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Where did I come from?
Testing the Hypotheses
The two hypotheses of human origins make different predictions about the evolutionary relationships among geographically separated groups of humans. Wilson and his colleagues tested the hypotheses by comparing their observations with the predictions. Support for a hypothesis is found when the observations match the predictions of that hypothesis.
The Multi-regional Hypothesis has two important aspects: the idea that archaic humans (Homo erectus) left Africa and dispersed into the different geographic regions a very long time ago (about 1 million years ago), and that there was sufficient genetic mixing (gene flow) between populations to maintain a single species of humans. This means that we should observe deep genetic differences between the different geographic regions. The common genetic ancestor of modern humans should be very old. Futher, the African populations are not unique and should have patterns of genetic variation similar to other regions.
When the genetic relationships are estimated, we might observe a phylogenetic tree similar to the following, where each colour represents a different region.
Recent African Origin Hypothesis
The Recent African Origin Hypothesis implies that genetic variation among African samples will be different from that observed within other geographic regions. There should be a deep divergence between two groups of Africans, those related to the ancient Africans who left the region and gave rise to modern humans elsewhere and those whose ancient relatives did not leave Africa. Consequently the common ancestor of modern humans must be among the ancestors of modern Africans. The recentness of the origin of modern humans means that the common ancestor will have occurred more recently than under the Multi-regional hypothesis. If ancient Africans dispersed beyond that region, then all of the non-African regions will show relatively recent relationships with modern Africans.
Performing the Test by Inferring Genealogies from Mitochondrial DNA
Two pieces of evidence are needed to decide between the hypotheses: an estimate of the evolutionary relationships of modern humans from different geographical regions, and an estimate of the time of the most recent common ancestor of modern humans.
To do this, Wilson and his colleagues needed to use methods with which they could get relatively unambiguous estimates of human evolutionary relationships. The results needed to be reliable, that is they needed to be unaffected by other factors. They also needed to be precise, that is they needed to enable Wilson and colleagues to be able to distinguish between different proposed relationships.
The approach chosen was to estimate the relationships of people from different geographic regions by studying the variation in their mitochondrial genome. Mitochondria have several features that make them valuable for studying evolutionary relationships:
Every region in our genomes will show a similar type of coalescing back to a common ancestor. A different ancestor is expected for each region in the nuclear genome. Only a single common ancestor is expected for the mitochondrial genome because it does not recombine. The most recent common ancestor of the mitochondrial genome is expected to be much more recent than for other genomic regions because of its pattern of inheritance.