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This story is a supplement to the feature "The Migration History of Humans: DNA Study Traces Human Origins Across the Continents" which was printed in the July 2008 issue of Scientific American.
Geneticists can track the path of ancient migrations by examining genetic markers in Y chromosomes from men who hail from different parts of the world. Each marker, such as M168 or M89, identifies a lineage of men and where the lineage originated. By building an evolutionary tree based on observing many living people with the markers, investigators can determine the approximate ages of the lineages.
The out-of-Africa theory postulates that humans with modern traits left Africa from 50,000 to 60,000 years ago to settle the world. Along the way, they replaced archaic hominids, such as Homo erectus, that left Africa as early as 1.8 million years ago. The competing multiregional theory holds that modern characteristics evolved not just in Africa but in archaic hominid populations in Asia and Europe. Interbreeding among all these groups (horizontal arrows below) ensured that they remained a single species.