Genome Study Provides a Census of Early Humans
Nicholas Wade, New York Times, 18/01/2010
From the composition of just two human genomes, geneticists have computed the size of the human population 1.2 million years ago from which everyone in the world is descended. They put the number at 18,500 people, but this refers only to breeding individuals, the “effective” population. The actual population would have been about three times as large, or 55,500.