According to a research in the link bellow, only around 2000 people left in the world some 70,000 years ago because of the draught. From the same report, the number of world population is around 6.6 billion now. Assuming these 2000 people stayed alive made up of equal reproducing men and women, or 1000 couples, each couple has contributed 6,600,000 people of today.
http://news.yahoo.com/s/ap/20080424/ap_on_sc/close_call









