The Toba volcano is located in Sumatra, in one of the most seismically active regions in the world. Its caldera (Lake Toba) is the largest Quaternary caldera on earth, spanning ~100x30 kilometers (~60x20 miles), and it is close to the junction of the Sumatra Fault Zone and the Investigator Fracture Zone. It has erupted four times in the last million years or so: the present shape of the caldera is from the latest eruption ~74,000 years ago. That one was a super-eruption, the largest volcanic event which has occurred on our planet in the past two million years.
The Toba super-eruption involved a minimum of 2,800 cubic kilometers (~670 cubic miles) of magma, and at least 800 km3 (~200 mi3) of volcanic gases and ash were directly injected into earth's atmosphere. Northern Sumatra was covered in a thick deposit over an area of 30,000 km2 (11,500 cubic miles). The enormously thick atmospheric ash plumes blanketed an area of land between the South China Sea to the Arabian Sea, probably within a period of days or weeks.
Toba and Human Evolution
The Toba eruption has been interpreted as a significant driver of severe climatic deterioration, and one of the most significant events associated with human evolution. At a minimum, the ashfall would have polluted the lower atmosphere and water sources for many years. Dips in average temperature associated with Toba have been seen in Greenland ice cores. The eruption and the subsequent cooling of the earth in Marine Isotope Stage 4 (MIS4) have been cited as responsible for the disappearance of our species from Eurasia.
Briefly, the first wave of early modern humans left Africa and arrived in Eurasia approximately 100,000 years ago and are in evidence from what is today the Czech Republic and Israel. But there is a large gap between those first sites and the flourishing of humans in Europe and Asia, about 45,000 years ago: that has been tentatively attributed to the effects of the Toba eruption.
- Read more about Early Modern Humans
Recent analysis, however, suggests that the Toba did not exude as much sulfur as previously estimated, and the climatic upheaval was not as drastic. Evidence of the impact on the human populations in India is variously interpreted. The earliest possible evidence for hominids in the Indian subcontinent is between 108,00 and 128,000 years ago: Middle Paleolithic technology is in evidence from 77,000 through 38,000 years ago.
The Youngest Toba Tuff
Approximately 10 centimeters (4 inches) of ash from Toba (called the Youngest Toba Tuff or YTT) covered the land of much of India, and also fell into the Bay of Bengal and the Indian Ocean. Primary ashfall tephra is also clearly associated with Middle Paleolithic occupations in the Jurreru Valley of southern India, and the Middle Son Valley in Madhya Pradesh of northern India. Archaeological and geological studies have been conducted in Madhya Pradesh since the 1980s, and in Andrah Pradesh at the Jwalapuram sites in the Kurnool district since 2003.
In the Jurreru valley, the mean thickness of tephra is approximately 1 m (3 ft), although there are discontinuous deposits of 2.5 m (8 ft) in thickness in the lower elevations. The tephra today is buried beneath alluvial deposits of up to 2 m (6 ft) in thickness: archaeological study indicates it extends over many square kilometers, and the volume is estimated to exceed 1 million cubic meters (35 million cubic feet). The oldest sites below the tephra in Jurreru are Acheulean in age, ~140,000-125,00 years ago; Middle Paleolithic sites are present before and after the ashfall within a thousand years or so.
The Middle Son Valley tuff deposits occur in a discontinuous belt over a distance of about 30 km (~20 mi), with well-preserved deposits at Ghoghara and Khuteli. Here the primary ash-fall is approximately 4-6 cm (2-3 in) in thickness, with redeposited and reworked ash piled in places to some 4.5 m (15 ft). The oldest sites are also Acheulean, between 100,000-140,000 years ago; but there (apparently) is no Middle Paleolithic occupation before the ash fall, and after the ash fall the Middle Son remained empty for some 50,000 years.
But Did YTT Cause a Human Genetic Bottleneck?
The evidence for the YTT eruption is plentiful, and the effects throughout Asia are inarguable--but were there impacts in Africa that affected human evolution? A sediment core analysis of deposits in the floor of Lake Malawi suggests that, in east Africa at least, there was no major climatic disruption associated with the ash fall.
Sediment samples of the deposits in Lake Malawi were recovered by the Lake Malawi Drilling Project (Lane et al. 2013) via long cores from two locations. Analysis identified the YTT as a cryptotephra layer, subtle horizons of microscopic volcanic glass sherds approximately 27-28 m (88-90 ft) below the lake floor. This deposit included a concentration of 3,500 glass sherds per gram within 1 cm (.4 in) depth of core; the glass has a chemical composition that is similar to that of YTT deposits at Jwalapuram and the Toba caldera.
However, the sediments above the ash fall zone do not show evidence for catastrophic climate change. XRF investigations of those sediments revealed a slightly elevated diatom productivity, but not what one would expect had regional temperatures cooled by ~4 degrees centrigrade, which had been estimated from climate models.
If Lane and colleagues are correct, this suggests that there was no climatic force effecting a "human bottleneck" in East Africa: there is other, genetic evidence that the bottleneck occurred, we just need to look to other causes.
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Clarkson C, Petraglia M, Korisettar R, Haslam M, Boivin N, Crowther A, Ditchfield P, Fuller D, Miracle P, Harris C et al. . 2009. The oldest and longest enduring microlithic sequence in India: 35 000 years of modern human occupation and change at the Jwalapuram Locality 9 rockshelter. Antiquity 83(320):326-348.
Haslam M, Clarkson C, Petraglia M, Korisettar R, Jones S, Shipton C, Ditchfield P, and Ambrose SH. 2010. The 74 ka Toba super-eruption and southern Indian hominins: archaeology, lithic technology and environments at Jwalapuram Locality 3. Journal of Archaeological Science 37(12):3370-3384.
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Lane CS, Chorn BT, and Johnson TC. 2013. Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka. Proceedings of the National Academy of Sciences Early edition.
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