Scientific Methods

Thursday, April 13, 2006


Ever wonder if humans only left Africa fairly recently why don't we all look more African? Also, when you look at humans from a genetic point of view we are very similar suggesting that some time in the past our numbers and therefore genetic diversity was greatly reduced by some global catastrophe. Professor Stanley Ambrose, an anthropologist working at the University Of Illinois believes that he has the answer.

According to him the largest volcanic eruption occurred about 71 000 - 73 000 years ago in Sumatra, Indonesia (1). It has been called the "Toba eruption" and its end product was a volcanic winter that lasted 6 years (proceeded by a 1000 year long uber-ice age) that almost wiped mankind off the face of the Earth (1, 2). Studies have shown that Toba ejaculated about 1015 g of ash and H2SO4 aerosols that dispersed into the stratosphere and across the globe (2). The aerosol cloud blocked out the Sun, which meant that there was a rapid drop in temperatures (by as much as 15 degrees Celcius regionally and more than 5 degrees globally) (2). It is believed (by those who study human genetics) that the effects that the Toba eruption had on the climate also had dramatic effects on the human population of the time (1, 2). It caused the population to undergo a dramatic decrease in numbers falling to about 10 000 adults (2). These humans survived in small isolated, tropical settlements in Africa and Eurasia (1, 2). The humans living in Europe and China would have been totally wiped out by the drop in temperature (1). This separation and isolation resulted in the surviving humans undergoing genetic drift and local adaptations and thus racial differentiation was born (1, 2). So hypothetically, the Mount Toba eruption may have caused modern races to have formed swiftly (70000 years ago) instead of progressively over one million as previously thought (1). Studies have also shown that many other species also underwent a population bottleneck, which would have also contributed to the drop in human population (no plants + no animals = no food and no clothes). The rapid drop in temperature hit tropical plants the worst as they were use to warm and wet conditions. There were severe droughts in the tropical rainforest and monsoon regions. After the bottleneck there was a rapid increases in numbers as conditions and technology improved (1, 2). This prompted more migrations to occur (2).

For a long time scientists have suspected that the human population had undergone a bottleneck but could not figure out how or why (3). The Toba eruption hypothesis provides an answer. The evidence for these climatic changes can still be found in coral reefs and ice cores (3).

On average the Earth experiences a volcanic winter due to a super-eruption (by super I mean global killer) every 50 000 and since the last one (mount Toba) occurred 73 000 years ago it means that we are 23 000 years overdue (I guess mother Earth didn't get the memo). This figure is an estimate based on past data and geological events so scientists cant say for sure when it will happen but chances are if the Earth continues to function the way it has for 4.5 billion years then its definitely going to happen again and maybe this time we won't be so lucky.


1. Ambrose SH (1998) Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans. J Hum. Evol. 34:623-651

2. Rampino MR, Ambrose MH (1999) Volcanic Winter in the Garden of Eden: The Toba super-eruption and the Late Pleistocene human population crash. World Archeological Congress.


Bradley Flynn
Department of biodiversity and conservation biology
University of the Western Cape
private bag X17
contact number: 083 380 2036


  • I sure am looking forward to your Power Point Presentation! This sounds very interesting... I have read about the Toba eruption and would like to learn more.

    By Anonymous Karen Marais, at Thursday, April 13, 2006 12:44:00 PM  

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