Around 70–75,000 years ago the Toba caldera in Sumatra Indonesia erupted. This was a supervulcano eruption, probably the largest eruption within the last two million years. The climatic effects of this event led to a decrease in average global temperatures by 3 to 3.5 degrees Celsius for several years. This massive environmental change made life very hard for many species and created population bottlenecks. This accelerated differentiation of the isolated human populations, eventually leading to the extinction of all the other human species except for the branch that became modern humans. Genetic evidence tells us that all today living humans descend from a population of about 1000-10000 individuals living at the time of the Toba event.
Population bottlenecks increase the rate of genetic drift. The Polar Bear for example could have evolved in this way. A Brown Bear group could have ended up on an arctic island at the end of the last Ice Age. Beneficial traits such as color, size, swimming ability, cold resistance, and aggressiveness could develop in just a few generations.
What kind of environment were these humans living in after the Toba disaster? Finding prey inland could be hard if many species were decimated. Could it be that it was this occasion that led humans to seek food predominately in coastal areas and those humans who were best suited for this survived? Maybe a strand of humans had already embarked on the aquatic path and went from being a very minor group to the one that became our ancestors.
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