Modern Humans Spread from Aden to the Antipodes With Passengers and When?

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Chapter 18

Modern Humans Spread from Aden to the Antipodes

With Passengers and When?

Stephen Oppenheimer

Introduction

The use of non-recombining DNA and the phylogeographic approach, based on reconstructed gene-trees , has illuminated the geography of the exit route of anatomically modern humans (AMH ), their further travels, and the logic of only one ultimately successful exit from sub-Saharan Africa – likely via the Yemen , around 72 ka (Oppenheimer 2012a ). However, there are competing archaeo-genetic views of multiple exits (Underhill et al. 2001 ; Rasmussen et al. 2011 ). Chronometric predictions based solely on genetic data have systematic problems, the most important being calibration and wide confi dence intervals, and so need testing against more accurate data from archaeology, palaeoanthropology and the earth sciences. Several recent papers and books off er new climatic, fossil, lithic and genetic evidence for earlier exits of AMH , before the great Toba volcanic eruption of 73 ka (Younger Toba Event [YTE]) (Petraglia et al. 2007 , 2011 ; Oppenheimer 2003 , 2009 ; Rosenberg et al. 2011 ), and even as long ago as the last interglacial , that is, up to 50,000 years before the YTE (Petraglia et al. 2010 , 2011 ; see also Groucutt & Petraglia, this volume; Liu et al. 2010b ; Armitage et al. 2011 ; Cabrera et al. 2009 [cited as genetic evidence in Petraglia et al. 2010 ]; Rose et al. 2011 ). In contrast, others argue for a later movement of modern humans north to Europe and east to India (e.g., Mellars 2006b ; 2006c )

Besides chronology, there are three basic questions, essential in any reconstruction of the African AMH exit, in the following logical order:

1. How many founding exits of AMH can be seen in the fossil or archaeological record? 2. Which of these are evidenced by genetic continuity into modern regional populations

(rather than by evanescent interglacial faunal spreads in the fossil record)? 3. Which route or routes were taken?

Given these issues, including the depression of relevant mtDNA haplogroup ages on recalibration, the time is ripe for a focussed review of the evidence both for and against a pre-Toba exit from Africa, and also for a re-ordering of the relative importance of the primary South Asian route to Australia versus the later Levantine-European dispersal.

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Regional Setting

There is a growing consensus that the dispersal of AMH from Africa occurred via a single south-ern exit across the mouth of Red Sea , followed by movement round the coasts of the Indian Ocean initially to Bali , but ultimately to Melanesia , Australia and the Americas ( Figure 18.1 ). The argument for a single exit (e.g., Oppenheimer 2003 ; 2009 ) rather than multiple exits (e.g., Lahr & Foley 1994 ; Underhill et al. 2001 ; Rasmussen et al. 2011 ) is mainly based on the fi nding of sin-gle ex-African uniparental lineages (both male and female) representing the entire non-African genetic diversity on those loci. Recent claims for AMH exits eastwards during the last interglacial (Petraglia et al. 2010 ; Liu et al. 2010b ; Armitage et al. 2011 ; Rose et al. 2011 ) need to be viewed in this context.

The presumption that modern human range extension out of Africa proceeded initially along coastlines has depended largely on the rapidity of this movement, as inferred from genetic phylo-geography, since only three key founder mtDNA haplotypes (M , N and R ) are indicated. These gave rise to multiple regionally specifi c branches en route, in eff ect three starbursts rooted on these haplotypes, seeding all the way from Arabia to Bali (e.g., fi g. 6 in Soares et al. 2009 ). For this speedy beach-travel model, these colonisers must have been able to exploit coastal marine resources . Shellfi sh, being mainly limited to the intertidal zone, are easily over-exploited, neces-sitating a continuous extension of foraging along the beach. This model clearly provides an immediate and continuous motive for uni-directional range extension in a linear fashion. Such a systematic pattern of littoral marine exploitation is characteristic of, if not unique to, mod-ern humans (but in one instance, Neanderthals ) starting from at least 160 ka (see McBrearty & Stringer 2007 ). Shell middens and other archaeological evidence from Africa show that early AMH populations were profi cient at such exploitation. Putative middens and other evidence of marine exploitation are found at the very earliest occupation sites in coastal Australia and most signifi cantly in Timor at one of the oldest Asian human coastal sites at the Australian threshold (O’Connor 2007b ; see also Balme & O’Connor, this volume). It is unlikely that this human skill-

46-50,000 years agoHomo sapiens entered Europe. MostEuropeans today can trace theirancestry to mtDNA lines that appearedbetween 50,000 and 13,000 years ago

20-30,000 years agoCentral Asians moved westtowards Europe and easttowards Beringia

Apporx. 15,000 years agoHumans crossed the Bering landbridge that connected Siberiaand Alaska

15-19,000 years agoArtefacts and tools found inPennsylvania give evidencethat humans had migratedinto the Americas before theIce Age

12,500 years agoEvidence of human habitationand artefacts found, MonteVerde, Chile

50-60,000 years agoHumans crossed fromTimor to Australia

40,000 years agoHumans trekked north fromPakistan up the Indus Riverand into Central Asia.

40,000 years agoHumans from the East-Asian coast moved westalong the Silk Road

Approx. 72,000 years agoA group of humanstravelled through thesouthern Arabianpeninsula towards India.All non-African people aredescended from this group

12,000 years agoA group of humanstravelled northwardthrough Egypt andIsrael but died out90,000 years ago

African originsOver 150,000 years agomodern humans - ourmtDNA ancestors - lived in Africa

Modern humansmoved east from IndiaInto Southeast Asiaand China

costal route

Figure 18.1. Narrative map of modern human dispersals, as reconstructed in this review (modifi ed from Oppenheimer 2003 ; 2009 ).


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complex and the ability to cross short stretches of water determined the date of expansion, since these ‘modern’ skills, were practiced by modern humans within Africa for twice as long as outside (McBrearty & Stringer 2007 ).

Materials and Methods Used in Reviews of Genetic Phylogeography

Phylogeography

Genetic phylogeography (literally the geography of gene trees ; see Richards et al. 2000b for a description of the method of Founder Analysis) is particularly useful for answering topographic questions of route and is the only discipline that reliably informs on routes and numbers of suc-cessful human exits. Genetic phylogeography is used extensively for dating migrations, although it is less precise or accurate than disciplines such as archaeology, hominin palaeontology and palaeo-climatology, which are most useful for testing dating regimes and routes suggested by the genetic models. A combined approach should give increased confi dence by triangulation.

The study of the geographic spread of gene lines involves three related types of inference: fi rst, an unambiguous gene tree (e.g., Figure 18.2 ) is built up by sequence study (preferably complete sequence [CS]) of non-recombining DNA in representative population samples. This review refers mainly to maternally transmitted mitochondrial DNA (mtDNA). The non-recombining part of the Y chromosome (male-NRY), however, produces very similar topographic results (Oppenheimer 2003 ; Richards et al. 2006 ; Hudjashov et al. 2007 ). Such trees because of their non-recombining DNA possess great detail and geographic specifi city, even 70,000 years after the African exit. Second, migrations are inferred from the geographic distribution of individual gene branches. Third, the diversity of clusters of gene lines is used to infer the time depth of

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ChukchiAustralianAustralianChinesePimanUzbekSamoanKoreanNew GuineaNew GuineaNew GuineaItalianGeorgianGermanSaamiEnglishCrimeanDutchFrenchNew GuineaAustralianEvenkiBuriatKhirgizWaraoWaraoAsian IndianChineseSiberian InuitGuaraniJapaneseJapanese

MkambaEwondoBamilekeLisongoYorubaYoruba

MandenkaEffikEffikIboIboMbenzeleBiakaBiakaMbenzeleKikuyuHausaMbutiMbutiSanSan

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Figure 18.2. Illustrative gene tree based on the fi rst mtDNA complete sequence data available in 2000 (from 52 individuals randomly selected around the world). The structure of the tree remains essentially the same today (modifi ed from fi g. 0.3 in Oppenheimer 2003 ; 2009 ). Note that the chimp-human coalescent date derives from analysis of coding region – that is, it is not assumed from fossil evidence.


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individual branches and founding events. Accurate dating of Y trees has lagged behind mtDNA analysis as a result of methodological problems, including the cost of complete sequencing, but a preliminary independent estimate for the African/non-African split using CS-NRY is 75 ka (Cruciani et al. 2011 ).These three forms of inference are combined in Founder Analysis, which is the identifi cation and dating of specifi c founder lines moving from a source to a target region (Richards et al. 2000 ).

Linearity of Dating

The linearity of the mtDNA molecular clock is a problem, since purifying selection preferentially removes minimally deleterious non-synonymous mutations over time, resulting in non-linearity of calibration for more recent times. Luckily the eff ect is regular, predictable and measurable and can be corrected for, giving demonstrable independent linearity between the synonymous and non-synonymous sequence data, thus allowing a more-precise complete sequence clock. A major recalibration paper (Soares et al. 2009 ) used a worldwide phylogeny of more than 2,000 complete mtDNA sequences to measure, predict and correct for this eff ect, calibrating on the chimp-human split of 6.5 ma, but cross-checking on recent archaeological founder events. For consistency, recalibrated mtDNA lineage dates are exclusively cited as evidence in this review.

Review of Out-of-Africa Models

How Many Exits of AMH Ancestors from Africa? The Genetic Evidence

Multiregional View versus Recent Replacement There are two contrasting models of how and when our species appeared in Europe and Asia. The multiregional hypothesis postulates that geographically diff erentiated populations of Homo sapiens arose independently in diff erent regions from a common ancestral population, but avoided speciation by gene fl ow with neighbouring populations. Replacement models (as proposed in this chapter) maintain that H. sapiens arose in Africa between 100 and 200 ka and then later left Africa and eff ectively replaced (with acknowledgment of the possibility for some admixture [Oppenheimer 2003 , 49]) indigenous populations in Asia and Europe.

Limited Archaic Interbreeding outside Africa Contrary to common perception, the replacement model does not depend on the assumption that there was a lack of interfertility, and thus zero gene-fl ow , between AMH and diff erent indig-enous regional human populations. The genetic evidence merely confi rms, with at least 95% probability, morphological arguments that AMH appears to have replaced indigenous popula-tions. If one ignores the degree of replacement, however, both multiregional and replacement models turn out to have a point. It is essential to clarify this point in order to lay to rest the mis-assumption, sometimes made by multiregionalists, that any evidence of archaic admixture (i.e., reticulation) outside Africa, however small, would necessarily disprove the recent replacement model (Wolpoff et al. 2000 ). It would not: replacement is simply implied by degree, which does not need to be 100%.

Interbreeding with Neanderthals The issue of interbreeding is raised by recent claims of a 1–4% introgression of the Neanderthal genome into modern Eurasians (Green et al. 2010 ). If valid, and this author is convinced there is at least a prima facie case, this evidence is not necessarily at variance with the simple view of apparently complete replacement or with the unambiguous lack of uniparental evidence for Neanderthal introgression amongst the vast number of uniparental mtDNA/Y genomes already


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studied. It should be remembered that the eff ective population size of the haploid uniparental genomes would be a quarter that of autosomes. The fi nding of only single African lineages (L3 and M168 ) in non-African mtDNA and NRY respectively not only excludes the multiregional model at these two loci but also demonstrates a severe founding drift event. This drift would likely have caused the extinction of minority lineages (e.g., the less than 4% maximum estimated initial introgression among putative uniparental genetic material acquired from pre-dispersal interbreeding).

Whether such hypothetical extinction of putative acquired uni-parental Neanderthal lineages among AMH would have occurred following admixture outside Africa (Green’s preferred geo-graphic option) is clearly debatable, but depends on where and how soon after exit admixture occurred and what population sizes were involved. However, it is diffi cult to imagine any geo-graphic scenario in which the ancestors of East Asians could have simultaneously acquired the same dose of Neanderthal autosomal genes as those of Europeans, unless it was soon after the exit and before East-West divergence. The authors themselves acknowledge, for this to happen, that “gene fl ow from Neanderthals into the ancestors of non-Africans occurred before the divergence of Eurasian groups from each other” (Green et al. 2010 , 710), thus explicitly depending on the single exit hypothesis for AMH , recruited in this case to explain autosomal miscegenation.

The current northwest Eurasian distribution of Neanderthals leaves large lacunae in South Asia . Because of the absence of fossil evidence from this region, the possibility cannot be dis-counted that Neanderthals lived in this region and also Arabia. If so, it may have been in these regions that admixture occurred with immigrant AMH populations, before the diff erentiation of Europeans from East Asians.

Interbreeding with Denisovans in Oceania An alternative admixture scenario that Green and colleagues ( 2010 ) did not consider for that publication but, as co-authors, did off er subsequently in the context of the recently sequenced hominin phalanx and molar tooth from a Middle Palaeolithic/Upper Palaeolithic transitional context in Denisova Cave , Southern Siberia (Krause et al. 2010 ; Reich et al. 2010 ), is the possibil-ity that AMH interbred with other non-Neanderthal populations, such as H. heidelbergensis , or H. erectus descendants moving from China to Central Asia .

In their fi rst report (Krause et al. 2010 ), the Leipzig group argued from the complete mtDNA sequence of the phalanx that the Denisova lineage had branched off long ago (1.04 Ma, CI: 0.779–1.3 Ma), leaving both the modern human and Neanderthal lineages more closely related to each other (0.493 Ma, CI: 0.3744–0.6121 Ma) than either were to Denisova . In their second publi-cation, they reported another complete mtDNA sequence (from the tooth) which was very close, but not identical to that of the phalanx from the same site, indicating two ‘Denisovan’ individu-als, very similar at the mitochondrial locus, and confi rming the phylogenetic age estimate, with a slightly reduced error (0.982 Ma CI: 0.7805–1.208 Ma). Morphologically, the Denisova tooth grouped tightly with early Homo specimens (Australopithecines, H. habilis and African H. erectus ), all of whose teeth were signifi cantly larger than a more ‘modern’ group containing Neanderthals , H. heidelbergensis and all other European humans (ancient and modern), all AMH , and Chinese H. erectus . Indonesian H. erectus specimens tended to take an intermediate position between the two groups. Another, non-metric, feature of the dental root, namely a massive, splayed lingual root, again grouped the Denisovan tooth with the erectus types. In offl ine material for two other genetic comparisons, the Leipzig group displayed Venn diagrams of autosomal segmental duplica-tions showing those held in common, compared fi rst between a present-day human (NA18507), a Neanderthal , Denisova and a chimpanzee, and then between a Neanderthal , Denisova and a present-day human. In each case the modern human and Neanderthal shared most segmental duplications with each other and least with Denisova (Reich et al. 2010 ).

These four comparisons, three genetic and two morphological, are all consistent with each other, fi rst in making Denisova the biological outgroup when compared with the relatively


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closely related European human groups (AMH , Neanderthals and H. heidelbergensis ), but further in implying that Denisova could be descended from an earlier African population. (The case for a Eurasian origin for the Denisova population is argued by Martin ó n-Torres et al. 2010 .)

In the second Denisovan paper, Reich and colleagues ( 2010 ) also performed extensive autoso-mal sequencing and claimed that 4–6% of the provisional autosomal signatures from the Denisova fi nger bone were found in three Melanesians (two Papuans and a Bougainvillean), but in none of the other modern human groups they tested (African: San, Yoruba and Mbuti; fi ve “Eurasians”: French, Han Chinese, Sardinian, Cambodian and Mongolian; and South American: Karitiana). Given all the genetic and morphological observations summarised thus far, this inclusive or exclusive geographical scenario could be consistent with ancestors of Oceanic AMH mixing, not with North Chinese H. erectus , but with H. erectus in the region of present-day Indonesia .

The regional distinction is important, since controversial evidence published by Swisher et al. ( 1996 ) suggested that ‘Solo man’ , a signifi cantly larger-brained version of Homo erectus from Ngandong and Sambungmacan , Central Java , may have survived to only 27 ± 2 to 53.3 ± 4 thou-sand years ago. The controversy over sources of error was such that it was revisited by some of the same researchers, resulting in higher but confl icting dates, the youngest of which was a electron spin resonance (ESR)/U-series of 143 ka (+20/ − 17) (Indriati et al. 2011 ; see Dennell, chapter 4 , this volume). Whether the fi rst dates were valid or not, these controversial reports could bring the evolved descendants of H. erectus much closer to the Upper Pleistocene.

Rather surprisingly, in view of the erectine links and the range of their own genetic and mor-phological evidence, Reich et al. ( 2010 ) argued instead for a closer relationship of descent between the Denisova hominin and Neanderthals than either had with AMH . This change in inference was made on the basis of another comparison, namely of autosomal SNPs (single nucleotide polymorphisms) between Denisovans with AMH , chimpanzees and Neanderthals , but was based on the relative degree of sharing of SNPs distinctive to Neanderthals . The results and implica-tions of the latter were quite diff erent, when compared with the four genetic and morphological comparisons mentioned previously, and there is a simple explanation for this anomaly, which they did not test for: Neanderthal-Denisovan admixture .

The strength of evidence for their assertion of more recent common descent for the two archaic groups was less convincing than that for the simple Neanderthal intrusion into AMH . This is because in this more complicated three-group interbreeding puzzle they did not test a more likely local explanation for the autosomal SNP associations between Neanderthals and Denisovans. That would be extended direct hybrid interbreeding between the two archaic human groups locally in Central Asia , prior to AMH arrival, as supported by their geographic, physical and temporal overlap, consistent with the extended presence of Neanderthal mtDNA in that part of Central Asia (shown in Okladnikov Cave , near Denisova Cave ; Krause et al. 2007b ). Such inter-breeding would constitute only admixture – not, of course, recent common descent – and might be one explanation for the extraordinary plesiomorphy shown in the recently discovered Late Pleistocene “ Mongolanthropus ” at Salkhit in Mongolia . With marked superciliary arches, this skull-cap fragment “shows multiple similarities with Neanderthals, Chinese Homo erectus , and West/Far East archaic Homo sapiens ” (Coppens et al. 2008 ; but see also Kaifu & Fujita 2012 , who mention its dating of 20 ka and see it as within the range of Late Pleistocene AMH).

Perhaps the most interesting result in the Reich et al. ( 2010 ) paper was from comparisons of autosomal SNPs in Denisovans with a variety of modern Eurasian populations, which revealed that Denisova shared 4–6% of its genetic material with present-day Melanesians in the Pacifi c, a quarter of the globe away, but not with any other more proximate populations. The possibil-ity of widespread archaic admixture in Oceania was reinforced by Rasmussen et al. ( 2011 ) who showed Denisovan admixture in a 100-year-old Australian Aboriginal hair, similar in degree to that in Melanesians. They also postulated two dispersals of African-derived AMH populations into Australia : the fi rst 62,000 to 72,000 years ago, and a later one 25,000 to 38,000 years ago. Unfortunately, their interpretations were infl uenced by a selective use of autosomal-dating of


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demographic events, unsupported archaeologically. Their analysis and interpretation of ances-tral population phylogeny depends not on any genetic phylogeny but, similar to the Green et al. ( 2010 ) paper, on a statistical association test based on only four complete genomes.

On the positive side, the Rasmussen et al. ( 2011 ) results still extend the infl uence of ‘Denisovan DNA’ in Oceania more broadly to the Sahul region (as was predicted, Oppenheimer 2012b ) and do not falsify the consensus single AMH exit model preferred here. Indeed, there is skeletal evi-dence that would reverse the morphological order of their “two Australian waves” since the ear-liest human crania in Australia are modern and gracile, while there is a late appearance of archaic, robust skulls in the Kow Swamp and Coobool Creek sites, radiocarbon-dating to no earlier than 14,300 years ago (Brown 1992 ).

This Antipodean Denisovan puzzle has been extended by a more recent and fascinating third publication (Reich et al. 2011 ) presenting results for 33 more locations in Asia, Southeast Asia and Oceania. Briefl y, all 15 populations tested to the east of Huxley’s Line ( Figure 18.3 ), including one Negrito and one non-Negrito group in the Southern Philippines , Near Oceania and Polynesia , show signifi cant evidence of Denisovan intrusion, up to but no more than that shown for Melanesians (i.e., Near Oceanians) and a now similar level for Aboriginal Australians . Conversely, of 27 Asian and Southeast Asian (SEA) populations west of the Huxley Line , includ-ing two Negrito groups, none shows any signifi cant evidence of such intrusion in Reich’s analysis. All admixture thus appears to have been off shore in Wallacea or Sahul since Huxley’s Line demar-cated the true eastern limit of the Asian Mainland until 10 ka. More westerly admixture with later local extinction is possible but much less likely.

Relevant questions about these fi ndings are how many times, from where, and when? For the fi rst, with the exception of the Philippines , all the results of relative admixture estimates are

Figure 18.3. Map showing a single southern route out of Africa and beachcomber arc route from the Red Sea along the Indo-Pacifi c coast to Australia, including likely extensions to China, Japan and New Guinea, from MIS 4. Vegetation and sea level shown as at LGM. Overlay: claimed fossil and cultural evidence for AMH in Eurasia during MIS 5 to MIS 4 (see text for dates) (modifi ed from fi g. 1.6 in Oppenheimer 2003 ; 2009 ).


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consistent with a single proximate common source of Denisovan admixture : Wallacea , Australia or New Guinea with subsequent fresh external diluting AMH gene fl ow . The non-Denisovan admixture analysis in Wallacea , Oceania and the Philippines show New Guinea and Australia as tightly correlated, major alternative candidate sources of AMH gene fl ow for the Pacifi c region, but Australia as consistently the richer one, overall by 40%. The apparent anomaly could still be compatible with Near Oceania being the primary dispersal source, but with the New Guinea Highlands (Gosden 2010 ; see Summerhayes and Ford, this volume) having gone through found-ing event(s), isolation and subsequent drift, a scenario already inferred from shared unique human leukocyte antigen (HLA) markers (Serjeantson & Hill 1989 ). However, Wallacea (Nusa Tenggarah and/or Moluccas) still seems the most parsimonious primary Denisovan source, from geographic considerations. This speculation is consistent with the plot of individuals’ Denisovan admixture against Near Oceanian (New Guinea) admixture (Reich et al. 2011 , fi g. S1), which shows tight correlation and high values of relative Denisovan admixture (ranges from 30% to 100%) for Wallacea and Fiji . In contrast, Polynesians , although grouping together with the lower end of the Wallacean distribution with lower values (20%–30%) and still correlating with Near Oceanian admixture , show consistently less Denisovan intrusion than expected from that admixture , likely resulting from later Southeast Asian admixture and drift.

The south Philippines are anomalous compared with the other 13 sites, since both samples show 3–4 times more Denisovan intrusion than would be expected from their Near Oceanian (i.e., non-Denisovan) admixture values and no clear correlation with them. This likely indicates a separate admixture event and subsequent dispersal, though the source region could still be in north Wallacea .

The similar, asymptotic levels of Denisova intrusion in Melanesians and Australians suggest that the main admixture event occurred in a single source population before the colonisation of Sahul , most likely in Wallacea . Caveats on the above fresh inferences are the paucity of regional samples from Sahul , which could allow for subsequent minor migrations or even fresh admixture events.

Interbreeding with Denisovans in China? The ‘exclusivity’ of the Oceanic Denisovan admixture was brought into doubt by Skoglund and Jakobsson ( 2011 ), who, by adjusting for founding genetic drift coupled with ascertainment bias, demonstrated an additional ‘Denisova-related’ intrusion centred in East Asia (mainly China but extending to two populations in South America). This result helps to support similar inferences for East Asia published four months before (Abi-Rached et al. 2011 , fi g. 2E).

A discrete third hominin (Denisova-like) admixture event in China would tempt speculation about which hominin was intrusive and a search for morphological evidence of hybrids. There is a dearth of unambiguous archaic candidates in that region dating to less than 100 ka. Xujiayao, Maba and Dali were late large-brained archaic individuals and morphologically intermediate between H. erectus s.s. and AMH , but none dates to later than the terminal Middle Pleistocene (Brown 1999 ; Kaifu & Fujita 2012 ). As to evidence of an unexplained morphological change among AMH , the clearest potential candidate change came surprisingly much later, and after the appearance of more conventional Late Pleistocene AMH fossils such as those from Upper Cave 101, Zhoukoudian and Liujiang in China (Brown 1999 ). This change was apparently the late and sudden appearance of Mongoloid fossil skulls throughout East Asia and the New World, fi rst clearly evidenced in the Asian Early Holocene–Early Neolithic record, with skulls such as Baoji (Brown 1999 ). These features were possibly anticipated by distinctive, older, ‘broad-cheeked’ AMH fossils such as the Minatogawa 1 skull (ca. 20 ka; Brown 1999 ) and, more recently, Longlin 1 , which dates to the Pleistocene-Holocene transition (Curnoe et al. 2012 ). “ Mongolanthropus ” also needs to be mentioned here for morphological and dating reasons, although its face is missing. One possibil-ity is that the onset of Late Glacial and Early Holocene warming allowed populations to expand from their refugia and interact with each other. More likely, hybridisation could have already occurred in Far Eastern LGM refugia (see elaboration of these two complementary expansion-


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contraction hybridisation scenarios – attributed respectively to Darwin, Hewitt and Arnold, in Stewart & Stringer 2012 ).

Looking for refuge locations, most known LGM hominin archaeological sites in China are north of the Yangtzi in modern Sinodont, northern Mongoloid regions that might fi t dental clines (see fi gs 6.3, 5.4, 5.3 in Oppenheimer 2003 ), though ascertainment bias is possible. However, either model would still, as mentioned for Neanderthal intrusion, have to assume a subsequent bottleneck period of selection or drift (perhaps during the Younger Dryas refuge period), fol-lowed by expansion. This would explain the lack of uniparental evidence of admixture , as with the Neanderthal story, but also the more uniform and gracile suites of cranial features found in modern southern and northern Mongoloid populations.

Several predictions of the recent replacement model can be tested by genetic phylogeogra-phy , using non-recombining DNA such as mtDNA and the NRY . Recent replacement outside Africa should show older lineages within Africa and younger lineages outside, depending on the degree of replacement. This was fi rst unequivocally demonstrated to be the case (Oppenheimer 2003 ) 16 years after the famous ‘Mitochondrial Eve’ paper was published (Cann et al. 1987 ), after knowledge of mtDNA variation had been improved by a set of complete sequence data from 52 individuals of worldwide distribution (Ingman et al. 2000 ).

This author dated the branches of the world mtDNA tree, reconstructed from those fi rst 52 complete sequences, showing that all non-Africans belonged to two sister twigs (M and N ), both aged at only around 70 ka, and arising from L3 , one of the youngest of more than a dozen branches originating in Africa (see Figure 18.2 ). This broad conclusion has not been subsequently falsifi ed by analysis of tens of thousands of mtDNA sequences.

Single versus Multiple Recent Exit Models for AMH Two further fundamental observations can be made from that worldwide mtDNA tree. First, of the dozen or more L branches of the tree found only in Africa most are, of course, much older than L3, M or N. The genetic antiquity of the deepest L branch point (192 ka; Soares et al. 2009 ; 190 ka in Figure 18.2 here) is consistent with new dated fossil fi nds of early AMH in East Africa (164 ka: White et al. 2003 ; 195 ka: McDougall et al. 2005 ).

Second, as mentioned, the most singular observation is that ‘L3 ’, the remaining African branch, is unique in that it also encompasses all non-Africans, with two branches originating outside Africa, namely M and N (Oppenheimer 2002 , 2003 ; Metspalu et al. 2004 ). In eff ect, this means that the rest of the world was colonised by two descendants of the ancestral haplotype for M and N (a subtype of the L3 root holding a195 nucleotide transition). The latter now dates by the cor-rected molecular clock to 71,600 (CI: 57,100; 86,600) years ago (Soares et al. 2009 ). The overall age estimate of L3 , including all the African branches, is the same and likely represents the upper age limit for the exit (i.e., L3 could be older but not younger than the exit). However, the confi -dence intervals on this estimate straddle the date of YTE , and thus prevent a genetic resolution of the issue of whether AMH entered South Asia before or after the Toba eruption .

A single mtDNA lineage exit is unambiguous evidence of a single successful exit of AMH from Africa and is paralleled closely in the Male-NRY tree and also clearly refl ected in the X chromosome and in several autosomal loci (Oppenheimer 2003 ; Hudjashov et al. 2007 ; Richards et al. 2006 ; Abu-Amero2009). That all non-African descendants, for both mtDNA and Y lines, each share one and the same close African ancestor is in itself evidence of a period of drift in a small isolated founding group. The chances of two or more founding exit episodes separated in time and place each giving rise, by drift, to the same source African lineage in all diff erent parts of the non-African world are negligible, leaving the single successful exit as default (Oppenheimer 2009 ).

Claims for additional earlier AMH exits during MIS 5e , based on fossil or lithic evidence, are not necessarily implausible in practical terms and need to be taken seriously. Given the dating evidence from Skh u l and Qafzeh , which variously approach and signifi cantly exceed 100 ka


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(Stringer et al. 1989 ) for at least one abortive AMH , exit near MIS 5e means that similar claims for the southern route may well turn out to be demonstrated in the fossil record but not in the mtDNA or NRY of modern populations: if so, such dispersal events must be presumed evanes-cent. Furthermore, although autosomal evidence, having far more loci and less drift, is much more sensitive than uniparental study and can thus show less than 5% Archaic admixture into AMH , outside Africa, it would be far less specifi c when comparing AMH with AMH, thus losing that advantage looking for AMH genetic survival from the last interglacial .

Which Route?

A single successful African exit for AMH has several clear implications. Because there was only one route of exit, the number of subsequent route options was also decreased. Whichever route was taken initially, however, a multidisciplinary model also has to explain how both Europe and Asia could have been colonised ultimately from the same single exit group.

Southern Rather than Northern Exit: Genetic Evidence As far as the genetic evidence is concerned, there are two logical reasons for identifying the south-ern route across the mouth of the Red Sea . First, of only two primary branches of L3 mtDNA outside Africa (M and N – Figure 18.2 ), both are represented in East Eurasia, including South Asia (where they have the highest number of autochthonous M and N lineages in the world), while N is represented only in West Eurasia. Second, Levantine and European N representatives are less diverse, more derived and younger than those of Arabia and Southwest Asia (see fuller discussion in Oppenheimer 2003 ; Richards et al. 2006 ; Fernandes et al. 2012 ).

Climatic Considerations and Archaeology of the Exit Route An implication of the single genetic exit, southern or northern, is that there must have been great diffi culty in establishing a beachhead outside sub-Saharan Africa (SSA) (e.g., the Arabian Gulf; Oppenheimer 2003 , 88) during the Upper Pleistocene, presumably as a result of the Saharan-Arabian arid zone barrier to migration out of SSA (see Figure 18.3 ). This barrier was freely permeable only during interglacials but may also have been open between 80 and 75 ka during early MIS 5a (see Rosenberg et al. 2011 ), a period characterised by a moist welcoming refugium , a ‘Gulf Oasis’ (Rose 2010 ; Fleitmann et al. 2011 ; see also Petraglia et al. 2011 ).

A Last Interglacial Exit into Southwest Asia? Two full interglacials have occurred during the past 150,000 years, the last, or Eemian, intergla-cial (MIS 5e ) and the most recent, during the Holocene . Each time a diverse fl ow of African savannah fauna occurred from SSA to the Levant and Arabia (Turner 1999 ). AMH used this cli-matic window for a northern exodus during MIS 5e , as evidenced by skeletal evidence at Skh u l and Qafzeh . However, this dates to between only 120 and 90 ka (Gr ü n & Stringer 1991 ). There is presently some archaeo-genetic consensus that this early exit left no descendants in modern populations (contra Petraglia et al. 2010 ). On the archaeological side, there is the long gap of up to 40,000 years between the Skh u l and Qafzeh AMH fossils and the reappearance of AMH , signalled by the Early Upper Palaeolithic in West Eurasia after 49 ka, a period that was interca-lated by the Neanderthal presence in the Levant (for a recent perspective on dates and causes of these switches, see Shea 2008 ). On the genetic side there are no mtDNA or Y lineages outside Africa that date even near an MIS 5e exit at 120 ka, or even the less relevant lower bracket of 90 ka, which evidence virtually excludes survival of this very real Levantine AMH colony into the modern ex-African gene pool (Oppenheimer 2003 ; Richards et al. 2006 ).

An identical issue applies to recent claims that AMH reached Jebel Faya in southern Arabia between the beginning of MIS 5 e and 90 ka (Petraglia et al. 2010 ; Armitage et al. 2011 ). Here,


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only lithic evidence is available. The tool kit showed affi nities to the late Middle Stone Age in northeast Africa. This evidence was used to infer the presence of AMH in the Arabian Peninsula at two diff erent times: an occupation phase during MIS 5e (two optically stimulated lumines-cence [OSL] dates: 123 ± 10 ka and 127 ± 16 ka, and another later one at 95 ka: OSL: 94.8 ± 13.0 ka; see Armitage et al. 2011 ; Petraglia et al. 2010 ). Rose et al. ( 2011 ) have also recently reported the fi nding of a Middle Stone Age Complex of Nubian typology in Dhofar , Oman , dated at 106 ka (OSL), again fossil-free.

Except for the absence of fossils, the chronology and implications for these putative, limited appearances of AMH outside Africa during MIS 5e and its disappearance before 90 ka are similar to Skh u l and Qafzeh in that the colonies seem to have died out with no issue. This author feels that it would be hardly surprising if the Skh u l and Qafzeh event via the northern route out of Africa had been duplicated via the southern route out of Africa as well, although given the likely low population densities of the period, it does not necessarily mean genetic survival of such a migration. However, because there is no human skeletal record from Pleistocene Arabia , the mak-ers of the Jebel Faya assemblages are unknown, and the possibility cannot be excluded that these were made by Neanderthals . Given that stone tools are a poor guide to identifying the hominin that made them, the African Middle Stone Age typology of the tools would not necessarily contradict this possibility. Furthermore, if Neanderthals were in Arabia , it provides a plausible scenario for a Neanderthal admixture into an AMH population that was ancestral to both West Eurasians and East Asians, as predicted by the single southern exit hypothesis .

Fossil Evidence in East Asia for an Exit during MIS 5e The recent fi nding of an anterior mandible with two molars at Zhiren Cave (Zhirendong ) in South China has been claimed as possible evidence for an expansion of AMH into East Asia during MIS 5 e (Liu et al. 2010b ). The authors argue that the mandible dates to more than 100 ka (but see Kaifu & Fujita 2012 ) and shows derived modern human features, distinct from any known late archaic human. However, they also place it close to later Pleistocene archaic humans (contra Kaifu & Fujita 2012 ). In a comment in Nature , Dennell ( 2010 , 513) agrees with the dates as “100,000–113,000 years old, and possibly older”, on the basis of stratigraphy and faunal associa-tions, but questions the need to postulate interbreeding, at least in relation to the molars: “The latter are small, and would be considered as modern in a Late Pleistocene west Eurasian (post-Neanderthal) sample”.

Westaway and colleagues ( 2007a ) also recently suggested the possibility of AMH presence in Java during MIS 5e on the basis of one tooth from Punung Cave (reported in Storm et al. 2005 ) that was small enough to be classifi ed as H. sapiens . This claim is necessarily very tentative until more evidence is available.

The Liujiang skull, a Late Pleistocene cave specimen also from South China , has had similar claims for such antiquity. It is clearly anatomically modern, and other items in the breccias in which it was found date to 111–139 ka (Shen et al. 2002 ), which could put it in the same time frame as the Zhirendong and Punung Cave fossils. These dates are very controversial, however, and the younger dates (all around 67 ka) based on the covering fl owstone seem a much safer minimum.

Summary of Genetic Arguments against a Genetically Successful Exit during MIS 5e The recalibrated age estimates and confi dence intervals (CI) of the out-of-Africa mtDNA branch L3 of 71,600 years ago (CI: 57,100; 86,600) (Soares et al. 2009 ) comfortably exclude the date ranges implied by an Asian exit during MIS 5e (as did previous L3 age estimates: Oppenheimer 2003 ; 2009 ). Furthermore, given the clear phylogenetic evidence for only one successful exit, it could be either an exit during only MIS 5e or later (e.g., during MIS 5a–4), but not both. Since the genetic dates and the bulk of the archaeological evidence and dates converge on MIS 4 or


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even later, a population that left Africa during the last interglacial and survived substantially into the modern gene pool looks extremely unlikely. So, one has to presume that, like the Skh u l and Qafzeh exit, any Asian exit by AMH during the last interglacial must have been similarly evanes-cent. This view is consistent with the archaeological record for the terminus of the putative early Africa -Arabia exits in Oman , which are older than the range of L3 age estimates.

Dating Migrations

Possible Dates of the Defi nitive Exit As mentioned above, a general guide to the earliest possible date for the expansion of H. sapiens into and beyond Arabia is the genetic age of lineage L3 that is now 71,600 years ago (CI: 57,100; 86,600) (Soares et al. 2009 ), if we exclude an exit in the last interglacial (MIS 5e ) on the grounds that it lacks a genetic progeny. For the lower limit, N , one of the two daughter lineages of L3 , the nearest outside Africa has been dated in South Asia, where it has a predominantly western distribution, to 71,200 years (CI: 55,800; 87,100). However, the ages of the M and N lineages estimated elsewhere are mainly around 60 ka. Branch M is surprisingly young in South Asia where it has a predomi-nantly eastern distribution, dating from 49,400 years (CI: 39,000; 60,200) (Soares et al. 2009 ).

Delayed Migration to the Near East and Europe Since the genetic evidence favours a single southern exit rather than a northern one, it is impor-tant to track and date the appearance of northwest Eurasians out of the southern route. The problem remains, however, as to how and when ancestors of northwest Eurasians crossed the arid zone between Southwest Asia and the Levant and Turkey in northwest Eurasia, and why they were delayed. A climate-permissive model can be reconstructed (Oppenheimer 2009 ) of AMH populations suddenly being able to move into northwest Eurasia from South Asia during Interstadial 14/1349–55 ka ago (see fi gs. 1.7, 1.8 and 3.4 in Oppenheimer 2003 ; also M ü ller et al. 2011 ). This dating would coincide with the earliest appearance of Upper Palaeolithic technology in the Levant and southeast Europe (calibrated to 47/49 ka and 46 ka respectively, Mellars 2006a ) and also with the age of U8, the oldest mtDNA ‘N’ lineage in the Near East and Europe (at 51.3 ka (CI: 44.0–58.8): Soares et al. 2010 ; see also Oppenheimer 2003 ; Richards et al. 2006 ).

Dating the arrival of AMH in India and East Asia While there may arguably be evanescent archaeological or physical evidence for AMH in Arabia and southern China soon after the last interglacial , so far there is little fossil or genetic evidence in India or Southeast Asia that dates to the period immediately before the Toba eruption . Instead, there is a congruence of proxy evidence to either just before or soon after this event, the former (pre-Toba) being only cultural and the latter (just post-Toba) now including (in East Asia) several fossils and the recalibrated genetic dates.

India and Toba Given the previous suggestion (Oppenheimer 2003 ) that AMH may have arrived in Southeast Asia before the super-eruption of the Toba volcano (73 ka ago), much archaeological interest has focussed on India, which has the most extensive deposits of younger Toba tuff (YTT) (Jones 2007 ). Although Middle Palaeolithic stone artefacts have been found in association with several Indian YTT deposits (both above and below), hominin fossils are absent, and whilst H. sapiens might have made these assemblages, other possibilities cannot be discounted.

Despite these issues, recent articles (Blinkhorn & Petraglia, this volume; Petraglia et al. 2007 ; see also Petraglia et al. 2010 , 2011 ; Clarkson et al. 2012 ) argue from lithic evidence both below and above Toba ash in the Jureru Valley in Southern India for the presence of AMH in India at the time of the YTT event, with survival afterward. Much of the argument impressively detailed


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and analysed by Clarkson et al. ( 2012 ; see also Clarkson, this volume), which supports claims that these tools (pre- and post-Toba) were made by AMH , rests on their tight statistical typological associations both with the South African Middle Stone Age , and Palaeolithic AMH cultures in Australia and Southeast Asia , and their diff erences from North African and Levantine cultures, archaic hominins, very early modern human and later Aurignacian assemblages. This nuanced perspective is obviously more consistent with the southern route of AMH dispersal, as is implicit in Clarkson’s statement: “There are no obvious diff erences between the Jurreru assemblages and newly reported assemblages from Arabia dating to MIS5a” (Clarkson et al. 2012 , 178). In the absence of hominin fossils, there are the usual problems of trying to match a technology to a par-ticular hominin, but when this work was presented at a meeting in Oxford in 2010, this author was convinced by the statistical and visual associations, as were several seasoned archaeologists, excepting Paul Mellars (reported in Balter 2010 ). Previously pre- and post-Toba-ash cultural dates for the fi rst site (Petraglia et al. 2007 ) were both close to the YTE, but in new results on nine paired Indian sites, only the pre-Toba dates were close to the YTE at 74,000 BP or earlier, whereas nearly all of the post-Toba dates were about 55,000 years BP or younger (Balter 2010 ), thus raising a question mark over their key claim of rapid local human recovery in India post YTE.

Indian Genetics and Toba A prediction of the ‘catastrophic’ interpretation of the conjunction of tools and ash in India, clearly not shared by Petraglia and co-workers, is that a deep and wide genetically sterile furrow would have split East from West Asia, with India eventually recovering from a genetic bottleneck either locally or by re-colonisation from either side (Oppenheimer 2009 ). Evidence for such a furrow was fi rst argued for in the genetic map of Asia (Oppenheimer 2003 , 180–184). Also consistent with the possibility of a local Indian bottleneck, M groups in Eastern India round the Bay of Bengal, although numerous, are notably much younger than elsewhere along the Indian Ocean trail (fi g. 3, Oppenheimer 2009 ; see also table 1, Sun et al. 2006 ; and table 3, Soares et al. 2009 ). However, dates for haplogroups N and R , which locate further west in South Asia, where less ash fell, are rela-tively higher ( fi g. 3 , Oppenheimer 2009 and table 3, Soares et al. 2009 ; but see also fi gs. 4.3 and 4.5, Oppenheimer 2003 ), consistent with local drift near the east coast of India and a pre-Toba exit.

Fossils and Genes in East and Southeast Asia Inevitably, as in Europe, fl aked stone artefacts are vastly more common than fossil hominin specimens. Nevertheless, the few dated AMH fossils from China and Southeast Asia (and even Australia), date from MIS 4–3 and show a much older age bracket (67–40 ka) than the oldest such fossils in Europe ( < 35 ka: Trinkaus 2003 ) ( Figure 18.4 ). However, with the possible exception of the Punung tooth, none predates Toba .

The small collection of East Asian dates for the early arrival of AMH are older than their equivalents in Europe (reviewed in Oppenheimer 2012a ; 2012b and elsewhere in this volume and need not be discussed here).They are limited by the predominant use of radiocarbon-based dat-ing with its low ceiling, the lack of diagnostic fossils even when the cultural dates are secure, and lack of secure direct dating even when the fossils are securely AMH .

There is also a problem with the newly recalibrated estimates of genetic ages for M, N and R haplogroups in East Asia , which now cluster at around 60,000 BP or less (Soares et al. 2009 ), 5,000–15,000 years lower than estimated before (Oppenheimer 2009 ), rather than the new archae-ological estimates of up to 67,000 BP. Confi dence intervals of the new genetic dates for all three haplogroups are, however, still suffi ciently wide to include 67,000 BP and even Toba .

Dating Pleistocene Arrivals of Humans in Sahul and Near Oceania Archaeology, Radiocarbon and Luminescence Dating For Melanesia , the fi rst archaeological evidence of occupation in the upland Ivane Valley , New Guinea , has been radiocarbon dated to between 49,000 and 43,000 cal BP and 44,890 and


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43,100 cal BP to the east, in the Pacifi c island of New Ireland (Summerhayes et al. 2010 ; see also Summerhayes & Ford, this volume). For Australia , the carbon ceiling has similarly been increased to around 48,000 years using the ABOX-SC method (Turney et al. 2001a ; also in Summerhayes et al. 2010 , table S2). Non-carbon dating techniques of cultural remains suggest substantially earlier human colonisation of Australia by 50–60 ka (Roberts et al. 1994 , 2001 ; Bird et al. 2002 , 2004 ). There is clearly more work required with non-radiocarbon-based techniques. This review is not intended to replace reviews that focussed on that controversy but merely points out that the present ‘oldest non-carbon dates’ are not inconsistent with Australian colonisation by 50–60 ka. Recent sea level curves, concentrating on troughs (Siddall et al. 2003 ), suggest that the best win-dow of opportunity for such a long crossing over the large Sahul shelf from Timor would have been circa 65 ka when sea levels were 100 m lower, with occupation in the Northern Territory becoming visible on modern littorals only from 60 to 50 ka onwards owing to higher modern sea levels (Oppenheimer 2004 ).

Genetics As mentioned, genetic phylogeography is a better tool for elucidating migration trails than for dating . Resolution on the main descent of Melanesians and Australian Aboriginals from the single exit is clear. The point, made initially in 2003 (Oppenheimer 2003 ), that Australian and Melanesian mtDNA all belonged to M or N lineages, is consistent with the single exit, and has since been confi rmed by complete mtDNA sequencing and NRY work on an expanded set of samples (Macaulay et al. 2005 ; Hudjashov et al. 2007 ).

Previous mtDNA genetic dates of arrival of the vanguard in the Sahul , estimated by this author, were consistent with a pause of around 10,000 years at Bali before crossing to Australia (Oppenheimer 2009 ). Combining the Roberts et al. ( 1994 ; 2001 ) colonisation dates for Australia with the recalibrated genetic ages of M , N and R in East Asia would give no time at all for such

Figure 18.4. Map with archaeological dating of early AMH evidence in ISEA and Sahul as mentioned in the text; for 14 C calibrated estimates (the rounded mean of the upper and lower fi gures), see Summerhayes et al. ( 2010 ). Two routes to Sahul involved island hopping (modifi ed from fi g. 4.1 in Oppenheimer 2003 ; 2009 ).


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a pause before arriving in Australia, possibly 50–60 ka. This would infer an extremely rapid transit from Africa to Australia, of circa 12,000 years given the recalibrated molecular age of the out of Africa L3 lineage at 71,600 years. This speed of colonisation is however consistent with the phylo-genetic observation that the great arc of dispersal was eff ected entirely by the three primary root founder haplotypes M, N and R , each giving rise to multiple sets of unique regional daughter branches in every sector round the Indian Ocean .

Conclusions

Although no hominin fossils have been found, the recent archaeological evidence from India is consistent with a southern exit of AMH from Africa shortly before the Toba eruption of circa 74 ka. However, Neanderthals or other archaic populations cannot be excluded as potential tool makers of their African-type Middle Stone Age assemblages. The preliminary evidence on the dating of the post-Toba cultural layers would suggest a prolonged period of recovery, consis-tent with a local Indian bottleneck. The parallel phylogeography of both uniparental lineages (mtDNA and male-NRY ) overwhelmingly suggests a single exit of AMH from Africa , most likely by the southern route .

Recent recalibration and correction for non-linearity of the mtDNA molecular clock reduces most previous genetically dated estimates (based on the age of L3 for the upper limit) of this exit to approximately 72 ka (CI: 57–87 ka), with rapid migration round the Indian Ocean coast to Australia . The lower limit of the estimate could be described by the hypothetical founding ancestor of the two primary non-African clades M and N at 71.5 ka (CI: 57.0; 86.4), or failing that by the age of the most southwesterly clade, N estimated in South Asia at 71.2 ka (CI: 55.8; 87.1), though the ages of haplotypes M and N elsewhere are younger than this. These genetic confi dence intervals straddle the Toba event and thus cannot resolve the issue of whether humans dispersed before the Toba eruption , but they do appear to exclude a signifi cant (> 5%) AMH survival from any earlier MIS 5e exit, as claimed from Jebel Faya , Zhirendong and a controversial older dating of the Liujiang skull.

As with the recent inference of Neanderthal and Denisovan admixture outside Africa, there is a possibility that uniparental studies, with their known problems of drift, could miss a less than 5% contribution from any other human demic source, including that from an MIS 5 AMH exit. However, given that the autosomal genomic methods recently used to infer archaic admixture were statistical and relatively non-specifi c and had no method of dating, these methods would themselves be unlikely to resolve this issue any time soon.

Palaeoclimatology, palaeoanthropology and archaeology potentially off er fi rmer dating mea-sures for dating the defi nitive AMH exit from Africa , although with no fully convincing evidence older than the Toba eruption . Dated, contextually uncontroversial, diagnostic fossil evidence in China and SEA goes back only to 50 ka. This asymptote is partly an artefact of method, which has been largely radiocarbon-based. Without that limitation of method, Liujiang in Southern China provides a defi nite AMH skull, and an accurate and contextually conservative minimal date of 67–68 ka, while Australia off ers cultural evidence between 50 and 60 ka. The ash fallout from the Toba super-eruption still off ers both a stratigraphic approach to dating sites in South and Southeast Asia and an ongoing source of controversy in the absence of any relevant fossil evidence. Diagnostic AMH fossils in Asia dating to the period 74–90 ka would help resolve the issue.


Modern Humans Spread from Aden to the Antipodes With Passengers and When?

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