Molecular Anthropology

The essential postulate on which molecular anthropology is based is that closer genetic similarity indicates a more recent common ancestry. All organisms are believed to have evolved from a single ancestor. As different life forms evolved, their DNA began to diverge through the processes of mutation, natural selection, and genetic drift. Even within the same species, populations that do not interbreed will accumulate genetic differences, which increase over time. The number of these differences is proportional to the amount of time since the two groups diverged.

There are several advantages to comparing DNA data instead of external physical characteristics (collectively called the phenotype). Environmental factors can shape the phenotype to make two individuals with the same genetic makeup look different. For instance, nutrition has a profound effect on height, and if we used average height to classify humans, we might mistakenly conclude that medieval humans represented a different sub-species because they were significantly shorter than modern humans. DNA comparisons, on the other hand, would show no significant difference between these groups.

Another advantage is that DNA sequence differences can be easily quantified—two base changes in a gene are more different than one. Despite being random events, mutations occur at a fairly steady rate, constituting a "molecular clock," and so the number of differences can be use to estimate the time since the two organisms shared a common ancestor.

Molecular anthropologic data has been used to argue that modern humans arose in Africa and migrated out to replace archaic populations elsewhere. The details and the timing are still controversial.

Finally, since all organisms contain DNA, the sequences of any two organisms can be compared. The same techniques used in molecular anthropology can also be applied to evolutionary questions in other species, to determine the evolutionary relations between different animal species, for instance, or even between bacteria and humans.