Skepticblog

According to some estimates, on Halloween of last year (2011), the population of humans on this planet  passed the 7 billion threshold. Today, humans (along with their domesticated animals) are the most abundant large vertebrates on the planet, and the problem of human overpopulation (and its effects on the overexploitation of the planet’s resources) is one that vexes people worldwide. It’s hard to imagine the idea that humans have not always been so numerous, or that we have not always been the dominant large species on the planet. But it was not always so. As I describe in my recent book Catastrophes!, about 74,000 years ago a volcanic eruption occurred on Mt. Toba in Sumatra which caused a global “volcanic winter” that nearly wiped out humans completely.

Studies of the ash deposits in the adjacent ocean floor around Sumatra show that Toba ejected 2800 cubic km of material. It was believed to be the largest volcanic explosion in the last 25 million years. It released the energy equivalent of 1 gigaton of TNT, forty times larger than our largest nuclear bomb explosion, and about 3000 times as powerful as the eruption of Mt. St. Helens. Toba injected so much ash into the stratosphere that the ash clouds blocked the sun’s radiation. It caused a “volcanic winter” that lasted almost 10 years, and caused global temperature to drop by 3-5°C (5-9°F), further amplifying the cold of the ongoing Ice Ages. The tree line and snow line dropped 3000 m (9000 feet) lower than today, making most high elevations uninhabitable. Global mean temperatures dropped to only 15°C after 3 years, and took a full decade to recover to pre-eruption temperatures. Ice cores from Greenland show the evidence of this dramatic cooling in the trapped ash and ancient air bubbles, although so far it has not been detected in Antarctic ice cores (Rampino and Self, 1993a, 1993b; Robock et al., 2009).

A number of scientists have argued that the Toba catastrophe nearly wiped out the human race, leaving a genetic bottleneck of only about 1,000 to 10,000 breeding pairs of humans worldwide (Rampino and Self, 1993b; Ambrose, 1998). In addition to the geologic evidence of Toba’s size and atmospheric effects, geneticists have found evidence from the molecular clocks in our genomes that human populations went through a genetic bottleneck at about this time.

Not only is there evidence of reduced human populations about 74,000 years ago, but there are many human-dependent organisms that show the same pattern. Scientists found a similar genetic bottleneck in the genes of human lice, and in our gut bacterium Helicobacter pylori, which causes human ulcers; both of these date back to the time of Toba, according to their molecular clocks (Rogers, 2004; Linz et al., 2007). There is also genetic evidence that a number of other large mammals, including chimpanzees (Goldberg, 1996), orangutans (Steiper, 2006), macaques (Hernandez et al., 2007), cheetahs and tigers (Luo et al., 2004), and gorillas (Thalman et al., 2007) suffered population bottlenecks about the time of the Toba eruption. Humans themselves in the region around southeast Asia apparently vanished, because the molecular clocks in the mitochondrial DNA shows that most humans in the region migrated there from Africa shortly after the Toba event.

The details of the Toba catastrophe theory is still being argued over, but it is not unreasonable to think that such a global catastrophe would have profound effects on the human population. It’s startling to realize how recently the most common ancestors of all modern human populations came to inhabit most of the Old World, and how quickly the “races” differentiated over less than 70,000 years. And the Toba story reminds us that no matter how dominant and destructive humans are now, our existence on this planet is precarious and fragile. As historian Will Durant put it, “Civilization exists by geologic consent, subject to change without notice.”