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By Tim Appenzeller

One day some 74,000 years ago, in a swampy valley in the south of India, dawn never came. In the half-light, greyish dust sifted down, blanketing the ground and turning trees to ghosts. Far to the east, a volcano called Toba on the Indonesian island of Sumatra had unleashed one of the greatest eruptions ever known, flinging thousands of cubic kilometres of rock into the atmosphere and spreading a pall of ash across southern Asia.

Clive Oppenheimer, a volcanologist at the University of Cambridge, UK, has studied the ash deposits in India's Jurreru Valley to reconstruct the events that followed. Within days, the trees shed their whitened foliage; rains later swept ash into layers several metres thick on the valley floor. Eventually, the lakes and swamps vanished, perhaps because the climate had become drier and cooler. Toba had transformed a lush habitat into a wasteland.

The catastrophe had witnesses. Archaeologists digging beneath the ash layer have found stone artefacts indicating that humans were living in the valley before the eruption. But were they modern humans — people like us — or some other, now extinct, branch of the human lineage?

Today, the thick ash deposits of the Jurreru Valley mark a division not only in the geological record but also between archaeologists debating one of the field's biggest questions: when and how did modern humans leave their African cradle and colonize Asia, Earth's largest landmass? It was the first great expansion of the human species, carrying people some 12,000 kilometres to Australia by about 50,000 years ago. But just how early the pioneers set out is controversial — as are the routes they followed, the tools they carried and, most fundamental of all, what triggered the migration. Were they enticed into the wider world by a favourable climate, or propelled by a revolution in technology and culture?

In archaeologists' shorthand, the debate boils down to a simple question: pre-Toba or post-Toba?...

The pre-Toba artefacts from the Jurreru Valley look nothing like the Arabian ones, says Anthony Marks of ÃÛÌÒ½´Methodist University in Dallas, Texas, who studied the Jebel Faya material. And the archaeologist who analysed the oldest relics from the Jurreru Valley and provided key support for the claim that they are the handiwork of modern humans is no longer so sure. Chris Clarkson of the University of Queensland in Brisbane, Australia, a frequent collaborator of Petraglia's, now thinks they might be the work of an unidentified population of archaic people....