Exactly when the first humans emerged from Africa to colonize the world has been a topic of heated debate. [Photos: Our Closest Human Ancestor]
All of the estimates hinge on one number: the gene mutation rates. By knowing how often genes change, and then counting up the number of genetic differences between different species or groups of people, scientists can create a “molecular clock” to decipher how long ago they shared a common ancestor.
Story: Tia Ghose, LiveScience | Photo: F. Demeter