Ancient campsite may show how humans survived Toba volcano super-eruption

An campsite in what is now Ethiopia may have been used for a few years before, during and after a huge volcanic eruption 74,000 years ago that altered Earth’s climate.

The eruption of Toba, a supervolcano on the Indonesian island of Sumatra, was the biggest eruption on Earth in the past 2 million years. Some researchers think it caused a volcanic winter that lasted several years and might have wiped out most humans alive at the time, but the magnitude of its impact is disputed.

Bones found at a site in Ethiopia suggest that the people living there had to adapt their diet to survive during a drier year or two after the eruption, but the impact appears to have been mild.

“It was a pretty lucky find,” says John Kappelman at the University of Texas at Austin, whose team discovered the site in 2002. “There is no question about it.”

Most early human sites are caves that were occupied for tens of thousands of years, he says. But this camp is an open-air site near the Shinfa river, a tributary of the Blue Nile. “Our hunch is that this site was occupied for maybe five to 10 years, something like that,” says Kappelman.

The team has found thousands of stone chips from the making of tools, along with some stone points that may be among the oldest arrowheads ever found. “We have evidence for archery in the form of these little stone points,” says Kappelman.

The researchers have also discovered ostrich egg shells and the bones of many animals, some of which have cut marks and signs of heating. So they think people were bringing animals back to the site to butcher and cook.

In the middle of the layer of sediment containing the stone chips and bones, the team also found volcanic ash in the form of minuscule pieces of glass known as cryptotephra. “They are just tiny, tiny glass shards,” says Kappelman – and their composition matches others from the Toba super-eruption.

An isotopic analysis of the ostrich shells suggests that the climate became drier after the eruption. This coincides with a quadrupling in the amount of fish remains seen and a decrease in other kinds of animal remains.

The team’s explanation for this is that the Shinfa river is seasonal and dries up, leaving waterholes in the dry season. Immediately after the Toba eruption, the dry season was longer, so the fish in the shrinking waterholes were easier to catch. This made up for the fall in terrestrial prey animals, the researchers suggest.

In the following years, food remains returned to pre-eruption levels, with no sign of a mass die-off, says Kappelman.

Other researchers have argued that when conditions got drier, early humans moved to places that were wetter, he says. For this reason, it is also thought that the migration of people out of Africa took place during periods when the climate was wetter, allowing them to survive in the usually arid regions between Africa and Eurasia.

“Our site shows that humans were adapted to seasonally arid conditions,” says Kappelman. This means that the movement of modern humans out of Africa, which may have taken place as recently as 65,000 or 60,000 years ago, could have happened during dry periods, he thinks.

However, Kappelman agrees that earlier migrations out of Africa by less sophisticated peoples may have been limited to wetter periods.

“This is an intriguing paper for many reasons – the possibly precise tie-in with the Toba super-eruption, the environmental evidence, subsistence behaviours including fishing, possible use of bow and arrow, and behavioural adaptations that might have facilitated dispersals from Africa,” says Chris Stringer at the Natural History Museum in London.

“I’m sure each of these propositions will fuel debate, but I think the authors have made a plausible – though not definitive – case for each scenario they propose,” he says.

The study also adds to the growing evidence that the global impact of the Toba super-eruption was relatively minor and short-lived, says Stringer.

But Stanley Ambrose at the University of Illinois Urbana-Champaign, one of the researchers who thinks Toba wiped out most humans, disagrees. He says the site may represent a much greater period of time than Kappelman’s team thinks, meaning the effects on people may have been much greater.

“Materials deposited by humans long before and long after the eruption – possibly centuries to more than a millennium earlier or later – could be juxtaposed with the ash layer by well-known processes of disturbance, such as burrowing rodents and cracks that form during the dry season,” says Ambrose.