As an international research team, publishing its study in Communications Earth & Environment , suggests, a catastrophic drop in atmospheric ozone levels around the tropics has likely contributed to a bottleneck in the human population .
This ozone loss would have occurred 70,000 years ago as a result of the eruption of the Toba supervolcano, located in present-day Indonesia.
Large volcanic eruptions emit gases and ash that create an aerosol layer that attenuates sunlight in the stratosphere, causing the Earth’s surface to cool. This "volcanic winter" has multiple side effects, such as colder oceans, prolonged El Niño events, poor harvests and disease .
The study suggests that the Toba cloud of SO2 depleted global ozone levels by as much as 50 percent. Furthermore, they found that the effects on ozone are significant, even in relatively small eruption scenarios. Health hazards resulting from increased ultraviolet radiation at the surface would have significantly affected human survival rates. As the author Sergey Osipov of the Max Planck Institute for Chemistry explains:
Toba has long been postulated as the cause of the bottleneck, but initial investigations into the climatic variables of temperature and precipitation did not provide concrete evidence of a devastating effect on humanity. We note that near-surface ultraviolet (UV) radiation is the driving evolutionary factor in the tropics. Climate becomes more relevant in the more volatile regions far from the tropics.
The theory states that human evolution was affected by a recent volcanic event. It was proposed in 1998 by Stanley H. Ambrose of the University of Illinois at Urbana-Champaign. The theory states that, when the climate and other factors were restored, humans began to spread out from Africa.