CRANIOMETRIC VARIATION, GENETIC THEORY, AND MODERN HUMAN ORIGINS

Recent controversies surrounding models of modern human origins have f ocused on among-group variation, particularly the reconstruction of ph ylogenetic trees from mitochondrial DNA (mtDNA) and the dating of popu lation divergence. Problems in tree estimation have been seen as weake ning the case for a replacement model and favoring a multiregional evo lution model. There has been less discussion of patterns of within-gro up variation, although the mtDNA evidence has consistently shown the g reatest diversity within African populations. Problems of interpretati on abound given the numerous factors that can influence within-group v ariation, including the possibility of earlier divergence, differences in population size, patterns of population expansion, and variation i n migration rates. We present a model of within-group phenotypic varia tion and apply it to a large set of craniometric data representing maj or Old World geographic regions (57 measurements for 1,159 cases in fo ur regions: Europe, Sub-Saharan Africa, Australasia, and the Far East) . The model predicts a linear relationship between variation within po pulations (the average within-group variance) and variation between po pulations (the genetic distance of populations to pooled phenotypic me ans). On a global level this relationship should hold if the long-term effective population sizes of each region are correctly specified. Ot her potential effects on within-group variation are accounted for by t he model. Comparison of observed and expected variances under the assu mption of equal effective sizes for four regions indicates significant ly greater within-group variation in Africa and significantly less wit hin-group variation in Europe. These results suggest that the long-ter m effective population size was greatest in Africa. Closer examination of the model suggests that the long-term African effective size was r oughly three times that of any other geographic region. Using these es timates of relative population size, we present a method for analyzing ancient population structure, which provides estimates of ancient mig ration. This method allows us to reconstruct migration history between geographic regions after adjustment for the effect of genetic drift o n interpopulational distances. Our results show a clear isolation of A frica from other regions. We then present a method that allows direct estimation of the ancient migration matrix, thus providing us with inf ormation on the actual extent of interregional migration. These method s also provide estimates of time frames necessary to reach genetic equ ilibrium. The ultimate goal is extracting as much information from pre sent-day patterns of human variation relevant to issues of human origi ns. Our results are in agreement with mismatch distribution analysis o f mtDNA, and they support a ''weak Garden of Eden'' model. In this mod el, modern-day variation can be explained by divergence from an initia l source (perhaps Africa) into a number of small isolated populations, followed by later population expansion throughout our species. The ma jor population expansions of Homo sapiens during and after the late Pl eistocene have had the effect of ''freezing'' ancient patterns of popu lation structure. While this is not the only possible scenario, we do note the close agreement with recent analyses of mtDNA mismatch distri butions. (C) 1994 Wiley-Liss, Inc.