DNA in mitochondria descends through the mother and does not undergo the shuffling associated with the pairing of parental DNA. It therefore has a slow and even mutation rate, and is used to date species divergence. Recent mitochondrial evidence also tends to confirm the theory that humans had a single origin in Africa. For a readable but detailed account of the geographical emergence of H.Sapiens (one which questions the implicit racism of earlier accounts) see Stringer, CB, and McKie, R, African Exodus: The Origins of Modern Humanity (London: Cape, 1996).