The Toba volcano and Out of Africa

17 Oct 2010

New Scientist 17th April, 2010 ... had an interesting story on the so called Toba super volcano which blew its head on what is now Sumatra - 74,000 years ago. Various articles appeared that seemed to suggest it was a massive catastrophic event that almost wiped out humanity from the face of the earth - creating a genetic bottlekneck in Africa. The severity of the Toba eruption was not actually based on reality but on a series of computer simulations. From these projections we learnt that Toba released 2500 cubic km of magma and was 5000 times more powerful than the 1980 eruption of Mt St Helens. It belched out 100 times more aerosols that the recent eruption of Pinatuba in the Philippines and it was suggested it caused global temperatures to drop by 10 degrees Celsius. This set in motion a ten year long 'volcanic winter'  and widespread catastrophe as trees died and animals became extinct due to loss of habitat etc. Modern humans, Homo sapiens, then thought to be confined to Africa, would have been whittled down to a few refugia. After the event and as they regrew in numbers, they may have sown the seeds of the genetic differences that we now see between different races once they had left Africa.

It's enough to give catastrophism a bad name and naturally the theory has a number of detractors, not least being Hans Graf, an atmospheric scientist at Cambridge University. He thought the climate change scenario was grossly exagerated - and suggested the magma produced was at least half of that claimed, with the implication it was even less. He said it was probably a giant in 'ash' production, nor sulphur, and said a lower figure in temperature was appropriate - possibly a drop of 2.5 degrees.

The simulators were of course miffed with this slicing down of their catastrophe but unfortunately for the computer modellers archaeologists and geologists looking for evidence of the Toba event in India have come up with evidence that would tend to support Graf. There is no evidence of a decline in woodlands but ash would have spoiled water supplies, it is suggested, but most importantly a layer of ash was found. No skeletal remains, in contrast, were dug up but many stone tools were found, in a layered strata. A deposit that appears to have been a camp and tool manufacturing site was buried under ash - but the layer above the ash suggests that life went on much as before, and this situation was found at more than one locale.

There has of course been spirited defense by the catastrophist camp - and by the consensus opinion makers regarding human origins, for if Toba was really not as powerful as the modelling portrayed this will affect current ideas on human evolution and dispersal. A lot depends on which species of humans made the tools found in India. According to the consensus view humans did not arrive in India until around 60,000 years ago. Before that date they are, or were thought to have been confined to Africa in a pristine untouched environment where a pure strain of humanity was eventually unleashed on the world - and quickly colonised it. This position has been maintained even though there is evidence of modern humans in the Levant as early as 125,000 years ago - but this was countered by the claim that they were pushed back into Africa by the succeeding Ice Age which was very dry, they think, in western Asia. At the moment it is thought the tools were the product of an earlier species of humanity such as Homo erectus - and that may well be right. Homo erectus are known to have been in India some 700,000 years (geochronology). However, the suspicion is that the tools were actually made by Homo sapiens and this is throwing a brick into the consensus workings. It is actually backed up by evidence from Australia which has been stuffed under the carpet and marginalised simply because it did not fit into the consensus model. Homo sapiens was in Australia by 60,000 years ago - possibly even before that. At Kota Tampan in Malaysia a stone tool culture has been found spanning the period between 74,000 and just 4000 years ago - when it disappeared. This would seem to indicate the dispersal into the Levant around 125,000 years ago was not a failure - as demanded by the consensus model. Hence, it is quite reasonable to assume Homo sapiens expanded during the last interglacial period reaching and colonising SE Asia and Australia, migrating via India.

However, the consensus model is not beaten as it is now being suggested the first wave of humans were killed off by the Toba eruption - and the later dispersal, which came after 74,000 years ago, replaced them. It remains to be seen if this compromise catches on or anthropologists cry foul.