MOLECULAR-GENETICS OF SPECIATION AND HUMAN ORIGINS

The major histocompatibility complex (MHC) plays a cardinal role in th e defense of vertebrates against parasites and other pathogens. In Som e genes there are extensive and ancient polymorphisms that have passed from ancestral to descendant species and are shared among contemporar y species. The polymorphism at the DRB1 locus, represented by 58 known alleles in humans, has existed far at least 30 million years and is s hared by humans, apes, and other primates. The coalescence theory of p opulation genetics leads to the conclusion that the DRB1 polymorphism requires that the population ancestral to modern humans has maintained a mean effective size of 100,000 individuals over the 30-million-year persistence of this polymorphism. We explore the possibility of occas ional population bottlenecks and conclude that the ancestral populatio n could not have at any time consisted of fewer than several thousand individuals. The MHC polymorphisms exclude the theory claiming, on the basis of mitochondrial DNA polymorphisms, that a constriction down to one or few women occurred in Africa, at the transition from archaic t o anatomically modern humans, some 200,000 years ago. The data are con sistent with, but do not provide specific support for, the claim that human populations throughout the World were at that time replaced by p opulations migrating from Africa. The MHC and other molecular polymorp hisms are consistent with a ''multiregional'' theory of Pleistocene hu man evolution that proposes regional continuity of human populations s ince the time of migrations of Home erectus to the present, with disti nctive regional selective pressures and occasional migrations between populations.