About 74,000 years ago, according to scientists, humans suffered the worst crisis our species has ever faced. A massive Indonesian volcano, Toba, exploded. Chunks of boiling lava and bushels of dust covered the earth for miles in every direction.
"Picture the entire Indian subcontinent covered with a sheet of white," says Hema Achyuthan, a professor at Anna University in Chennai.
"That's what it would have looked like." The early humans who had by then settled in southern India were probably all but wiped out, suffocated by the dust cloud. After a few rains, the white ash washed away. Eventually, all signs of the eruption were buried by later civilisations.
Mystery of humans in India
But for Ravi Korisettar, an archaeology professor at Karnatak University, the forgotten Toba disaster could be the key to understanding mankind's earliest past, as well as how humans might respond to the climate crises of the future.
"It could have been a critical moment in human history and evolution," he says.
In order to solve that mystery, Korisettar had to figure out exactly when humans first arrived in India. Here too, the ash proved to be a critical link.
Korisettar says, "If you look at global prehistory, India doesn't takes up only a couple of pages. By looking at Toba ash, we've filled in some of the blanks."
Archaeologists who study human origins face a formidable challenge when they come to India. Unlike in Africa and Australia, where excavations have dug up early human skeletons, there are no fossil remains in India that go back 74,000 years or longer.
As a result, there's a great deal of uncertainty about how the first humans arrived in India.
Genetic testing suggests that the first humans to reach India came to the Andaman Islands about 65,000 years ago.
From there, they moved along India's southern coast into Southeast Asia and then Australia. But Korisettar's research has challenged that hypothesis. He first discovered the Toba ash at a prehistoric human settlement near Pune.
Dating revealed that the white ash that had once covered the site came from the Toba eruption. Which means archaeologists could assume that humans had been at the Pune site when the volcano exploded, far earlier than geneticists previously believed.
"At that time nobody knew that we had this type of ash in the Indian geologic record," Korisettar said. "I became obsessed with identifying ashes."
Korisettar and a fellow archaeologist, Mike Petraglia from Oxford University in England, got funding from Oxford to start a special project dedicated to studying the remains of the Toba explosion and its impact on the early people of the Indian subcontinent. They recruited a team, including Achyuthan, from all over the world to study the soils, climate changes, and ashes associated with that long-ago explosion.
They discovered buried caches of Toba ash in the Narmada Valley in central India, at the Bori and Morgaon sites near Pune in western India, and in the southern Cuddapah basin in Andhra Pradesh, at a well-preserved site in the Jurrerru River Valley. They even found ash as far away as the Thar Desert.
But the group's most interesting findings concern the future, rather the past. They believe that a close study of the Toba eruption could help predict how human society might respond to large-scale climate changes in the future.
"It's a big project," says SB Ota, regional director for the Archeological Survey of India's Bhopal office. "We don't know whether or to what degree the eruption affected human culture."
Recently, the archaeologists found early human settlements — marked by prehistoric stone tools — that suggest humans weren't wiped out by the disasters like previously believed.
"We use an interdisciplinary approach," says Korisettar. The Toba team includes specialists in archaeology, soil chemistry, climate modelling and early stone art.
They are already planning their next missions — further north, upwards into Rajasthan — a journey that could lead all the way to India's first-ever human settlement. As always, they'll look first for the telltale Toba ash, the white powder that is an archaeologist's buried treasure.
"With this kind of tantalising results, we are sure that India has many such archeological contexts," says Korisettar.
"Our findings have created history."