How Did Humans Survive Our Near Extinction?

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Roughly 75,000 years ago, Indonesia exploded. A great supervolcano in Sumatra erupted, spewing an estimated 2,800 cubic kilometers of magma and rock into the air. Ash may have been deposited as far as the Indian Ocean, the South China Sea, and even Lake Malawi in Africa, perhaps farther. Gases ejected into the atmosphere may have caused global temperatures to drop by as much as 18 degrees for several years after the eruption, with a smaller degree of cooling continuing for a thousand years after.

The Toba supereruption, as this event is now called, was a hundred times larger than the eruption of Mount Tambora in 1815, the worst volcanic eruption on record, which caused the notorious "Year Without a Summer." Toba's infamous place in history is etched into the Earth itself as a crater lake (seen top). And as Toba scarred the Earth and altered global climates, it could have devastated life on Earth.

In his recently released book, End Times: A Brief Guide to the End of the World, science journalist Bryan Walsh summarized a 2009 research paper on Toba's potential climate effects.

"Precipitation would have fallen by 45 percent, and vegetation cover would have shrunk dramatically, with broadleaf evergreen trees and tropical deciduous trees dying out. Imagine a winter that lasted for years, like something out of Game of Thrones, shriveling life on land."

For our Homo sapien ancestors, concentrated almost entirely in Africa and southern Asia, life could have become very challenging indeed. In fact, genetic evidence suggests that between 50,000 and 100,000 years ago, our species experienced an extreme population bottleneck, plummeting to as few as 2,000 to 10,000 individuals from a population that once numbered in the many tens of thousands. After existing for perhaps 200,000 years on Earth, humans almost went extinct, and Toba may have been to blame.

How did our forebears survive? Stanley Ambrose, an anthropologist at the University of Illinois and one of the chief architects of the Toba catastrophe theory, hypothesizes that cooperation may have been the difference between life and death.

"You might think that in an apocalypse event that people would be stealing from each other," Ambrose told Walsh. "It's not true. In a stable environment, when population density is high, you don't need to rely on your neighbors to get the next meal, and you may even need to defend yourself from them. But in a situation where it's the equivalent of a small lifeboat and everyone needs to cooperate, selfish people will be weeded out. You end up with a population that is more sharing and caring."

The Toba catastrophe theory is controversial, and there are very legitimate scientific critiques. A number of researchers question whether a worldwide ecological disaster actually occurred. They cite evidence suggesting that humans in Africa were not affected. Other studies hint that African vegetation did not suffer catastrophic die-offs. Moreover, neanderthals in Europe seemed to suffer no ill-effects from the eruption and any associated climatic effects. Needless to say, proponents of the Toba catastrophe theory vehemently disagree with the conclusions of these studies, countering with evidence that other large mammals also experienced population bottlenecks around the Toba supereruption.

We may never know conclusively how Toba affected our early ancestors, but as Walsh made clear in his book, the idea that our species endured a worldwide cataclysm is certainly seductive.

"You and I and everyone we know – everyone who came before us and everyone who might come after us – are here because the human beings who lived through Toba found a way to survive the eruption and its long, cold aftermath. Without their resourcefulness, the human story could have ended in its earliest chapters. Extinction was a possibility."

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