The best answers to your question are prehistoric, so you may want to ask /r/askscience .

Professor Stanley H. Ambrose argues that the Mount Toba supervolcano eruption some 70 thousand years ago caused a significant amount of volcanic matter to enter the Earth's atmosphere. This matter blocked out a portion of the sunlight which would have hit the Earth's surface, causing the Earth to become much cooler than it otherwise would have been. The eruption and subsequent millenium-long cool period caused the human population to crash down to less than 10,000 individuals. It is thought that all of these individuals lived in Africa, but there may have been some living in parts of Eurasia. Genetic studies continue to uncover new evidence.