People survived the Toba supervolcano’s global winter after all

Landsat image of Lake Toba
Landsat image of Lake Toba

A supervolcano eruption 74,000 years ago wasn’t enough to stop humanity in its tracks, artifacts at a Paleolithic site in central India suggest. The study is the latest strike against a hotly debated proposal that suggests the eruption of Indonesia’s Toba supervolcano had a huge influence on human evolution. The idea is that the eruption caused global cooling that killed most of the humans who had spread from Africa into Europe and Asia. But at the Dhaba site in Madhya Pradesh, India, archaeologists found stone tools in sediment layers spanning thousands of years before and after the eruption—evidence that human life went on.

An ancient apocalypse?

Today, the Toba supervolcano lies beneath the strikingly scenic Lake Toba on the Indonesian island of Sumatra. Seventy-four thousand years ago, it erupted in the middle of an important chapter in humanity’s takeover of the world. One of archaeology’s biggest questions in recent years has been when and how people first spread beyond Africa into different areas of the world; the answers lie in fossilized skeletons, objects left behind, and the DNA of modern people.

Fossil evidence suggests that people had reached the Levant by around 200,000 years ago, the Arabian Peninsula by around 85,000 years ago, and northern Australia by around 65,000 years ago. But the genomes of modern people suggest that the ancestors of modern African and non-African peoples branched off from a common ancestor around 70,000 years ago. At first glance, those lines of evidence don’t seem to agree, and some paleoanthropologists say that’s because a sudden, lengthy period of global cooling changed environments around the world in very drastic ways. The resulting crisis allegedly killed off most of the people alive at the time, leaving only a few thousand survivors.

The smoking gun, according to proponents of the hypothesis, is the Toba supervolcano. When Toba erupted 74,000 years ago, it spread a thick layer of ash as far away as Tanzania, 7,400km (4,600 miles) to the west. Volcanologists use a scale called the volcanic explosivity index to measure the explosive force of an eruption. Vesuvius’ eruption in 79 AD scored a 5 on that scale, while Toba rated an 8 (out of a possible score of… 8), on par with the Yellowstone Supervolcano’s eruption 630,000 years ago. Toba’s eruption would have released billions of tons of sulfur dioxide and other chemicals into the atmosphere and triggered several years of relentless volcanic winter—and that’s a best-case scenario.

Impact issues

But geologists, paleontologists, and paleoanthropologists are still debating how much of an impact Toba had on Earth’s climate, how long it lasted, and what happened to humans and other species. Ice cores from Greenland suggest that Earth’s climate abruptly turned colder around the time of the eruption and stayed chilly for around 1,000 years, but scientists don’t agree on whether to blame Toba or other climate processes. Some models suggest at least a few decades of colder climate in the wake of the eruption, while still others suggest just a few years. Unsurprisingly, scientists in several fields are still debating how that would have impacted life. The same fossil species appear above and below the Toba ash layer in lake sediments from Lake Malawi in East Africa, which suggests that conditions didn’t change enough to disrupt life.

And the archaeological evidence from Dhaba suggests that people in India weren’t dying off in droves, either. In fact, the layers of sediment just above the Toba ash layer contain as many artifacts as those just below it.

Layers of sediment at Dhaba span 80,000 years of human occupation.
Layers of sediment at Dhaba span 80,000 years of human occupation.
Christina Nuedorf

Adapt and survive

At Dhaba, near the banks of the Son River in central India, you can trace the development of stone-tool technology by looking at the artifacts buried, layer by layer, in 80,000 years’ worth of accumulated sediment. Those layers are the key to answering an important question about Toba’s role in humanity’s story: did people arrive in India before the eruption, carrying the same stone-tool technology they had brought from Africa, or did they arrive tens of thousands of years afterward with more recently developed tool kits?

According to luminescence dating of nearby sediments, the oldest stone tools at Dhaba are between 75,000 and 82,000 years old, pre-dating the Toba eruption by a few millennia. And on the other side of the layer of tuff deposited by Toba (which argon isotope dating puts at 74,000 to 76,000 years old), the same stone tools keep showing up, uninterrupted by anything that looks like a sudden apocalyptic depopulation. They persist for another 34,000 years before gradually giving way to a different stone tool technology which involves much smaller “microblades” a few centimeters long. Overall, Dhaba seems to suggest that people arrived in central India before the Toba eruption, survived its aftermath, and stuck around—which doesn’t bode too well for the population bottleneck idea.

The tools people were making and using at Dhaba in the millennia before the Toba eruption are strikingly similar to the tools found at sites in Africa starting around 280,000 years ago, in Arabia 100,000 to 74,000 years ago, and in northern Australia around 65,000 years ago (that timeline should look pretty familiar). They’re sharp flakes, blades, and scrapers made by knapping pieces off prepared stone cores—a technology archaeologists call Levallois. That puts India firmly in the timeline of human migration as “an important bridge linking regions with similar archaeology to the east and west.”

Of course, it’s likely that conditions at Dhaba changed for at least a few years after the eruption—a 2009 study of ancient pollen grains suggests that there was some deforestation in Southern Asia at that time. But adapting to changing climates and new landscapes has always been a human strong point, and that’s what the archaeological record at Dhaba shows: not that nothing changed, but that people probably adapted, coped, and kept going.

Nature Communications, 2020 DOI: 10.1038/s41467-020-14668-4 (About DOIs).


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