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Becoming Human

Eruption early in human prehistory may have been more whimper than bang

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If Hollywood’s right, the apocalypse will be brutal. Aliens, nuclear war, zombies, plague, enslavement by supersmart robots — none of them are good endings. Some archaeologists, however, believe an apocalypse has already come and gone. About 75,000 years ago, they say, a monster volcanic eruption nearly wiped out humankind, leaving behind only a few thousand people to repopulate the world.

The explosion of Indonesia’s Toba volcano was the largest eruption of the last 2 million years. The volcano coughed up some 2,000 to 3,000 cubic kilometers of ash, enough to fill almost three-quarters of the Grand Canyon. Unleashing hordes of light-blocking particles, an eruption that size should have cooled the planet and reduced rainfall, killing off plants and creating food shortages.

Archaeologists recognized in the late 1990s that the disaster might explain a population bottleneck recorded in modern people’s DNA. Genetic evidence indicates that the human population drastically declined and variation in the gene pool plummeted around the same time Toba exploded.

The story sounds neat and tidy. But researchers are still trying to verify whether our species’ near-extinction experience was fact or fiction.

One way scientists have approached the issue is by simulating the climate after Toba’s eruption. In 2009, researchers reported in the Journal of Geophysical Research: Atmospheres that global temperatures could have dropped by 8 to 17 degrees Celsius and taken a few decades to get back to normal. That extended volcanic winter, they noted, would have had “devastating consequences for humanity and global ecosystems.”

The tricky part of these simulations is estimating how much sulfur — the main cause of volcanic climate cooling — Toba expelled. Another problem is incorporating all of the processes that affect the atmospheric life cycle of sulfur particles. Last year, a team attempted to account for these factors and concluded that the consequences of Toba weren’t so dire and varied regionally. In East Africa, for example, temperatures would have fallen only 4.5 degrees C while in India they would have dropped almost 9 degrees C; in both areas, temperatures would have returned to normal within five years, the team reported in Quaternary International. In these simulations, dry grasslands would have replaced forests in many places, but that alone wouldn’t have led to widespread starvation and death.

The geologic record also offers mixed messages. Ice cores from Greenland and Antarctica contain a spike in sulfate concentrations dated to approximately the same time that Toba erupted. Six years of elevated sulfate levels coincide with the onset of a global cool period recorded in the cores. But critics point out that no volcanic ash has been found in the cores to directly link the chill to Toba.

Other geologic clues also suggest the volcano didn’t cause much of a climate calamity. In April, two geologists and an archaeologist announced that they had discovered traces of Toba’s eruption in a sediment core from East Africa’s Lake Malawi. The core contained shards of microscopic volcanic glass with the same chemical composition as glass known to have come from Toba. The core also indicated that temperatures declined by no more than 1.5 degrees C after the eruption, the researchers report in the Proceedings of the National Academy of Sciences. That wouldn’t have had a big ecological impact on East Africa, they say, where a big bulk of the human population presumably lived 75,000 years ago.

Humans elsewhere seem to have survived unscathed as well. In southern India, archaeologists have found collections of stone tools from before and after the eruption that look nearly identical. The tools resemble those made by people living in Africa at the time, circumstantial evidence that Homo sapiens, not some other hominid species, was living in the area pre- and posteruption. But without fossils, which are lacking in India, archaeologists can’t be 100 percent sure that a different Homo species didn’t make the artifacts.

In recent years, geneticists have started questioning their own evidence for a population bottleneck. While some researchers estimate that the decline in the human population happened but preceded Toba by tens of thousands of years, others think people never actually approached extinction. In some situations, cultural behaviors and customs that limit mating between different groups can result in genetic patterns that match those produced by population bottlenecks, anthropologists reported in 2009 in the Proceedings of the National Academy of Sciences.

Finding more climate records that can be directly tied to the Toba eruption and the period when it erupted should help researchers reach more definitive answers. That’s a difficult task given that the timing of the eruption itself isn’t quite nailed down yet, with dates ranging from 73,000 to 75,000 years ago. A precise age is important because the volcano’s actions would have altered climate in a matter of months and years, not centuries and millennia.

Unfortunately, even the best scientific stories are never as easily resolved as a Hollywood blockbuster.

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