The human population currently stands at approximately 7 billion indivudals, the largest, most successful mammalian population ever. But this was not always the case - in fact, approximately 70,000 years ago the H. sapien population was reduced to less than 10,000 individuals worldwide. Why? What event caused this population bottleneck? What were the consequent impacts on the human gene pool? And (arguably) most importantly, how did we survive?
Toba - The Supervolcano Approximately 70,000 years ago, the 'supervolcano' Toba (located in what is now Sumatra, Indonesia) erupted, spilling 2,800 cubic kilometres of volcanic material into the atmosphere, almost 1,000 times the size of the Mount Vesuvius eruption that wiped out Pompeii. The resultant crater is what is now Lake Toba. |
Figure 4.1 A simulation of the Toba eruption
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The eruption coincides with the onset of the last glacial period, and it is believed that the eruption caused a "brief, dramatic cooling, or 'volcanic winter'" (Rampino, M. L. and Self, S., 1992). This dramatic climate change, rather than the eruption itself, is what caused the sudden population bottleneck in the human population, reducing their numbers to less than 10,000. The eruption deposited an enormous ash cloud, which hung over the Earth for 6 years, choking streams, destroying vegetation, and cooling the frozen Earth a further 3-5 degrees Celsius. There is evidence to suggest that the Toba catastrophe also led to genetic bottlenecks in the populations of other species , such as the Eastern African chimpanzee, Bornean orangutan, central Indian macaque, the cheetah, and the tiger. In addition to this, species hunted by H. sapiens were also wiped out or reduced in number, further contributing to the difficulty of survival during this time period.
Figure 4.2 A diagram showing genetic bottleneck
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Impacts on the human gene pool As in any population bottleneck, the Toba catastrophe had significant impacts on the human gene pool and genetic diversity. The diversity in allele frequencies was reduced significantly, with some alleles disappearing from the gene pool entirely. Geneticists believe that this is the reason humans today show such little genetic variation. |
However, as we are aware, it was not only H. sapiens which existed at this time, but also H. neanderthalensis and H. floresiensis. The impacts of the Toba catastrophe on these hominins is unclear, however it may be the case that a similar reduction in allele frequencies among these populations led to a reduced ability to adapt to changing environments that they potentially could have adapted to prior to the bottleneck. Despite this, it is unknown whether the genetic bottlenecks as a result of Toba led to the extinction of H. neanderthalensis and H. floresiensis, and, if so, how H. sapiens managed to survive when other species did not. Although the exact geographic distribution of H. sapiens at the time of the eruption is unknown, analyses of mitochondrial DNA have revealed that the first major migration of H. sapiens occurred around 70,000-60,000 years ago, further lending evidence to the Toba theory.