According to NPR, around the year 70,000 B.C., a volcano called Toba on Sumatra Island in Indonesia, erupted, blowing about a thousand kilometres of vaporized rock into the air. It is by far the largest volcanic eruption we know of.
That eruption dropped roughly six centimeters of ash over all of South Asia, the Indian Ocean, the Arabian and South China Sea. With so much ash, dust and vapor in the air, science writer Sam Kean says Toba "dimmed the sun for six years, disrupted seasonal rains, choked off streams and scattered whole cubic miles of hot ash (imagine wading through a giant ashtray) across acres and acres of plants." Berries, fruits, trees, African game became rare. Early humans, living in East Africa just across the Indian Ocean from Mount Toba, probably starved, or at least, he says, "It's not hard to imagine the population plummeting."
It is estimated that the population of Homo Sapiens—or humans—decreased to 5,000 or 10,000 and our species nearly went extinct. Little clumps of people hunted and gathered for thousands of years until, in the late Stone Age, we began to recover. But for a time there, says Sam Kean, "We damn near went extinct."