When Mount Toba erupted on the Indonesian island of Sumatra some 74,000 years ago, ash fell like snow on the Indian subcontinent, including on human toolmakers who had shaped stone flakes into sharp cutting instruments. Debate over the identity of these craftspeople—and whether a cataclysmic “volcanic winter” wiped them out—has raged for decades, because it has implications for when our species first left Africa. A new study of these people’s tools suggests they not only survived the eruption, but thrived for another 50,000 years. Others, however, say there isn’t enough evidence that the tools were made by Homo sapiens at all.
The researchers may have found “an early wave of modern humans … or it might be another kind of early human altogether,” says Martin Richards, an archaeogeneticist at the University of Huddersfield, Queensgate, who was not involved with the work.
Some scientists have argued that the massive ash cloud from the Mount Toba eruption would have partially blotted out the Sun, sending global temperatures plummeting and threatening the survival of numerous species, including our own.
In 2007, though, anthropologists found evidence of stone tools in southern India that dated to before and after the eruption, suggesting the event may not have been as devastating as previously thought. But critics said it wasn’t clear whether the tools were made by our species or another archaic human such as Neanderthals or Denisovans.
In the new study, the team behind the 2007 study returned to the subcontinent, to a site known as Dhaba in central India on the banks of the Son River. The researchers uncovered thousands of stone flake tools used for cutting and scraping.
Most of the tools in the lower sediment layers are consistent with a specific production method known as the Levallois technique, which was used by both modern humans and Neanderthals during the Middle Stone Age, about 250,000 to 25,000 years ago. In the upper layers, however, the tools transition to smaller, more complex stone blades that are unquestionably made by our own species, the researchers argue.
Using a technique called optically stimulated luminescence, which measures electrons to infer when layers of sediment were last exposed to light, the researchers dated the site to a continuous occupation stretching from about 80,000 to 25,000 years ago. The dates, combined with the uninterrupted manufacture of increasingly complex tools, suggests modern humans were not only present in the region when Toba exploded, but that they survived for many thousands more years, the researchers report today in Nature Communications.
The findings bolster arguments by others, including geochronologist Kira Westaway at Macquarie University, that some members of H. sapiens made it as far as northern Australia some 65,000 years ago—though these early migrants apparently died out without leaving any traces in living people. “This paper finally connects the dots between India, southeast Asia, and Australia for modern human dispersal,” Westaway says.
Team leader Michael Petraglia, an anthropologist at the Max Planck Institute for the Science of Human History, suggests the eruption wasn’t so devastating. “The impact of the Toba eruption on climate has been overestimated,” agrees Chad Yost, a geoscientist at Arizona State University, Tempe, who was not involved with the work.
Stanley Ambrose, an anthropologist at the University of Illinois, Urbana-Champaign, and a fierce critic of the team’s 2007 study, remains unconvinced. He says it’s impossible to tell which species made the tools based on the evidence presented. What’s more, he questions the dates because the team found no well-defined layer of ash in the sediment, which would have more conclusively shown that the toolmakers’ occupation spanned the eruption.
Richards doesn’t doubt the team’s dates, but says identifying these toolmakers will require human fossils. “Without them, we can’t really be confident that we’re talking about modern humans [or] some other archaic hominin.”