70,000 years ago, the human population hit a major bottleneck, and scientists aren't exactly sure why.
One of the more popular explanations has been the Toba explosion, the massive explosion of the Toba supervolcano that occurred at roughly that same time, but has been mostly disproven.
DNA evidence shows that African populations, especially the ancient Khoi and San people of Africa, show far more genetic diversity than those who came out of Africa according to the "out of Africa" theory. The study showed that those that came out of Africa descended from between 1000 and 10,000 breeding pairs.
The reason for this remains a mystery.