ON THE ISLAND of Sumatra some 74,000 years ago, an erupting supervolcano wreaked havoc, sending up plumes of ash and debris that spread for thousands of kilometres and caused temperatures to plummet.
The eruption’s effects were felt as far away as southern Africa, where they would have impacted early humans. Some scientists have even suggested that the Toba supereruption was so powerful, it pushed our ancestors to the brink of extinction around the time humans were first venturing out of Africa.
“Toba is the largest eruption on Earth in the last two million years,” says Gene Smith, a geologist at the University of Nevada Las Vegas.
However, some humans not only survived but thrived after Toba, judging by the artefacts they created during and after the eruption. The discovery, announced on Monday in the journal Nature, strengthens the case that the intense eruption didn’t pose an existential threat to our ancestors, perhaps because humans in Africa took refuge along the coast.
Genetic evidence shows that modern humans descend from a population that numbered only in the thousands when it ventured out of Africa some 60,000 years ago. Why so meagre? Some scientists had suggested that our ancestors were devastated by the Toba eruption.
In a flash, the supervolcano belched out a 4,170 cubic kilometres of rock and dust, sending debris kilometres into the air and leaving a scar in the earth more than 97 kilometres wide. Toba also threw huge amounts of dust and sulfur into the atmosphere, potentially cooling Earth’s surface and building on a cooling event that was already creating glaciers and lowering the planet’s sea levels.
Given Toba’s potential role in shaping humankind, researchers have worked to understand precisely how early humans reacted to it.
Previously, researchers in India had found evidence of early humans (though not necessarily anatomically modern Homo sapiens) living through the eruption. Sediments from Africa’s Lake Malawi also suggest that the eruption didn’t markedly change the region’s climate. But to better understand what was happening in Africa, researchers needed to find archaeological sites interspersed with Toba ash.
In 2011, Smith and his wife were taking a National Geographic-organised trip to South Africa’s Pinnacle Point, an archaeological site overlooking the Indian Ocean, when they met Arizona State University archaeologist Curtis Marean.
Marean showed Smith an enigmatic soil sample from the area, which Smith immediately recognised as containing ash from a volcano. Soon thereafter, Smith joined Marean’s team; the next summer, he and colleagues collected samples.
First, the team needed to figure out which volcano blanketed Pinnacle Point and Vleesbaai, a nearby site. They scoured samples to find microscopic shards of glass, which act as volcanic eruptions’ calling cards. The glass samples chemically matched those found at Toba and other Toba ashfall sites.
Researchers also needed to date the ash layer, a tricky task performed by coauthor Zenobia Jacobs, a researcher at the University of Wollongong. After analysing the ash, Jacobs estimates that the layer dates back 74,000 years, give or take 5,000 years—a good match to the Toba eruption.
Below, within, and above this ash layer, researchers then found more than 400,000 artefacts left behind by humans, ranging from heat-treated stone tools to animal bones and signs of fire. Based on the evidence, the team argues that modern human populations on the South African coast thrived after the eruption, staying at the site for thousands of years and even developing innovations in their tools.
Marean and his colleagues suggest that the South African coast may have acted as a refuge—perhaps the refuge—for modern humans weathering the Toba eruption. They point to one 2009 study that suggested the eruption could have lowered global temperatures by 7 degrees Celsius, which would have made survival tough elsewhere in Africa.
“If there was a volcanic winter that really got cold, it would not have gotten as cold along the coastline,” says Rutgers University climate scientist Alan Robock, who coauthored the 2009 Toba study.
Robock points out, however, that newer studies downwardly revise the climate impacts of Toba’s epic eruption. In 2010, climate scientist Claudia Timmreck found that Toba injected so much sulfur into the atmosphere, the resulting aerosols had a fairly high chance of sticking together and settling out of the atmosphere, limiting their cooling influence in the long term.
Immediately after the eruption, temperatures would have plummeted in some regions, her work suggests. But after three years or so, the eruption’s effects would have died down to levels indistinguishable from the climate’s natural variability, limiting the danger to humans.
“Are these the only people that survived?” Robock asks. “That’s the question.”
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Lead Image: Some 74,000 years ago, the largest volcanic eruption of the last two million years rocked the island of Sumatra. The 60-mile-wide caldera formed in the explosion is now a large volcanic lake. PHOTOGRAPH BY ANDIA, UIG VIA GETTY IMAGES