One, Multiregional evolution, posits that humans arose in Africa some two million years ago, evolved as a single species spread across the Old World and were linked through interbreeding and cultural exchange. Although that original Eve study was later shown to contain fatal flaws, Out of Africa has continued to enjoy much molecular affirmation, as researchers have increasingly turned to DNA to decipher the history of our species. But a closer look at these genetic studies has led some researchers to question whether the molecular data really do bolster the Out of Africa model. And striking new fossil data from Portugal and Australia appear to fit much more neatly with the theory of Multiregional evolution. The DNA from mitochondria, the cell's energy-producing organelles, has been key Out of Africa evidence.
Mitochondria are maternally inherited, so genetic variation arises largely from mutation alone.
But the significance of each finding has been questioned. The date is suspect because the molecular clock depends on problematic assumptions, such as the calibration date and mutation rate. And if natural selection has shaped mtDNA, as some studies suggest, then the rate of mutation accumulation may have differed at different times. The African root for the mtDNA gene tree is compatible with Out of Africa, but it does not exclude Multiregionalism, which predicts that the common ancestor lived somewhere in the Old World, probably Africa. And neither does the high mtDNA variation in African populations as compared with non-Africans uniquely support Out of Africa, according to anthropologist John H. Relethford of the State University of New York College at Oneonta.