Critics questioned the appropriateness of the mtDNA samples used in the study and argued that computer analysis of the data was flawed and that Wilson's conclusions were not supported by the fossil record. A further study using more diverse mtDNA samples and supporting Wilson's theory was published in 1991, but other computer analyses of mtDNA samples have indicated that several different "family trees" can be constructed from the same data and that the order in which samples are analyzed by the computer program affects the results.
A 1995 study supported the idea that modern humans originate from a single source by identifying identical stretches of DNA on the Y chromosomes of a sample of men taken from different racial and geographical groups worldwide. This study looked at the zinc finger y, or ZFY, gene, a gene that is passed only from father to son, and concluded that humans evolved from a common ancestor (i.e., a small group) around 270,000 years ago. More recently, Spencer Wells, in The Journey of Man (2002), has argued that all men are descended from a single African man who lived 60,000 years ago.
Those who argue against the Eve, or "out-of-Africa," hypothesis, are led by Dr. Milford Wolpoff and support an alternate hypothesis known as "regional continuity." It contends that human evolution was a much slower process (covering a million or more years) that occurred simultaneously in many areas of the Old World after the migration of Homo erectus, the species generally recognized as immediately preceding Homo sapiens, from Africa.