18. On the verge of extinction

 

A key moment in the dispersion of AMHs over the rest of the globe happened 74,000 years ago with the disastrous explosion of the supervolcano Toba, located in northern Sumatra. This was the largest volcanic event on earth in the past two million years. It involved the explosive eruption of at least 2,800 square kilometers of tephra (Greek word for ‘ash’) , causing ash plumes to cover a large part of the world, from the south China Sea to the Arabian Sea. It left behind what still is the world’s largest caldera: Lake Toba.

For comparison: the largest volcano event in historical times was the Tambora eruption on the Indonesian island Sumbawa in 1815. It produced the ‘year without summer’ in 1816. But Toba ejected about 300 times more volcanic ash than the eruption of Tambora and caused six years of climatic deterioration (‘volcanic winter’) which in turn caused the decimation of animal and human life over very large areas.

That most paleo authors do not mention the Toba bottleneck – in the discussion between Richard Klein and his critics Hensilwood and d’Errico e.a. we don’t find any reference to the Toba event – can perhaps be ascribed to the fact that thorough research on this catastrophe is fairly recent.[1] How severe the impact has been on the total human population is still unclear, but geneticists Jore and Harpending[2] propose that all humans alive today are descendants from a very small part of the earlier population: perhaps some 8000 breeding pairs about 70.000 years ago[3]. The impact on the northern hemisphere was more severe than in the south; nevertheless also Neanderthals survived the supposed six years of volcanic winter. The AMHs had even better chances, not only because they lived farther south, but also because of their broader subsistence strategy based on hunting and gathering coastal resources (shellfish, fish, sea lions and rodents, as well as bovids and antelope).

In Jwalapuram in India, archaeologists dug through the several metres-thick Toba ash layer (see photo). First, above the Toba tuff, they found many stone artefacts from about 74,000 years ago, made from limestone, chert, chalcedony and quartzite, with blades and bladelets representing a Late Pleistocene assemblage also assigned to the Middle Paleolithic. Digging deeper, below the thick Toba tuff layer, they found many more stone tools that could be dated at about 77,000 years ago, of limestone, quarzite, and chert: scrapers, blades and a burin also identifyable as Indian Middle Paleolithic. This lithic assemblage clusters together with the lithic technology of sub-Saharan Africa, and indicates that the AMHs belonged to the first migration wave out of Africa. We will talk about this in the next paragraph.

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The Toba ash deposit layers, as excavated at Jwalapuram in Southern India. The ash layers reach from just above the bottom of the pit to halfway up the ladder. Only the bottom layer dates from immediately after the eruption: the higher layers originate from aeolian transport and slope-wash during annual monsoons into this low location.

According to the excavators, before the explosion this place was a ‘paradise’ on the shore of a lake. Then it got buried under two-to-five metres of volcanic ash. The next six years, with the atmosphere still full of ash particles, were a volcanic winter. During the next thousand years, the climate still was extreme cold: a glacial maximum[4]. But perhaps after some decennia of annual monsoons, big parts of the area became sufficiently overgrown to harbourgrass eaters again. So archaic AMHs must have lived in this area, and after the Toba explosion survivors reappeared as soon as grass vegetation and grazing animals reappeared.

The Jwalapuran excavation is not the only site showing evidence of the Out of Africa (OoA)-migration of the AMHs, both before and after the Toba explosion. Another recent excavation site is Jebel Faya[5], where 125,000 year old hand axes are found, showing a pattern of flaking seen only in early Africa. Indicating an out-of-Africa migration 20.000 earlier than thought before. Indeed, 130,000 years ago, there was a window of climate change. The Arabian Peninsula was more habitable than today.


[1] in an article in October 1993 Ann Gibbons, a staff journalist for Science, first suggested that a bottleneck in human evolution about 50,000 years ago could be linked to the Toba eruption. Rampino and Self backed up this idea in a letter to the journal later that year. The bottleneck theory was further developed by Ambrose in 1998 and Rampino & Ambrose in 2000, who invoked the Toba eruption to explain a severe culling of the human population. (Wikipedia)

[2] Population Bottleneck from Macmillan Science Library: Genetics. Copyright © 2001-2006

[3] Wikipedia “Toba catastrophe theory”

[4] OIS 3 – 63,000 to 45,000 years ago (warm)
OIS 4 – 73,000 to 63,000 years ago (cold)
OIS 5 – 130,000 to 74,000 years ago (warm)

[5] Anthropology Net 27 Jan 2011: Hans-Peter Uerpmann of the University of Tubingen and his team excavating the Jebel Faya site in the United Arab Emirates

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