This blog provides a forum for presenting and discussing the latest findings relating to the ancient Indian Ocean, from archaeology, molecular genetics, historical linguistics and other disciplines. It takes a long-term view of the Indian Ocean region, exploring the processes that shaped its cultures, societies and environments from the Pleistocene to the historical period.

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Friday, 3 September 2010

Toba Supervolcano – Impact on Indian Environments Overblown

Palaeo-3 is the latest forum for a debate on the Toba volcanic super-eruption of 74,000 years ago and its role in shaping terrestrial environments and evolutionary events. The most recent debate stems from an article published by Martin Williams and his team which contends that the Toba eruption was followed by climatic cooling and the deforestation of India (see also press release by co-author Stan Ambrose, and sensational media attention). The researchers base this claim on study of carbon isotopes from ash profiles in the Son and Narmada River Valleys in India and pollen from a marine core in the Bay of Bengal. The team of earth scientists and archaeologists argued that Toba led to major and prolonged alterations in ecological settings in India, such as the replacement of forests by grasslands and woodlands. The investigators reasoned that these dramatic landscape alterations would have taken a heavy toll on tropical ecosystems and even the survival of certain mammals and humans in India and elsewhere, as supposedly demonstrated by genetic bottleneck data in a number of species. In their article, they say, "Our results demonstrate that the Toba eruption caused climatic cooling and prolonged deforestation in South Asia, and challenge claims of minimal impact on tropical ecosystems and human populations" (emphasis ours).

Though we are keen to learn more about the effects of the Toba super-eruption on ecosystems, especially given the paucity of such studies in India, we make the case that Martin Williams and his team have overstated their evidence. In a Palaeo-3 comment by Michael Haslam and myself, we assert that the new study provides no compelling evidence that clearly and convincingly links Toba to major and catastrophic impacts on ecological settings in India. For starters, as revealed by the illustration provided in Haslam and Petraglia (above), we point out that climatic cooling was well underway before the volcano exploded, thereby questioning the cause and effect relationship between Toba and environmental deterioration. Moreover, we argue that the so-called forest to grassland transition may be the consequence of natural changes in climatic regimes or even to other local depositional processes, such as the growth of grasses on top of ash. Indeed, we are highly skeptical that all vegetative communities across India responded in the same way after the ash was laid down. Instead, we suggest that a mosaic of ecological conditions existed across the subcontinent after the Toba event. On the basis of the current evidence, we see no reason to accept the claim, as forcefully made by Williams and his colleagues, that Toba altered the course of human and mammalian evolution. And, on this score, it is somewhat surprising to see that Williams and his team admit in a reply that the evidence to demonstrate a causal link between Toba and genetic bottlenecks is tenuous.

The Toba debate will not end here. But, stay tuned for additional publications that will soon appear on Toba and the archaeology of human populations residing along the Indian Ocean rim.

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