Feature

Supervolcano: How humanity
survived its darkest hour
(New Scientist)

THE first sign that something had gone terribly wrong was a deep rumbling roar. Hours later the choking ash arrived, falling like snow in a relentless storm that raged for over two weeks. Despite being more than 2000 kilometres from the eruption, hominins living as far away as eastern India would have felt Toba's fury.

Toba is a supervolcano on the Indonesian island of Sumatra. It has blown its top many times but this eruption, 74,000 years ago, was exceptional. Releasing 2500 cubic kilometres of magma - nearly twice the volume of mount Everest - the eruption was more than 5000 times as large as the 1980 eruption of mount St Helens in the US, making it the largest eruption on Earth in the last 2 million years.

The disaster is particularly significant since it occurred at a crucial period in human prehistory - when Neanderthals and other hominins roamed much of Asia and Europe, and around the time our direct ancestors, Homo sapiens, were first leaving Africa to ultimately conquer the world. Yet with no recent eruptions for easy comparison, the full extent of its fallout and impact on early humans has been shrouded in mystery.

Now dramatic finds from archaeological digs in India, presented in February at a conference at the University of Oxford, are finally clarifying the picture of the eruption and its effects, and how it shaped human evolution and migration. Further results from the digs may even rewrite the timing and route that modern humans took out of Africa.

The new work portrays a somewhat different view of the eruption from the most popular current theory. Previous computer models of the eruption had suggested the event was truly cataclysmic - very nearly a doomsday for early humankind. With calculations based on the assumption that Toba belched out 100 times more aerosols than the 1991 eruption of mount Pinatubo in the Philippines, and scaling the environmental effects accordingly, the models suggested global temperatures dropped by about 10 °C following the blast. This supports the idea of a decade-long "volcanic winter" and widespread catastrophe (Journal of Geophysical Research - Atmospheres, vol 114, p D10107).

To make matters worse, the aerosols would have blocked out life-giving sunlight and absorbed water vapour in the atmosphere, causing a dryer global climate for the next few years. This would have resulted in a rapid decline in tree cover and a concomitant expansion of grasslands, leading to the extinction of many mammals and nearly wiping our ancestors out. The few primitive humans that did survive the eruption would have had to act fast, quickly adapting their way of life to suit the new conditions, travelling further to find food and cooperating with neighbouring populations in the battle for survival.

Indeed, the event may have drastically altered the path of evolution for our own species, Homo sapiens. Modern humans, who were still thought to be living in Africa, would have been whittled down to just a few thousand breeding pairs scattered in dispersed refugia - creating a so-called "genetic bottleneck" in evolution. As the separate colonies developed independently of one another, they would have sown the seeds for the genetic differences between races once these separate groups eventually left Africa.

Yet this theory has drawn some criticism since it was first put forward 17 years ago, with scholars such as Hans Graf, an atmospheric scientist at the University of Cambridge, believing that the climate change following the explosion has been wildly overestimated.

For Graf, the crux of the argument concerns the precise cooling effect of the sulphur dioxide released by the explosion. During smaller eruptions, like that of mount Pinatubo, most of the released sulphur dioxide reacts with hydroxide (OH) ions from water molecules in the atmosphere to form particles of sulphate - a highly reflective substance that bounces sunlight back into space before it can warm the Earth.

Previous estimates had placed Toba's sulphur dioxide production at 100 times that of mount Pinatubo's output. Graf thinks this figure is misguided: recent chemical analyses of Toba's fossilised magma suggests it should be roughly half that. "We think Toba was more of a giant in ash production, not sulphur," says Graf.

What's more, he says, the atmospheric effect of a super-eruption is incomparable to a smaller, typical eruption. Whereas most of the sulphur dioxide from mount Pinatubo would have been rapidly converted to sulphate particles, there simply would not have been enough OH ions in the atmosphere available to react with all the sulphur dioxide released by Toba, delaying the formation of these reflective particles.