“Sperm pass on none of their mitochondria during conception, so all mitochondrial information is transferred from generation to generation through mothers alone. Such a system means that there will be many extinctions along the way. A woman endows all her children with her mitochondria, but only her daughters have the mechanism to pass it onward to future generations. So if a woman has only sons or no children at all—and that happens quite often, of course—her personal mitochondrial line will die with her. All her descendants will still have mitochondria, but it will come from other mothers on other genetic lines. In consequence, the human mitochondrial pool shrinks a little with every generation because of these localized extinctions. Over time, the mitochondrial pool for humans has shrunk so much that, almost unbelievably but rather wonderfully, we are all now descended from a single mitochondrial ancestor—a woman who lived in Africa about 200,000 years ago. You might have heard her referred to as Mitochondrial Eve. She is, in a sense, mother of us all.”