ALU INSERTION POLYMORPHISMS AND HUMAN-EVOLUTION - EVIDENCE FOR A LARGER POPULATION-SIZE IN AFRICA

Alu insertion polymorphisms (polymorphisms consisting of the presence/ absence of an Alu element at a particular chromosomal location) offer several advantages over other nuclear DNA polymorphisms for human evol ution studies. First, they are typed by rapid, simple, PCR-based assay s; second, they are stable polymorphisms-newly inserted Alu elements r arely undergo deletion; third, the presence of an Alu element represen ts identity by descent-the probability that different Alu elements wou ld independently insert into the exact same chromosomal location is ne gligible; and fourth, the ancestral state is known with certainty to b e the absence of an Alu element. We report here a study of 8 loci in 1 500 individuals from 34 worldwide populations. African populations exh ibit the most between-population differentiation, and the population t ree is rooted in Africa; moreover, the estimated effective time of sep aration of African versus non-African populations is 137,000 +/- 15,00 0 years ago, in accordance with other genetic data. However, a princip al coordinates analysis indicates that populations from Sahul (Austral ia and New Guinea) are nearly as close to the hypothetical ancestor as are African populations, suggesting that there was an early expansion of tropical populations of our species. An analysis of heterozygosity versus genetic distance suggests that African populations have had a larger effective population size than non-African populations. Overall , these results support the African origin of modern humans in that an earlier expansion of the ancestors of African populations is indicate d.