The great arc of dispersal of modern humans: Africa to AustraliaIntroductionRegional settingMaterials and methods used in review of genetic phylogeographyPhylogeographyComplete sequence data: sources, phylogeny and datingReview of out of Africa modelsHow many AMH exits from Africa? The genetic evidenceSingle exit modelsModels with multiple exits: the Cambridge modelQuo vadis?Southern rather than northern exit: genetic evidenceClimatic considerations: constraints and imperatives for an exit routeDating migrationsPossible dates of exitDelayed migration to West EurasiaDating arrival of AMH in India and Southeast AsiaDating Pleistocene arrivals of humans in Sahul and near OceaniaConclusionsAcknowledgementsReferencesThe great arc of dispersal of modern humans: Africa to AustraliaStephen Oppenheimer*School of Anthropology and Museum Ethnography, Oxford University, 51 Banbury Road, Oxford, OX2 6PE, UKarticle infoArticle history:Available online 26 July 2008abstractDuring the Late Pleistocene, anatomically modern humans (AMH) dispersed out of Africa across thecontinents. Their routes obeyed the limitations placed on any large terrestrial mammal dependent ondaily drinking water, following certain climate-permissive corridors. AMH first spread north, with game,across the Sahara to the Levant during the Eemian interglacial (c.125 ka), but failed to continue to Europe,then occupied by Neanderthals. The savannah ecosystem in North Africa and the Middle East then driedup, and AMH vanished from the Levantine fossil record, being replaced there by Neanderthals. Later,AMH successfully left Africa as a single group by the southern route to India. The added ability to makeshort but deliberate open water crossings allowed them first to cross the mouth of the Red Sea fromEritrea, and subsequently Wallace’s Line to reach the isolated Sahul continent at least by 48,000 yearsago and possibly by 60–50,000 years ago. They only finally arrived in Europe from South Asia around 45–50,00 0 years ago, probably linked to climatic amelioration during OIS-3.Ó 2008 Elsevier Ltd and INQUA. All rights reserved.1. IntroductionThe use of non-recombining DNA and the phylogeographicapproach, based on reconstructed gene-trees has been vital inilluminating the geography of the exit route of anatomicallymodern humans (AMH), their further travels and the logic of onlyone ultimately successful exit from sub-Saharan Africa – via theYemen. When did such events take place? A multidisciplinary casecan be made for an exit prior to the Younger Toba volcanic eruptionof 74 ka (YTE). Chronometric predictions based solely on thegenetic data have systematic problems and should be tested usingdata from archaeology, Palaeo-anthropology and the EarthSciences. The latter, often having the best available dating andaccuracy for periods earlier than the radiocarbon ceiling, also offerinsights for windows of opportunity, based on sea level, climatefluctuations and even the occasional volcanic ash time marker.There are three basic questions, essential in any reconstructionof the African Exodus, in the following logical order: how manysuccessful exits, which routes and when? Two further questionsride on the second two and are: how and why?2. Regional settingIn geographic terms, the great arc of human dispersal was reallya double ocean arc extending from the southern tip of Africa to thesouthern tip of South America, but this article refers to the primarymodern human expansion from Africa round the coasts of theIndian Ocean to Melanesia and Australia in the Southwest Pacific(Fig. 1). The presumption that modern human range extension outof Africa proceeded initially along coastlines has depended largelyon the rapidity of this movement, as inferred from the geneticphylogeography and the very early dates of colonisation of South-east Asia and Australia, which will be discussed in this review. Forthis speedy beach-travel and the infferred mode and routes ofexpansion to be possible, these explorers must have been able toexploit coastal marine resources. Shellfish, being limited in theintertidal zone, have the tendency to be progressively over-exploited necessitating extension along the beach. This modelclearly provides an immediate and continuous motive for rangeextension in a linear fashion. Marine exploitation is characteristicof, if not unique to, modern humans (and in one instance, Nean-derthals) starting from at least 160,000 years ago (Marean et al.,2007; dates recently reviewed in McBrearty and Stringer, 2007).Shell middens and other archaeological evidence show that theexplorers were proficient at such exploitation. Middens and otherevidence of marine exploitation are found on the African thresholdat Abdur on the west coast of the Red Sea from 125 ka (Walter et al.,2000), at the very earliest occupation sites in Australia and mostsignificantly in Timor at one of the oldest Asian human coastaloccupation sites at the Australian threshold (O’Connor, 2007).It is tempting to suggest that this new human skill-complexand the ability to cross short stretches of water was an enabling,even essential factor in expansion, and could ‘explain’ and datemodern human expansion out of Africa. Yet such determinist*Fax: þ44 1865 437743.E-mail address: [email protected] lists available at ScienceDirectQuaternary Internationaljournal homepage: www.elsevier.com/locate/quaint1040-6182/$ – see front matter Ó 2008 Elsevier Ltd and INQUA. All rights reserved.doi:10.1016/j.quaint.2008.05.015Quaternary International 202 (2009) 2–13conclusions should be tempered by knowledge of the earliestspread of Homo erectus to similar sites in Southeast Asia and thepresence of Homo floresiensis in Flores near to Timor. The factthat marine exploitation, and a variety of other ‘modern’ skills,were practiced by modern humans in Africa twice as long asoutside (McBrearty and Stringer, 2007) and, owing to sea levelchanges, cannot be found as dated evidence until further alongthe route, rather blunts the value of such markers in deter-mining motives, dates, routes and number of exits from Africa. Ifthese behavioural milestones made it so much easier to leaveAfrica why did anatomically modern humans not leave Africamuch earlier and even many times over? In this perspective,regional and temporal climatic constraints are a more importantsource of information than any perceived ‘behavioural revolu-tion’ in determining dates for
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