Sumatra exploded 74 TYA. 2,500 km3 of magma flowed from the Toba volcano there. The area covered in lava was twice the size of New York City, equivalent to 76% of Rhode Island. Toba was the largest volcanic eruption of the past 2 million years. The 74 TYA Toba explosion was last of 3 major eruptions in the last million years.
Ash spread north and west by prevailing winds, covering the Indian subcontinent and raining down upon the oceans from the Arabian Sea to the South China Sea. All of south Asia was covered in a 15-cm ash blanket.
The Toba explosion coincided with the onset of Earth’s last glacial period, providing additional chill: a volcanic winter which fell global surface temperatures 3–5 ºC.
The effect of Toba on global human populations may have been devastating. Toba seems to have shrunk the gene pool. Genetic evidence suggests that humans today descended from a population base of a few thousand breeding pairs that existed 70 TYA.
Other animals – in at least Africa, Borneo, and India – recovered from low population numbers 70–55 TYA. Humans flowed out of Africa in a major migration wave 70–60 TYA.