Mount Toba is a volcano
. Its eruption
about 70 000 years ago was one of the most violent in the geological
history of the earth. It caused a volcanic winter
for about six years and an Ice Age
lasting a thousand years. The forests of South-East Asia
were totally destroyed.
It is increasingly believed that it was a bottleneck in human evolution. If this is true, the human race was driven close to extinction, and the survivors emerged from it, perhaps having evolved modern language because of it. This small African group fanned out across the world and replaced older human populations: if the Out of Africa theory is true.
The largest volcanic explosion in recorded history was that of Tambora, also in Indonesia, in 1815. The third-largest explosion of the twentieth century was Mount St. Helens in 1980. Here are the relative sizes of these eruptions in terms of the amount of ash they displaced:
- Mount St. Helens: 0.2 km3
- Tambora: 20 km3
- Toba: 800 km3
Beds of volcanic ash from Toba are found in India, several metres thick, in some cases 6 m thick. The atmosphere would have been dark from ash for several months. After that, sulfuric acid is believed to have subsisted in the upper atmosphere for perhaps six years, drastically reducing radiation reaching the earth. Sea temperatures dropped by 3°C or more.
Oxygen isotope ratios for ice cores from the millennium immediately after the explosion show that temperatures were then the lowest at any time in all the Ice Ages of the last 100 000 years. Summer temperatures were 12°C lower than in the previous periods.
DNA evidence in humans suggests that our ancestral population was at some point a mere 10 000 individuals or so. The Toba event is a perfect candidate.
Equatorial Africa would have been one of the few places affording a habitable niche. Most human genetic diversity today is among Africans. All non-Africans form a single sub-branch of one of the African branches.
Linguistics is also consistent with it. Modern language families can not be traced back with any certainty more than, at most, 10 000 years, but speculations about deeper relationships would also perfectly match a single common origin at this time.
Something profoundly changed in human cognition at around this time; whatever sets us off both from Neanderthals and from anatomically earlier Homo sapiens. They had culture and quite possibly primitive language; but after the change the survivors were the same as people of today.
The Toba hypothesis is related to, but is not the same as, the DNA analysis of human evolution nicknamed Mitochondrial Eve. It is possible that Mitochondrial Eve lived a long time before the Toba bottleneck.
The main proponent of the so-called Weak Garden of Eden model, has been Stanley Ambrose, who published it in 1998. His paper says it was 71 000 years ago: other sources say 72 500 and 75 000.)
The caldera now forms a lake 100 km long and 30 km wide, with an island called Samosir in the centre, formed by upwelling magma. There have been no eruptions of Toba in historical times. Its volcanic nature was discovered in 1949.
Update, July 2003. Work by Professor Zuraina Majid in Malaysia suggests modern humans were there before the Toba explosion. There is a seemingly primitive tool-making culture found at Kota Tampan in the Lenggong Valley, and an associated modern skeleton called "Perak Man", found in 1990. Dating was dubious for these, but they now think the volcanic ash layer at the Kota Tampan site belongs to the Toba explosion, which they date at 74 000 years ago. The ash deposit from the cataclysm must have totally wiped out any humans in the region of India, isolating modern humans in the Far East from those in Africa. This is consistent with mitochondrial DNA evidence for maternal lines in Asia today. A forthcoming book discussing this is Out of Eden: The Peopling of the World, Stephen Oppenheimer, Constable.
Another possible piece of evidence against it is an artefact found in Blombos Cave in South Africa, a piece of red ochre which appears to show tally marks, but which has been dated to about 77 000 years ago.
Ambrose, Stanley H. 1998. Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans. Journal of Human Evolution 35:115-118.