The 'Replacement' or 'Out of Africa 2' Hypothesis
The Recent African Genesis of Humans
Several alternative models for the origin of anatomically modern humans are currently proposed by paleoanthropologists. The Regional Continuity or Multiregional Evolution models are generally based on interpretations of fossil evidence. Three recent African origin models, Replacement, Weak Garden of Eden and Multiple Dispersals, are based on combinations of evidence from fossils, archaeology and, especially, genetic studies.
Do genetic studies reveal that an African woman of 200,000 years ago was our common ancestor? This idea was proposed in 1992 (Wilson and Cann, 1992). The Out of Africa 2 hypothesis for the origin of anatomically modern humans posits the replacement of the original populations of Homo with a second dispersal (hence 2) of near modern humans from Africa, a dispersal that purportedly replaced the archaic east Asian and Neanderthal populations without gene exchange. Some versions of the Replacement hypothesis allow for some gene flow.
The alternative to the Replacement hypothesis is the Multiregional Evolution hypothesis, discussed on the next page in this series. A more recent alternative hypothesis posits the impact of volcanic winter and a population bottleneck due to the eruption of Toba, Sumatra, around 70,000 years ago. The bottleneck model is presented after the Multiregional Evolution model. The remainder of this page is a summation of the so-called "Eve hypothesis" version of the Replacement Hypothesis, as presented by Wilson and Cann in 1992.
Genetic comparisons provide evidence that all living human populations can be traced along maternal lines of descent to a woman who lived in Africa about 200,000 years ago. The genetic information of living subjects does not explain precisely how, when, and where populations originate. But living genes have ancestors, and their relationships can be assessed. A genome holds the inherited biological information of an individual. Variants within a population can be studied and gene sequences determined.
In 1967 Vincent M. Sarich measured the evolutionary distance between humans and chimpanzees by studying their blood proteins. The accumulated differences reflect mutations since species divergence. His findings refined the genetic distance, dating it to between five and seven million years ago, compared to a previously estimated 15 million. That work used blood proteins. Since the 1980's we are able to sequence DNA. Wilson and Cann studied mitochondrial DNA (mtDNA).
Genetic studies have an underlying assumption that the rates of genetic change from point mutations are steady over long periods of time. In the case of mtDNA it is also presumed that there is no recombination of the DNA during reproduction. Study of the mitochondrial DNA allows tracing of maternal lineages. Mitochondrial DNA encodes 37 essential genes. The mtDNA is useful for study because the mutations accumulate steadily and rapidly, and they are effectively neutral, and therefore not eliminated by natural selection. Humans are so alike in their DNA sequences that evolution can best be measured using the genes that mutate fastest.
The degree of mtDNA relatedness declines step wise, and the farther back the genealogy goes the greater the variation. Also, all human mtDNA must have a common female ancestor. Some lineages die out. The time for this coalescence is a function of population. In 1988 Thomas D. Kocher examined the interrelatedness of the mtDNA of 14 subjects from around the world. He determined that 13 branching points could account for the differences found. He concluded that Africa was the human homeland. He also noted that all 14 human sequences were nearly equidistant from chimpanzee sequences, implying that the rates of change among humans are uniform. The chimpanzees showed as much as 10 times more genetic variation than humans, suggesting that humanity sprang from a small group of ancestors.
Wilson and Cann examined 182 distinct types of mtDNA from 241 individuals. The genetic tree they constructed had two main branches leading to Africa. They also found that people from a given continent do not generally all belong to the same maternal lineage. The New Guineans are typical, showing up on several different branches. Wilson and Cann relied on black Americans as substitutes for Africans. Linda Vigilant has since redone the study using mtDNA data from 120 Africans, representing six diverse parts of the sub-Saharan region. Vigilant traced a genealogical tree whose 14 deepest branches lead exclusively to Africans and whose 15th branch leads to both Africans and non-Africans. The probability that the 14 deepest branches would be exclusively African was one in 10,000.
Wilson and Cann calculated how much humans had diverged from one another relative to how much they had diverged from chimpanzees, and determined the ratio was less than 1:25. Assuming five million years since human/chimp divergence results in an estimate of 200,000 years to our common maternal ancestor. They also measured how much the mtDNA has evolved in the aboriginal populations of New Guinea and Australia. The result, about one third that of the species, infers coalescence 150,000 to 180,000 years ago given presently understood settlement dates of those areas.
Wilson and Cann conclude that how one human population replaced archaic humans is still a mystery. Cann suspected infectious diseases contributing to the process. They also looked ahead to the possibility of recovering DNA from fossils. Since their writing in 1992 this has been accomplished with Neanderthal fossils. The Neanderthal DNA findings will also be addressed in this series of articles.
Wilson, Allan C. and Rebecca L. Cann. 1992. The Recent African Genesis of Humans. Scientific American 266:68-73.