What you see here is Lake Toba, a donut-shaped lake which now fills the crater left in Sumatra, Indonesia, when a supervolcanic eruption happened about 77,000 years ago. This eruption blasted volcanic ash into the Earth's atmosphere, coating the entirety of South Asia in a 15-centimeter blanket of ash, as well as depositing ash over nearby oceans and seas. This resulted in a volcanic winter over the Earth, which lasted for as long as a decade, and potentially brought about a millennium-long global cooling event as well.
During this time, it appears that most of the human race was wiped out, with the population count getting as low as 1000 breeding pairs of humans.
In 1993, scientist Ann Gibbons first hypothesized that the bottleneck in human population, followed by the Pleistocene population explosion, could have been linked to the Toba event. Geneticist Lynn Jorde, in this BBC interview on supervolcanoes, echoes this idea:
"Our population may have been in such a precarious position that only a few thousand of us may have been alive on the whole face of the Earth at one point in time, that we almost went extinct, that some event was so catastrophic as to nearly cause our species to cease to exist completely."
In a population bottleneck, as the graph on this Wiki shows, a vast majority of a species is killed off, leaving only a few stragglers to either adapt and proser, or perish.
The possibility exists that such a global catastrophe could nudge along natural selection in favor of intelligence. It does stand to reason, after all, that if some disaster wipes out most of humanity, that the smarter folk will have a better chance of survival, by both being better prepared and being better at adapting new survival strategies during the ensuing fallout.
