What is a population bottleneck?
A population bottleneck is a severe reduction in population size for any given species which reduces the genetic diversity of that species.
Now given the amount of time human beings have existed on this planet there should be a reasonably wide range of genetic variation, yet human DNA is remarkably uniform throughout the world. Hence it is concluded that at some point in the past, the human species suffered its own population bottleneck, a dramatic decline in population caused by some unspecified natural disaster.1
For example, Lynn Jorde and Henry Harpending 2 from the University of Utah have studied the patterns of mitochondrial DNA (mtDNA) within the human population and have concluded that it is consistent with a dramatic reduction in population size at some point in our past, which they calculate occurred roughly 70-80,000 years ago when the human population was reduced to as few as 5 or 10,000 individuals.
Volcanoes and supervolcanoes
A normal volcano is formed by a column of magma rising from deep within the Earth, erupting on the surface and hardening in layers down the sides, forming the familiar cone-shaped mountain.
A supervolcano is formed when the magma, instead of breaking through the surface, pools beneath the surface melting the crust to form more magma and forms a depression in the ground. A vast reservoir of molten rock is formed, trapping volcanic gases which gradually build up colossal pressures over thousands of years. When supervolcanoes erupt they do so on a grand scale; on the volcanic explosivity index that goes from one to eight, they rate the full eight.
The Toba eruption
Now this is all theory, as no living human has ever witnessed a supervolcanic eruption, but it is believed that the last supervolcano to erupt was at Toba in Sumatra, the biggest volcanic eruption the world had ever seen, 10,000 times the size of the 1980 Mount St. Helens eruption, blowing a big 100 kilometres by 60 kilometres hole in the earth that is now known as Lake Toba.
It blasted vast clouds of ash across the world and released a large quantity of sulphur dioxide into the atmosphere. The global temperature dropped by a full five degrees centigrades, sufficient to cause the summers to freeze in Europe and to create a global volcanic winter (cf nuclear winter).
And when did this happen? 74,000 years ago, right within the 70 to 80,000 year time frame that messrs Jorde and Harpending believe that the human population bottleneck occured.
Hence the conclusion; around 74,000 years ago Toba erupted and created a volcanic winter so severe that it almost wiped out the entire human population on the planet. As it was humankind was reduced to a mere 5 or 10,000 souls and only just about managed to cling onto life.
The End of the World as We Know It
No one is sure where all the supervolcanoes are in the world but they do know that there's one right under the Yellowstone National Park, in the USA. The geological record show that it has erupted three times before, once every 600,000 years. The last one was 600,000 years ago, so the next one is due soon.
When it blows it is going to wipe out most of North America and create a volcanic winter that will stop global agriculture in its tracks for a good few decades. It might not be as bad as the Toba eruption but it'll be bad enough.
Fortunately 'soon' in geological terms means anytime in the next ten thousand or so years. But it might be tomorrow. Or the day after. But then maybe asteroid 1950 AD will get us first.
NOTES
1 This of course excites the creationists who see the cause of the population bottleneck as quite obviously the biblical flood, but that's another argument entirely.
2 Respectively the Professor of Human Genetics and the Professor of Anthropology at the University of Utah
SOURCES
James Q Jacobs Population Bottlenecks and Volcanic Winter at
http://www.jqjacobs.net/anthro/paleo/bottleneck.html quoting its source as;
Ambrose, Stanley H. 1998. Late Pleistocene human population bottlenecks, volcanic winter, and differenciation of modern humans. Journal of Human Evolution 35:115-118.
The transcript of the BBC Horizon programme on Supervolcanoes at
http://www.bbc.co.uk/science/horizon/1999/supervolcanoes_script.shtml