More than 5,500 miles from the site of the Toba supereruption in Southeast Asia, Curtis Marean, an anthropologist at Arizona State University, and his colleagues discovered signs of its debris at two archaeological sites on South Africa’s southern coast. The appearance of microscopic glass shards once ejected by the Toba event amid layers of ancient bones, complex stone tools and evidence of human fires allowed the team to directly observe the volcano’s impact on the human population for the first time.

The results surprised Dr. Marean. Should Dr. Ambrose’s theory be correct, there would be fewer signs of human occupation in the layer of soil above the one with the signs of the Toba supereruption. Dr. Marean’s team saw the opposite: After the catastrophic event, there were more signs of human occupation. Not only did humans appear to adapt to the trauma caused by the event, they thrived, said Eugene Smith, an author of the study and a retired geologist.

That doesn’t mean Toba’s volcanic winter never occurred. Dr. Marean speculates that an ensuing global chill may have driven these prehistoric humans to the coast where they were able to survive.

But not all experts agree with that interpretation.

Although Dr. Petraglia praised Dr. Marean’s work, he said it did not buttress the case for a global climate catastrophe following the Toba eruption. He pointed to a study published this year of a similar ash layer within Lake Malawi in East Africa. There, scientists found no signs that the lake’s temperature dropped significantly after the event — suggesting that there was no volcanic winter, and further challenging the idea of a human population decline resulting from the Toba eruption.

And he’s not alone.

“I personally lean toward the idea that Toba just didn’t have sufficient impact to have a significant impact on Homo sapiens in East Africa, period,” says Thomas Johnson, a retired paleoclimatologist at the University of Minnesota, Duluth, who was not involved in the study. “The large majority of the information that keeps coming out keeps putting nails in the Toba coffin.”