Can we retire the 60,000-year old coastal Out of Africa?

A widely popularized story has become part of the mainstream view of modern human origins. I have seen it repeated countless times in print, and it has been picked up and repeated in popular magazines and documentaries. One day, it may be worth for someone to chart how this idea came to prominence and wide acceptance, despite having very little in its favor. You can read the final death cries of its supporters in a recent Nature special issue.

The idea can be expressed as follows:

"A very small group of humans left Africa 60 thousand years ago; they were on the brink of extinction, but they managed to follow the coastline of Asia, eventually reaching Australia, and subsequently expanding to colonize the interior of the Eurasian landmass"

I have recently become aware of several new papers that directly challenge this view. Instead of writing a new post about each of them, I've decided to combine them all in one post.

The first one by Delagnes et al. in the Journal of Human Evolution describes Shi’bat Dihya 1 and 2, a couple of ~55,000-year old inland human settlements in the Wadi Surdud basin (Yemen). 


From the paper:

Our fieldwork at the Wadi Surdud in Yemen demonstrates that during the period of the supposed expansion of modern humans out of Africa ca. 60-50 ka, and their rapid dispersal toward southeastern Asia along the western and southern Arabian coastlines, the interior of this region was, in fact, occupied by well-adapted human groups who developed their own local technological tradition, deeply rooted in the Middle Paleolithic. Future research will likely reveal whether the archaeological assemblages recovered from the Wadi Surdud can be associated with the descendents of anatomically modern human groups who occupied the Arabian Peninsula during MIS 5 or the southernmost expansion of the Neanderthals.

There is a paucity of old human remains from Arabia due to local conditions, so we cannot know for sure what morphology was associated with these archaeological cultures. If we take the finds that we do have at face value, we would conclude that the Neandertal range did not reach much to the south of Israel, since (i) no Neandertal remains have been found there, while (ii) modern human remains have.

This is further reinforced by the discovery of the Nubian Complex in southern Arabia with its clear African links; it is extremely difficult to assign that to Neandertals, because that would strongly imply presence of Neandertals in Africa for which there is also no evidence.

But, I think we can infer at least three things:

• People thrived inland during the supposed Out of Africa; if there were any Out-of-Africans around this time did not have to "follow the coast".
• It is becoming increasingly difficult to discount the pre-100ka Mt. Carmel (Qafzeh/Skhul) modern humans as the Out-of-Africa that failed: together with Jebel Faya, and the Nubian Complex we are now getting a picture of thriving populations in the Near East and Arabia prior to 60ka. There is every reason to believe that pre-100ka modern humans in the Levant and probably elsewhere did not disappear to make way for the hypothesized second 60ka Out-of-Africa wave.
• If people without a modern technological package were living in the Wadi Surdud, the idea that they are descended from recent Out-of-Africans with all the technological bells and whistles of modernity disappears: these are people rooted in the local traditions, without any signs of recent African descent.

The second article is a review of the prehistory of the Arabian peninsula by Groucutt and Petraglia in Evolutionary Anthropology. One of the points made by the author, quite often forgotten in recent attempts to discover patterns of human dispersals from modern populations, is that populations don't stay put for tens of thousands of years:

As genetic studies of Arabian populations have increased in scale, they reveal a complex pattern.22–25 Such studies show that modern Arabian populations are mostly derived from Western Asia, reflecting dispersals since the Last Glacial Maximum (LGM). In some areas, however, there are relatively high levels of ‘‘African’’ lineages, which have generally been attributed to historical processes such as slavery.26,27 Genetic evidence, then, is poorly placed to elucidate the position of Arabia in the dispersal of hominin populations. Likewise, the extinction of regional populations means they will not be represented in contemporary genetic structure. In this situation, archeology, in the context of paleoenvironmental fluctuation, offers a key way to elucidate the dispersal of hominin populations into Arabia and their subsequent evolutionary and cultural trajectories.

In Europe where we have the best ancient DNA record, there is substantial evidence for major genetic change over the span of a few thousand years. We have evidence of substantial gene flow into East Africa over the last few thousand years. And, of course, in Africa itself we have substantial evidence for gene flow whether it is related to Bantu farmers or Nilo-Saharan pastoralists.

The authors' overall conclusion:

In general, indications of possible population connections to surrounding regions remain rather speculative, and this is compounded by the absence of fossil evidence. We suggest, however, that the emerging picture suggests a general lack of connections between Africa and Arabia after MIS 5, the last interglacial. Instead, there are perhaps indications that Arabia sometimes saw connections to the Levant.

The third paper is a book chapter by Petraglia, Groucutt, and Blinkhorn. There is plenty of interest here, but what caught my attention is this map of paleorivers in the Arabian and Thar deserts. We so often think of deserts as eternal entitities, and looking at the present-day landscape one would of course conclude that migrating humans would be stark mad to venture far from the coast.

An interesting bit from the paper:

Upper Palaeolithic stone tool industries (typified by the production of long,thin ‘blades’ as well as, often, evidence of ‘art’ and symbolism), appear in the Levant at about 47–45 ka but seem to be rare to absent in Arabia. Maher (2009) comprehensively discusses potentially later Pleistocene sites in Arabia, most of which she identifies as being located in the north-west and south-west of the peninsula. The Qaryat al-Faw site is a possible Late Pleistocene locality, containing laminar technology reminiscent of Levantine Upper and Epi-Palaeolithic assemblages (Edens 2001). The overall lack of Upper Palaeolithic technologies either suggests that human populations were not presentin MIS 3, or that Middle Palaeolithic technologies were used until more sophisticatedtechnologies were developed in the Holocene (Crassardet al.2006, Crassard 2007). Ineither case, the nature of environmental conditions is a critical factor in determining the character of human occupations and material culture. 

and:

Upper Palaeolithic industries are present in the Thar but the ages of many of these sites remain unknown. An assemblage dated to ca. 26 ka has been identified as UpperPalaeolithic at 16R Dune, although the sample size is too low for easy technological comparisons to be made (Achyuthanet al.2007). It can be reasonably hypothesised thatmany Upper Palaeolithic assemblages, containing blade and microblade technologies,date to ca. 35 ka and after (Petraglia et al.2009). Microblade innovations have beentied to more efficient hunting strategies in the face of environmental deterioration andpopulation increase at ca. 35 ka (Petraglia et al.2009). 

There is a widespread belief in the simultaneous spread of Upper Paleolithic technology and modern human populations in Eurasia. But, this is not what the actual evidence shows us. However, the evidence for an earlier appearance of UP industries in the Levant than either Arabia or India makes this hypothesis problematic. It renders the coastal migration hypothesis doubly so, because if modern humans with new technology followed the coast all the way to Australia then it is troublesome that the MP stone tools persist in the regions most adjacent to this migration route. A better explanation may be that the UP revolution should be decoupled from the migration paths of modern humans, and that it was a cultural phenomenon which originated probably in the Levant, rather than being part and parcel of the early migrating humans wherever they were found.

The final article by Dennell and Petraglia argues that the modern humans appear in South Asia long before the conventional 60-40ky time frame. A quote from the paper is useful to remember, and echoes the above discussion:

In the Levant, both Neanderthals and H.sapiens used Mousterian stone-tool assemblages, and in East and North Africa, early H. sapiens used the same lithic tool-kits as their predecessors. This indicates that stone tools are a poor indicator of the species of the hominin that made them: changes in hominin type did not necessarily result in changes in lithic technology. Conversely, unchanging lithic traditions need not imply that the type of hom-inin that used them remained the same. As example, in mainland and island Southeast Asia, there is no equivalent of the Upper Palaeolithic, and“Mode 1”, technologically-simpleflake and core assemblages persist into the late Pleistocene and even inplaces the Holocene, even though the species of hominin that made them changed from H. erectus to H. sapiens. This is in sharp contrast to a region such as western Europe, where the shift from the Middle to Upper Palaeolithic was associated with the replacement of Neanderthals by H. sapiens, leaving aside a much-contested debate over the status of so-called “transitional industries” that could have been made by either. Changes in lithic technology in southern Asia are likely to have been more subtle, and discernible only through more rigorous quantitative analysis than merely noting the size andfrequency of a few common elements

I would say that "Mode 4-5 => Modern human" is quite probably true, but "Mode 1-3 => Not modern human" is not. In a sense, this is a little sad, because the more advanced modes appear much later than the hypothesized appearance of modern humans in most places of the world, so they are not of actually much use in determining when they arrived. The paleoanthropological record is a better guide, but, it is also quite sparse or hotly contested in many areas of Eurasia.

To conclude a long post: the evidence for 60,000 year old Out-of-Africa is weak:

• No specific ties with Africa at this or later times
• Clearest evidence of ties between Arabia and Africa during MIS5
• Evidence for strong populations in Asia at this time, inconsistent with the failure of the modern humans sampled at Mt. Carmel in the Levant prior to 100,000 years
• Presence of primitive stone tools does not necessarily indicate absence of modern humans
• Genetic studies of modern populations should not be used as if these populations are direct descendants of first modern humans in same locales

All in all, I'd say that my "two deserts" theory whereby a Green Sahara pumped early modern humans to Asia prior to 100,000 years ago and then a deteriorating Arabian desert pumped them out post-70,000 years ago is not obviously wrong. Perhaps the pre-100ky wave went much further to the east, to India and Southeast Asia.

But, early modern humans may not have been as dominant as we often think, irrespective of when they dispersed: evidence of archaic humans persists in parts of the world down to the Holocene, and so do the simple stone tools of the Middle Paleolithic. Plus the fact that 2/2 of archaic hominins sampled so far show differential relationships to modern human groups, some of them quite unexpected (Denisova with Melanesians) ought to clue us in to the very real possibility that our distant past was quite more complex than we could ever imagine.

Journal of Human Evolution http://dx.doi.org/10.1016/j.jhevol.2012.03.008

Inland human settlement in southern Arabia 55,000 years ago. New evidence from the Wadi Surdud Middle Paleolithic site complex, western Yemen

Anne Delagnes et al.

The recovery at Shi’bat Dihya 1 (SD1) of a dense Middle Paleolithic human occupation dated to 55 ka BP sheds new light on the role of the Arabian Peninsula at the time of the alleged expansion of modern humans out of Africa. SD1 is part of a complex of Middle Paleolithic sites cut by the Wadi Surdud and interstratified within an alluvial sedimentary basin in the foothills that connect the Yemeni highlands with the Tihama coastal plain. A number of environmental proxies indicate arid conditions throughout a sequence that extends between 63 and 42 ka BP. The lithic industry is geared toward the production of a variety of end products: blades, pointed blades, pointed flakes and Levallois-like flakes with long unmodified cutting edges, made from locally available rhyolite. The occasional exploitation of other local raw materials, that fulfill distinct complementary needs, highlights the multi-functional nature of the occupation. The slightly younger Shi’bat Dihya 2 (SD2) site is characterized by a less elaborate production of flakes, together with some elements (blades and pointed flakes) similar to those found at SD1, and may indicate a cultural continuity between the two sites. The technological behaviors of the SD1 toolmakers present similarities with those documented from a number of nearly contemporaneous assemblages from southern Arabia, the Levant, the Horn of Africa and North Africa. However, they do not directly conform to any of the techno-complexes typical of the late Middle Paleolithic or late Middle Stone Age from these regions. This period would have witnessed the development of local Middle Paleolithic traditions in the Arabian Peninsula, which suggests more complex settlement dynamics and possible population interactions than commonly inferred by the current models of modern human expansion out of Africa.

Link

Evolutionary Anthropology DOI: 10.1002/evan.21308

The prehistory of the Arabian peninsula: Deserts, dispersals, and demography

Huw S. Groucutt, Michael D. Petraglia

As a geographic connection between Africa and the rest of Eurasia, the Arabian Peninsula occupies a central position in elucidating hominin evolution and dispersals. Arabia has been characterized by extreme environmental fluctuation in the Quaternary, with profound evolutionary and demographic consequences. Despite the importance of the region, Arabia remains understudied. Recent years, however, have seen major developments in environmental studies and archeology, revealing that the region contains important records that should play a significant role in future paleoanthropological narratives.1–3 The emerging picture of Arabia suggests that numerous dispersals of hominin populations into the region occurred. Populations subsequently followed autochthonous trajectories, creating a distinctive regional archeological record. Debates continue on the respective roles of regional hominin extinctions and population continuity, with the latter suggesting adaptation to arid conditions.

Link

HOMININ EVOLUTIONARY HISTORY IN THE ARABIAN DESERT AND THE THAR DESERT

Michael D. Petraglia, Huw Groucutt and James Blinkhorn

Link

Quaternary Science Reviews Volume 47, 30 July 2012, Pages 15–22

The dispersal of Homo sapiens across southern Asia: how early, how often, how complex?

Robin Dennell, Michael D. Petraglia


The timing and the paths of colonization of southern Asia by Homo sapiens are poorly known, though many population geneticists, paleoanthropologists, and archaeologists have contended that this process began with dispersal from East Africa, and occurred between 60,000 and 40,000 years ago. However, the evidence for this scenario is very weak, particularly the lack of human skeletal evidence between the Levant and Borneo before 40 ka, and other explanations are possible. Here we argue that environmental and archaeological information is increasingly indicating the likelihood that H. sapiens exited Africa much earlier than commonly thought, and may have colonized much of southern Asia well before 60,000 years ago. Additionally, we cannot exclude the possibility that several dispersal events occurred, from both North and East Africa, nor the likelihood that early populations of H. sapiens in southern Asia interbred with indigenous populations of Neanderthals, Denisovans and Homoerectus. The population history of southern Asia during the Upper Pleistocene is likely far more complex than currently envisaged.


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