The Toba Catastrophe Theory

Around 74,000 years ago, the Earth witnessed a massive explosive eruption of Toba. It is speculated that the event resulted in a large-scale – but not global – volcanic winter during the next six to ten years. The resulting caldera still exists (100 by 30 kilometers) and is now filled with water.

The erupted mass 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 Summer' in the Northern Hemisphere.

But the Eruption of Toba may have had an even greater effect. Our species, Homo sapiens, ventured out of Africa around 100,000 years ago, spread across Asia around 60,000 years ago and arrived on new continents and islands since then.

Hotly debated by scientists, the 'Toba catastrophe theory' claims that the Toba eruption is linked to a genetic bottleneck in human evolution about 50,000 years ago, which may have resulted from a severe reduction in the size of the total (world-wide) human population due to the effects of the eruption on the global climate. According to the genetic bottleneck theory, between 50,000 and 100,000 years ago, human populations sharply decreased to just 3,000–10,000 surviving individuals. The Toba eruption supposedly resulted in a near-global ecological disaster, including destruction of vegetation along with severe drought in the tropical rainforest belt and in monsoonal regions. For example, a 10-year volcanic winter triggered by the eruption could have largely destroyed the food sources of humans and caused a severe reduction in population sizes[1].

Well, because this is science (and not religion), people can dig up evidence to support or discredit it.

The eruption of Toba in 72,000 BC did produce voluminous ash deposits found across much of the Indian Ocean, Indian Peninsula, and South China Sea. A major climatic downturn was observed within the Greenland ice cores. But the eruption did not significantly impact the climate of East Africa and was therefore not the cause of a human genetic bottleneck at that time[2].

[1] Ambrose: Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans in Journal of Human Evolution – 1998
[2] Lane et al: Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka. in Proceedings of the National Academy of Sciences – 2013