Around 74,000 years ago, the Toba supervolcano erupted on the Indonesian island of Sumatra. It was the biggest volcanic eruption of the last 2 million years, unleashing 2,800 cubic kilometers of magma. That’s enough to bury the entire United States in a foot-thick layer of ash and rock.
In the 1990s, several scientists argued that Toba’s unprecedented outburst radically changed the world’s climate, blocking out sunlight and lowering global temperatures by several degrees for many decades. This “volcanic winter,” it is said, almost drove humans to extinction, leaving behind a measly group of a few thousand survivors, from whom we today are descended. The “Toba catastrophe theory” is highly controversial, and other researchers have argued that it greatly overestimates both the degree of climate change that the volcano inflicted, and its effect on our ancestors.
Now, into the fray comes a new study from an unlikely location. In a cliff near Mossel Bay, a town on South Africa’s south coast, scientists have discovered a layer of microscopic glass shards. Known as cryptotephra, these shards are the products of Toba’s wrath, created when the volcano superheated the silica within its expunged rock. They drifted in the air over 5,500 miles and fell on southern Africa as the sparsest of drizzles. And they settled among bones, tools, and other signs of human occupation.
The fact that these artifacts exist in plentiful numbers both above and below the shards suggests that the humans who once lived in South Africa weren’t affected by Toba’s wrath as one might expect if the supervolcano had truly brought on a global decades-long winter. If anything, they thrived. “We showed that after the input of the shards, human occupation at the site actually increased dramatically,” says Curtis Marean, from Arizona State University. “We never expected that.”
“This is the first time we can say: Here is what humans were doing before and after [the eruption],” adds Christine Lane, a researcher from the University of Cambridge who helped to study the cryptotephra. “And I think we were doing really well.”
“If Toba had triggered a major global climate event, Africa probably would have been affected, and they see no evidence of that,” says Britta Jensen, a tephra expert from the University of Alberta who wasn’t involved in the new study.
Marean’s team has spent years working at Pinnacle Point—a rocky headland from which they’ve uncovered 400,000 artifacts. Finding the cryptotephra was much harder. There are just 10 of them in each gram of sediment, and each one is just 0.04 millimeters wide. To recover them, Gene Smith, from the University of Nevada at Las Vegas, designed new techniques that use special liquids to separate particles of a specific size. The shards come out with other gunk, and specialists pick them out under a microscope. “Then you can do a chemical analysis on them, which is a whole new struggle because you’re trying to target something that’s 40 microns wide with a beam that then destroys it,” says Marean. “It’s crazy. It took us years. I first proposed this study in 2005.”
They found that the shards are almost chemically identical to others that have been found in India, Malawi, and Toba itself, all of which have been traced to the super-eruption. They were also found in sediments that were roughly the right age—around 74,000 years old. They must have come from Toba.
The team tracked down even more shards at a half-moon bay that’s just a few miles away, and that’s also replete with human-made artifacts. Usually, it would be impossible to say whether people who lived in two separate archaeological sites actually lived at the same time, because the dating methods that archaeologists use come with large error ranges. But the cryptotephra solve that problem because they were deposited in a two-week period. They unequivocally show that the people from the headland and the bay must have belonged to the same social group. They probably knew each other. They may even have been the same people, foraging in the bay by day and sleeping in the headland at night.
“This is a Holy Grail moment in geochronology,” says Marean. “It’s so, so rare for us to be able to speak about things at that temporal resolution.”
But it’s harder to say what the shards tell us about the Toba catastrophe theory, and even the scientists who worked on the new study don’t entirely agree. Lane, for example, says, “We need to put the idea of a global volcanic winter to bed. If there was a climatic impact of the eruption, it was much smaller than people had hypothesized.”
She bases that not just on the new South African evidence, but on other Toba shards that she found at Lake Malawi in East Africa. By analyzing the sediment around those shards, Lane concluded that the climate in the region didn’t substantially change after the eruption. Chad Yost, from the University of Arizona, also came to similar conclusions after studying Lake Malawi: He found that the eruption barely affected the vegetation around the lake. “It is becoming increasingly hard to find evidence in favor of the Toba catastrophe hypothesis,” he wrote just last month.
Yost adds that other groups of hominids survived the eruption, including the Neanderthals of Europe and the hobbits of Flores, who all lived far closer to Toba than humans in Africa. He also says that the global-winter idea is based on studies that used data from modern eruptions to simulate the consequences of Toba. Those simulations likely overestimate the amount of climate-changing sulfates released by the volcano by 10 to 100 times. “Each eruption is unique and its potential impact on climate is dependent on numerous variables that are just beginning to be understood,” he says. “The persistence of the Toba catastrophe hypothesis has essentially been perpetuated by bad data.”
Marean interprets the evidence from Lake Malawi differently, and notes that there are some signs of changing climate, such as an increase in wildfires. The question is how big those changes were, and he thinks that his new data from South Africa neither confirm nor refute the idea of a volcanic winter. Such an event would have hit people in landlocked regions hardest, so Marean speculates that they could have sought refuge in the coasts. Were the inhabitants of Pinnacle Point the only survivors to thrive after a continental catastrophe?
Stanley Ambrose, from the University of Illinois, isn’t even convinced that the Pinnacle Point community was flourishing. He says that the sediment layer just above the cryptotephra, which was deposited immediately after Toba erupted, has far fewer stone artifacts in them. “That does not mean ‘thrive’ to me,” says Ambrose, who first proposed the idea that Toba killed off most of humanity. “The actual data supports what I’ve been proposing for decades. I’m quite happy with it.”
Not quite, says Marean. He explains that the cryptotephra and the accompanying human artifacts were deposited in a layer of dune sand that was rapidly building up. The layer that Ambrose is talking about represents just a few weeks of time, and maybe even just one day. The fact that there are few artifacts in it means very little.
The problem with Toba is that it coincides with a lot of very interesting events in our history. Shortly afterward, there’s a big migration of humans out of Africa. Our stone technology becomes more advanced. We show more artistic expressions of symbolic behavior. “Were these related to Toba?” Marean asks. “We don’t know.” After all, Toba erupted when the world was already swinging into a glacial phase. That, says Yost, is more likely to have precipitated the various changes in human behavior than some hypothetical volcanic winter.
Ultimately, the South Africa study “is probably just the beginning,” says Siwan Davies, from Swansea University. The fact that the team found cryptotephra so far from their source “opens the door for many more groundbreaking studies to assess the impact of this eruption on other populations, in Africa and farther afield.” Scientists could look for similarly tiny amounts of cryptotephra in other parts of Africa. If people abandoned inland regions post-Toba, it would support Marean’s idea of a coastal refuge. If every population was thriving, the catastrophe hypothesis would wilt.
That’s the most exciting thing about this study—and it has little to do with how bad Toba was. It provides techniques that will allow scientists to match patterns of human behavior to changes in the planet’s climate with unprecedented precision.
For example, researchers have detected sulfates from the Toba explosion in ice cores from Greenland and the Antarctic—opposite sides of the planet. By lining up the sulfate data with the cryptotephra records, scientists can match what’s happening to humans in Africa to what’s changing with the climate at both poles. “We can see how people’s behavior is responding to worldwide stimuli,” says Marean. “That’s never been possible to a high degree of accuracy. Now we can do it right.”