Genetic evidence and the modern human origins debateIntroductionThe fossil record of evolution in the genus HomoModels of modern human originsThe fossil evidenceThe genetic evidence: gene treesThe genetic evidence: regional differences in genetic diversityThe genetic evidence: a cline in genetic diversityThe genetic evidence: the estimated number of human ancestorsThe genetic evidence: ancient DNA and the fate of the NeandertalsThe genetic evidence: adaptive genetic introgressionConclusionsReferencesSHORT REVIEWGenetic evidence and the modern human originsdebateJH RelethfordDepartment of Anthropology, State University of New York College at Oneonta, Oneonta, NY, USAA continued debate in anthropology concerns the evolu-tionary origin of ‘anatomically modern humans’ (Homosapiens sapiens). Different models have been proposed toexamine the related questions of (1) where and whenanatomically modern humans first appeared and (2) thegenetic and evolutionary relationship between modern hu-mans and earlier human populations. Genetic data havebeen increasingly used to address these questions. Geneticdata on living human populations have been used toreconstruct the evolutionary history of the human speciesby considering how global patterns of human variation couldbe produced given different evolutionary scenarios. Ofparticular interest are gene trees that reconstruct the timeand place of the most recent common ancestor of humanityfor a given haplotype and the analysis of regional differencesin genetic diversity. Ancient DNA has also allowed a directassessment of genetic variation in European Neandertals.Together with the fossil record, genetic data provide insightinto the origin of modern humans. The evidence points to anAfrican origin of modern humans dating back to 200 000years followed by later expansions of moderns out of Africaacross the Old World. What is less clear is what happenedwhen these early modern humans met preexisting ‘archaichuman’ populations outside of Africa. At present, it is difficultto distinguish between a model of total genetic replacementand a model that includes some degree of genetic mixture.Heredity (2008) 100, 555–563; doi:10.1038/hdy.2008.14;published online 5 March 2008Keywords: modern human origins; population genetics; paleoanthropologyIntroductionA long-standing debate in anthropology concerns ourown origins as anatomically modern humans (Homosapiens sapiens). The qualifying term ‘anatomically mod-ern’ is used to identify our early ancestors that werephysically much the same as living humans, but also todistinguish them from earlier hominins that could becalled ‘human’ at some level based on features such as anincreased brain size relative to body size and thepossession of a material culture (including stone tools).The fossil record of the past 2 Myr shows modernhumans evolving from earlier humans, often referred toas ‘archaic humans’, a broad group that includes thespecies H. heidelbergensis as well as the Neandertals ofEurope and the Middle East. What is less clear is theevolutionary relationship of modern humans to thevarious archaic human populations, as well as to earlierancestors. Did modern humans evolve via anagenesisfrom a single archaic species across the Old World, or didthey first arise in Africa? If the latter, then did modernpopulations expanding out of Africa replace the archaichuman populations that lived outside of Africa, or didthey interbreed with them? Were the Neandertals aseparate species from modern humans and, if so, did anyhybridization take place?These and other questions fall under what has beentermed the ‘modern human origins debate’. Althoughthis debate is often focused on the fossil and arche-ological records, studies of genetic variation havebecome increasingly important as a source of insight.Much of the work in this area has consisted of detailedanalyses of patterns of genetic variation in living humanpopulations. The strategy here is based on the realizationthat whatever our species’ evolutionary past, it has leftvisible signatures on our genome. Expectations ofcurrent genetic variation under different evolutionaryscenarios are compared with observed genetic variationin our species in order to test various origin models. Inaddition, the last decade has also seen an increase in theanalysis of ancient DNA, such that mitochondrial andnuclear DNA sequences are now available for theNeandertals, an archaic human group. The purpose ofthis review is to highlight some of the major findings ofgenetic analysis (using both living and ancient DNA)and their use (and misuse) in the modern human originsdebate. The focus here is primarily on recent findingsand the status of the debate as I perceive it.The fossil record of evolution in the genusHomoIn order to understand the contributions of geneticresearch to the modern human origins debate, it is firstnecessary to provide a brief review of the fossil record forhuman evolution over the past 2 Myr. Only a brief reviewis given here; more detail is available in many currenttexts on human evolution (e.g., Conroy, 2005; StringerReceived 5 September 2007; revised 25 January 2008; accepted1 February 2008; published online 5 March 2008Correspondence: Dr JH Relethford, Department of Anthropology, StateUniversity of New York College at Oneonta, Fitzelle Hall 311, Oneonta,NY 13820, USA.E-mail: [email protected] (2008) 100, 555–563&2008 Nature Publishing Group All rights reserved 0018-067X/08 $30.00www.nature.com/hdyand Andrews, 2005). Molecular evidence suggests thatthe hominin and African ape lines diverged about6–7 Myr ago. The fossil record of the first possible bipedsdates back over 6 Myr ago in Africa. By 4.2 Myr ago,there is definite evidence of bipedal hominins in Africa(Australopithecus anamensis). These early homininswalked upright (at least on the ground), had ape-sizedbrains and larger protruding faces and teeth. The speciesH. erectus appeared in Africa 1.8 Myr ago and ischaracterized by modern limb proportions, increasedbrain size, reduction in the size of the teeth anddevelopments in stone tool technology. Until this pointin time, hominin evolution had taken place exclusively inAfrica, but populations of H. erectus dispersed to EasternEurope and Southeast Asia about 1.7 Myr ago (note:some anthropologists refer to the initial African popula-tion as the species H. ergaster and reserve the nameH. erectus for the Southeast Asian populations).
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