The Toba supervolcano erupted on a Sumatran island in Indonesia around 74,000 years ago. Scientists believed that this eruption released enough ash, rock and gases to block out the Sun and cause a volcanic winter that may have pushed humanity to the brink of extinction. But recent evidence from excavations in South Africa suggest that human settlements may have thrived during this difficult period.
The eruption, touted to have been the largest in two million years, released more than 2,800 cubic km of magma. According to one report, this is enough to bury the entire US under a foot of rock and ash.
Scientists predicted that the volcanic particles released by the eruption blocked the Sun for many decades, drastically altering Earth’s climate. They speculated that this reduction in sunlight killed many plants, which in turn led to a decrease in the population of animals, including humans, that were dependent on them.
Some proponents of this ‘Toba catastrophe theory’ even believed that this pushed humans to the brink of extinction, reducing their numbers to mere thousands. But many experts argued that the catastrophe theory overestimated the environmental impact of Toba.
The resolution of this argument came from tiny volcanic glass shards called cryptotephra. These microscopic shards were flung out in all directions by the eruption. Deposits of glass shards from the Toba eruption were earlier discovered in India, Malawi and on Toba itself.
Some of these shards travelled around 9,000 km to fall on South African soil. It is the presence of the these shards among artefacts of human settlements that challenged the Toba catastrophe theory in a big way. Analyses revealed that these shards were chemically identical to the earlier ones found in India and on Toba, thus confirming that they came from the Toba eruption.
An international team of researchers, led by Curtis Marean from Arizona State University, excavated two sites near South Africa’s Mossel Bay, Pinnacle Point and Vleesbaai, and discovered a layer of these microscopic glass shards. What was more interesting was that they found abundant artefacts of human settlements, like bones and tools, both above and below this layer of glass.
If the Toba catastrophe theory was true and humans died in large numbers after its eruption, one would expect little or no signs of human activity in the period immediately following the eruption.
The researchers, however, found that the signs of habitation were “intense and continuous” both before and after the eruption. In fact, according to Zenobia Jacobs, a professor University of Wollongong who was also involved in this study, the population seemed to have gotten better at making tools after the eruption.
In a similar study, analysis of sediments found around the cryptotephra layer in Lake Malawi showed no evidence for substantial climate change. This further supports the theory that the Toba eruption may not have caused as much climatic shift as expected.
Right place, right time
While agreeing that massive changes were improbable, some experts confess that the populations off Mossel Bay might have had an inherent advantage that helped them survive.
According to Kira Westaway, professor at Macquarie University, who was not involved in this study, these settlements along the coast had an upper hand because they could rely on shellfish for food. “Shellfish in particular aren’t as affected by changes in climate as with terrestrial flora and fauna”, she said.
She also added that inland populations were more susceptible to climatic changes since extinction of flora and fauna might have made it harder to find resources.
The researchers said that studying different sites across Africa can help understand if inland and coastal human settlements had differential survival in the wake of the eruption.
Modern human beings were not the only ones to survive the aftermath of the Toba eruption, according to Chad Yost from the University of Arizona. Other hominids like the Neanderthals of Europe and hobbits of Flores survived it, despite being closer to Toba than Africa. He warned against using knowledge of modern day eruptions to predict the nature of Toba because each eruption is unique. He said that continuing to do so would only lead to false estimates and subsequent predictions of catastrophe.
Microscopic timepieces
The Toba eruption and the material that it spewed out had a major impact on geochronology, the branch of geology that deals with the dating of rocks and sediments.
Archaeologists usually find it difficult to compare the time periods when settlements from two different sites existed since archaeological methods come with huge margins of error. With deposits of these glass shards from Toba, experts can narrow down time scales to the order of weeks, since they know that the cryptotephra were deposited over a two-week period.
These glass shards and their ability to greatly narrow down time estimates is seen as the most exciting thing to emerge from Marean’s study. For Siwan Davies, professor at Swansea University, this “opens the door for many more groundbreaking studies to assess the impact of this eruption on other populations, in Africa and farther afield”. Such studies can then help to see how different populations fared during the period immediately after Toba’s eruption.