the bottleneck. Furthermore, a 2010 geneticists' study seems to question the foundations of the Toba bottleneck theory: analysis of Alu sequences across the entire human genome has shown that the effective human population was already less than 26,000 as far back as 1.2 million years ago, suggesting that no Toba bottleneck was necessary. Possible explanations for the low population size of human ancestors may include repeated population bottlenecks or periodic replacement events from competing Homo subspecies.