The triumphant trellis (and the transubstantiation of modern humans)

Genetics and the Search for Modern Human Originsby J.H. Relethford. John Wiley and Sons, 2001. £53.95 hbk. (252 pages) ISBN 0 471 38413 5View Large Image | Download PowerPoint SlideCould human evolution possibly become more complicated than it already is? This feeling overwhelms me each time I read a paper, by a proponent of multiregionalism, that hinges on recurrent gene flow over a vast geographical expanse of physical distance for an immense duration of time acting on unknown aspects of behavior, morphology and physiology. As a geneticist, I believe that the main answer to ‘What makes us human?’ will come from analysis of expression arrays that compare gene loci from human chromosome regions 2p and 2q with homologous loci on chimpanzee chromosomes 12 and 13. But will this provide the complete answer? To paraphrase John Maynard Smith, my 30 years as a geneticist, and 20 years as a physical anthropologist have convinced me that the problems in understanding human evolution are not in genetics or fossils, but in how to fit them together.This book purportedly shares this mind set. Although Relethford states his sympathies for the objective evaluation of new evidence, his main aim is to show that there is weak genetic support for a recent African origin for all modern humans, and that multiregionalism is alive and well, rescued by our ignorance of how fast and how far people moved when they did expand out of Africa. His goal is to convince the reader that phylogenetic analysis of sequence or haplotype diversity is an inappropriate way to portray the genetic population structure of interbreeding groups.Relethford confesses that he was swayed, at first, by the evidence from multiple genetic loci indicating that an effective population size of ∼10 000 individuals could not have remained in migrational equilibrium for 1.7 million years over five continents. However, he has since changed his mind and now envisions that migration pulses of 25–50 people every 40–125 generations could probably explain current diversity patterns. Local demes nearly went extinct and were swamped occasionally by adjacent immigrant peoples, somehow related to the original occupants. That sort of model requires the use of the ‘r’ word, and I do not mean rare alleles. His sympathies are apparent when he writes of his distaste for dogmatic statements made by molecular geneticists. This left me wondering about how many paleoanthropology news conferences he has attended.He covers topics such as head shapes in Irish counties, ecological estimates of carrying capacity for hunter–gatherers, age structure, Wright's statistics, and genetic diversity estimates. The strong points are the chapters on population size and the search for common ancestors. The weakest include Relethford's equation of simulation for rigorous mathematical modelling, a failure to discuss assumptions of coalescent theory, and his summary of important ancient DNA analyses. William Boyd rightly stated, in 1950, that, if we go back only 500 years, we arrive at a time when there must have been many of our ancestors from whom we cannot have inherited even so much as a single gene [1xSee all References][1].According to Relethford, mtDNA and Y models of population replacement based on phylogenetic analysis are inconsistent with the total evidence. However, a model that he favors, based on larger ancestral populations in Africa and a migration matrix favoring immigration into Asia, clearly predicts that 2400 (20%) of the 12 000 typed Asian Ys [2xAfrican origin of modern humans in East Asia: a tale of 12,000 Y chromosomes. Ke, Y. et al. Science. 2001; 292: 1151–1153Crossref | PubMed | Scopus (85)See all References][2] should be identifiable distinctly as old and Asian, unless selection (natural or sexual?) has eliminated them completely from current populations. In my mind, the answer to what made us modern humans must wait for a technology to analyse DNA remaining in bones that are older than the current 100 000-year ceiling. No amount of simulation will replace bands on gels and fossils in hand.