The Toba Catastrophe Theory

Lake Toba (photo by Andrey Samsonov)

Lake Toba (photo by Andrey Samsonov)

Werner Herzog’s quite enjoyable, and typically discursive, recent documentary Into the Volcano featured a short vignette on a specific volcanic eruption, and a theory associated with it, that I hadn’t encountered before.

Around 74,000 years ago, one of the Earth’s largest-ever volcanic eruptions, the Toba super-eruption, happened at what is now Lake Toba, in the northern part of the Indonesian island of Sumatra. According to the Toba Catastrophe Theory, this eruption caused a planet-wide volcanic winter that lasted six years or more, as well as global cooling that extended over a thousand years. Because of this, according to a related theory, the ancestors of modern humans nearly died out, or at the very least were severely challenged.

This super-eruption, sometimes called the “Youngest Toba Tuff” or YTT eruption, has been called “the largest known volcanic eruption in the history of the human species,” and is generally believed to have been the largest of the last 2.5 million years. Its estimated volcanic explosivity index was 8, the maximum possible. By way of comparison, the Toba super-eruption was about 100 times larger than the largest recent eruption, the one in 1815 at Mount Tambora in Indonesia, which resulted in 1816’s “Year Without a Summer” in the Northern Hemisphere. Current research says that Toba expelled an amazing 700 cubic miles of magma – one article put that number in context by saying that this mass is roughly equivalent to 19 million Empire State Buildings. The famous Krakatoa eruption of 1883 released only about 3 cubic miles of magma.

All life in the immediate area of the Toba super-eruption must have been destroyed. At least six inches, and perhaps even several feet, of ash were likely deposited over the entirety of South Asia, including the Indian Ocean and South China Sea. The volcano itself collapsed to form a huge caldera now home to Lake Toba, the fifteenth-deepest lake, and largest volcanic lake, in the world.

Michael Rampino and Stephen Self have argued that the Toba super-eruption caused a “brief, dramatic cooling or ‘volcanic winter.’” Temperatures around the world would have dropped, they suggest, by several degrees, and contributed to the beginning of the last glacial period, the Würm glaciation, which had probably already begun but was helped along by Toba.

This eruption might also have prompted what has been called a “population bottleneck” in the course of human evolution. Somewhere between 50,000 and 100,000 years ago, so the argument goes, the worldwide human population decreased sharply, perhaps as low as 3,000. There is some genetic evidence that all humans alive today are descended from just 1,000 to 10,000 breeding couples that lived around 70,000 years ago, and that the bulk of genetic differences between modern human populations dates from that time, rather than a more gradual process spread out over hundreds of thousands of years.

According to the “population bottleneck” theory, the Toba eruption and the resulting volcanic winter led to a global ecological crisis that could have destroyed most of the food sources available to the human population, thereby resulting in that much smaller population. Some evidence can be marshaled to support this idea. Mitochondrial DNA have shown that the major human migration from Africa happened between 60,000 and 70,000 years ago, which fits with the Toba timeline. There is also some evidence of genetic bottlenecks in other species from certain regions at that time, including chimpanzees, orangutans, tigers, and cheetahs.

On the other hand, a seven-year project led by Oxford University along with several Indian institutions found that many forms of life then existing in India survived the Toba super-eruption. This included human populations that seem to have been there at that time, whose stone tool assemblage, very similar to that of the human populations in East Africa, remained consistent before and after the super-eruption. (This is controversial, however, because there is argument about exactly when modern humans first arrived in India.) Also, in East Africa, where most or all humans lived at that point, there was apparently no volcanic winter, or even much of a change in climate. Farther afield, in Europe, Neanderthals certainly survived whatever global impact the Toba eruption had. However, it has been suggested that the eruption might have forced humans to adapt to a new environment, which helped them ultimately to replace the Neanderthals.

It is clear that as a result of the Toba super-eruption, some areas were totally devastated. Others, however, experienced minor changes and recovered very quickly. Not many people embrace the Toba Catastrophe Theory anymore, it would seem. Nor is it generally thought that there is a connection between the super-eruption and major changes in the human population that existed at that time. But the evidence either way isn’t extensive, and the theories are certainly interesting.

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