Everyone assumes that from the earliest times humanity has enjoyed steady and inexorable population increase, but 70 millennia ago Homo sapiens very nearly died out.

About 3000 generations ago – a mere blink of an eye in the evolutionary timescale – the human population of Earth dwindled to the point where everyone on the planet could have fitted inside a small football stadium. Mounting DNA evidence indicates that the human population was reduced to perhaps fewer than 10,000. One study even suggests that the number of women dropped to just 500 individuals. For centuries, each new generation of Homo sapiens could easily have been the last.

The evidence for this population crisis (or ‘bottleneck’) lies within each and every one of us – in the DNA that we all carry. There is very little variation in the modern human gene pool, a fact that researchers recently sought to explain by tracing our genetic heritage back through time. The amount of variation that they measured pointed to a period when there can only have been a tiny number of procreating females in the species. Other DNA studies supported the finding and began to offer more certainty on its timing: the crisis had occurred around 70,000 years ago. After hundreds of thousands of years of evolution, the human species came close to falling at the final hurdle.
 
Global catastrophe

Although geneticists were able to pinpoint the scale and timing of the population crisis, they could not offer any explanation for it. In fact the reason for the collapse of our species might be right under our feet. Deposits of ash and debris have provided evidence for one of the most severe natural disasters in the history of life on Earth. It has been dated by paleoclimatologists and geoscientists to exactly 71,000 years ago.

The cause of this disaster was the eruption of the Toba volcano on the Indonesian island of Sumatra. The largest volcanic explosion of the previous two million years, and one of the biggest of the past 450 million years, its effects were felt right across the earth. The crater it left behind measured 100km (60 miles) across.