This is just something I got while watching my favorite station (Discovery Channel).
Apparantly based on the lack of any significant differences in human DNA they surmised that at one point in history the human was near extinction. Apparantly there should be more variation in DNA. Using the knowledge of mitochondrial DNA and that it has a nearly set mutation rate they have been able to back track and give this near extinction time at sometime 70-80k years ago. It is surmised that the human race was cut down to roughly 10k members and that is what accounts for the lack of variation. They attribute this to a "super volcano."