The Toba eruption suspended volcanic gas and sulfuric acid in the stratosphere for years, reflecting warm sunlight away from Earth.
Ice cores reveal that the world was cooler by 5.4 to 9 degrees Fahrenheit (3 to 5 degrees Celsius) for several centuries following the event.
"It would have been more challenging times," said Will Harcourt-Smith, a paleontologist at the American Museum of Natural History in New York who was not involved in the study.
"My point is that humans were complex enough at this point to have dealt with that."
In Africa at this time, humans were exhibiting early symbolism, complex toolmaking behavior, and sophisticated social behavior, he said.
(Read related story: "Is Bead Find Proof Modern Thought Began in Africa?" [March 31, 2004].)
"These people are not behaviorally like you or I, but they are modern humans and they have many of the vestiges of humanity," Harcourt-Smith said.
Humans or Neandertals?
Some experts caution that telling the difference between human-made stone tools and those made by the now extinct Neandertals (or Neanderthals) is tricky business.
"Previous research on this subject has shown that South African and Neanderthal European [stone tool] assemblages are technologically and typologically indistinguishable," said Stanley Ambrose, an archaeologist at the University of Illinois at Urbana-Champaign in Urbana who was not involved in the current study.
"[The study authors] have a bagful of artifacts on which they're drawing conclusions that can only be confirmed by fossil evidence, and they don't have any fossils."
Chris Clarkson, an archaeologist at Australia's University of Queensland and one of the study's authors, said a large piece of ground ochre was found below the ash with the stone tools. Ochre was used by early humans for art, symbols, curing hides, or helping to attach stone tools to build a wooden shaft.
"All of these potential uses hint at more complex behaviors than are usually attributed to earlier extinct hominin species, although we know European Neanderthals also used ochre a lot," Clarkson said.
"More excavation will help us resolve whether this assemblage belongs to modern humans or not."
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