Human Populations: Origins and Evolution.

The origins of modern human diversity have long been debated within a framework set by two hypotheses: ‘Outof- Africa’ versus multiregional evolution. Humans are genetically closer to each other than members of all other primate species,mostof our genetic diversity is accounted for by individual differences within populations, and only a small fraction of the species’ genetic variance represents differences among populations or geographic groups. All these findings are hard to reconcile with the notion of parallel evolution in different continents, implicit in the multiregional model; the alternative hypotheses are now better described as either complete replacement of archaic human populations, or as partial assimilation into anatomically modern populations. Critical information about human demographic history has emerged from analyses of genomic diversity, clearly supporting an African origin of our species, followed by dispersal of rather small groups of people in the other continents. However, ancient deoxyribonucleic acid from fossil specimens seems to suggest low, but significant, levels of hybridisation between anatomically archaic and modern humans in the course of the latter’s expansion from Africa, although alternative explanations cannot be ruled out.