A supervolcano erupted 74,000 years ago. Here’s how humans survived it.

Microscopic shards of glass that rained down from an ancient supervolcano eruption reveal how early modern humans adapted to dramatic climate change, according to a new study of a prehistoric site in northwestern Ethiopia.

For decades, scientists have debated just how apocalyptic it was when Toba, a supervolcano located in Sumatra, Indonesia, erupted some 74,000 years ago. Some proposed that the biggest eruption in millions of years triggered a catastrophic volcanic winter that nearly wiped out Homo sapiens. Others say that climate impacts varied greatly by region and weren’t extreme enough to have a major impact on human evolution.

In the new study, published in the journal Nature, scientists discovered tiny bits of volcanic glass from Toba buried alongside ancient arrowheads and fossilized remains of animals that were bygone meals, indicating that humans were there before, during and after the eruption.

The find adds to a growing body of evidence that the mega-eruption was not a near-extinction event for humans.

Even more intriguing, scientists found that humans shifted their diets in the extremely arid conditions that followed the eruption in the lowlands of Ethiopia. They ate more fish, which researchers think could have been readily harvested as the Shinfa River dried up, leaving shallow waterholes.

“It is sophisticated behavior … to fish, instead of hunting terrestrial mammals. That kind of behavioral flexibility is kind of a hallmark of modern humans today,” said John W. Kappelman Jr., a professor of anthropology at the University of Texas at Austin who began working at the site, called Shinfa-Metema 1, more than two decades ago.

The study also challenges a dominant idea about early human dispersal out of Africa: Experts have long thought that humans weren’t able to survive in extremely arid climates, and would have retreated to higher elevations and stayed in place rather than continuing to move through – and ultimately leave – the continent.

“They could handle seasonally arid sites,” Kappelman said, “so why would they have to retreat?”

Cryptic volcanic glass

Kappelman and teams of American and Ethiopian students have been excavating at Shinfa-Metema 1 since 2002. The remote field site is in the lowlands of Ethiopia, near the border with Sudan. They go when it’s dry – and hot. Every drop of water is brought in, and the basalt rocks on the ground can get so hot they melt the soles of his boots.

They discovered stone points that they believe were arrowheads, evidence these people hunted with bows. They also found fossilized remains with bone breakage patterns that suggest they were brought to the site to be processed into food, including remnants of warthogs, antelope, a python and fish.

But for years, the scientists were hung up on a common problem in archaeology: dating the site. They tried carbon dating ostrich eggshells they excavated, but ran into a slew of technical problems. Once they overcame those, they found the eggs were beyond the range of reliable carbon dating – more than 45,000 years old. Another dating technique put the ostrich eggs at around 75,700 years old, plus or minus 4,700 years.

Then, in 2018, a separate team of researchers dated two archaeological sites in South Africa using a new technique that involves looking for “cryptotephra.” Tephra are glass fragments that rain down after a volcanic eruption. Cryptotephra are versions so small they are invisible to the naked eye. They are “cryptic” or hidden.

Or they were, until that team developed a process to isolate these tiny fragments and probe their geochemical makeup, which allowed them to determine which eruption created them. The microfragments of glass in South Africa came from the Toba, showing that ancient humans in that region survived the cataclysm.

“I began looking around for a site that would illustrate the power of the technique,” said Curtis Marean, an anthropologist at Arizona State University. He joined forces with Kappelman, and to their delight they found microscopic shards that also traced to Toba in Ethiopia.

Michael Petraglia, director of the Australian Research Centre for Human Evolution, who was not involved in the study, said in an email that the study was “fascinating and convincing,” adding to other evidence that people survived Toba in South Africa and India.

“This on-the-ground evidence contradicts the popular model that the ‘volcanic winter’ caused by the Toba eruption almost drove humans and our closely related ancestors to extinction,” Petraglia wrote.

“Instead, all evidence from Shinfa-Metema and elsewhere now indicates that human populations were flexible enough in their adaptations to overcome environmental challenges, even those introduced by the Toba volcanic super-eruption of 74,000 years ago.”

Green corridors vs. blue highways

The date of the site is only one piece of the puzzle. Scientists also chemically analyzed fossilized remains of animals and ostrich eggs to reconstruct what the environment was like. They found that after Toba, it became extremely arid, and the diet people followed shifted away from land animals and more toward fish.

This flexibility in behavior is important because many experts had assumed that humans couldn’t have survived such arid conditions, and that in turn would have shaped the human journey out of Africa.

Humans left Africa many times before the last 100,000 years, but genetic studies suggest that non-African people can trace their ancestry to a dispersal event between roughly 90,000 and 50,000 years ago. One theory for how those people traveled is that they waited for humid periods when more hospitable “green corridors” would have opened in desert environments.

But the evidence that humans could adapt to arid conditions, and even exploit their environment in new ways by shifting their diets, led the researchers to propose an alternate route: What if seasonal rivers formed “blue highways” – a siphon, in which people were drawn from one water hole to another, using up resources as they moved along the river’s path?

Rachel Lupien, a geoscientist at Aarhus University who was not involved in the work, is not yet convinced. She said that drawing comparisons between the short-term climate at this site post-eruption and the climate thousands of years later, when humans probably left Africa, ignores many variables that drive climate and rainfall. It’s also not clear that the specific arid conditions they discovered in Ethiopia would be similar to those over vast distances.

Petraglia said the idea of “blue highways” is interesting, but that given the wide range of ecological settings across Africa, it is unlikely to be a single explanation for how humans moved. He also noted that massive deserts existed in the Sahara and Arabia between 71,000 and 57,000 years ago, meaning “blue highways” would have been shut down or nonexistent over these large areas.