Out-of-Africa in the Mid-Pleistocene: A New Interdisciplinary Paradigm or a New Myth?
In the comments section on this blog, Dienekes raises the issue of interdisciplinary support for the out-of-America theory. Since I’m a big proponent of interdisciplinarity, the seeming convergence of genetics, archeology and paleobiology on the origin of modern humans in Africa, and not in America, may look like my theories are falsified by my own convictions. Dienekes’s query connected in my mind with a common reaction to the out-of-America model that usually goes like this: in order for out-of-America to make sense, we first need to make sure there’s something wrong with the current theories for the origins of modern humans in the Old World. I think it’s worthwhile to review what kind of consensus between genetics, archaeology and paleobiology we have on the out-of-Africa theory. The time is opportune, as a few recent papers go to the heart of these issues.
Over the past two years, the science of human origins has been struggling to achieve a new out-of-Africa synthesis. Geneticists have presented evidence that modern humans interbred with archaic hominins inside and outside (Neandertals, Denisovans) of Africa. Evidence of Neandertal admixture is found across all continental human populations outside of Africa but not in Africa suggesting to out-of-Africa theorists that this event took place after modern humans had departed from Africa but before they split into Western and Eastern Eurasian groups. This indicates that there must have been only one founding migration out of Africa.
The concentration of a bulk of basal and deep routing mtDNA and Y-DNA lineages in East Asia, the Sahul and East South Asia and the presence of derived mtDNA U lineages in Upper Paleolithic Europe (Kostenki, 30,000, hg U2) suggests that if there was a single founding migration out of Africa, it would proceed down a southern route via South Asia into East Asia and the Sahul with the subsequent return of some derived lineages to Europe (mtDNA hg U), North and East Africa (mtDNA hgs U6, M1). The molecular clock has generated over and over again the 60,000-40,000 year old estimates for the founding migration out-of-Africa via Arabia and India. Paleobiology seems to concur with this timeframe, as there’s no clear record of modern human presence in Europe, India, East Asia, Southeast Asia, Australia, or Papua New Guinea before 40-50,000 YA.
However, archaeologists rallied to oppose these dates and instead proposed an earlier, Mid-Pleistocene (roughly 120,000 YBP) exodus from Africa. For instance, Anne Delagnes et al. (“Inland Human Settlement in Southern Arabia 55,000 years ago. New Evidence from the Wadi Surdud Middle Paleolithic Site Complex, Western Yemen,” Journal of Human Evolution xxx (2012), 20-21) writes about the challenges related to the attribution of the Wadi Surdud technocomplex to Neandertals or modern humans:
“As no sound technological markers exist for distinguishing Neanderthals from early modern humans (Zilhão, 2001, 2010; d’Errico, 2003; Hovers, 2009a; Shea, 2010), neither of these alternatives can be ruled out. What is certain is that the people who settled in Wadi Surdud did not belong to migrating groups who left Africa around 60 ka and introduced new modern behaviors into Arabia, as they were still firmly embedded in a Middle Paleolithic tradition.”
“…During the period of the supposed expansion of modern humans out of Africa ca. 60-50 ka, and their rapid dispersal toward south-eastern 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.”
Jeffery Rose et al. (“The Nubian Complex of Dhofar, Oman: An African Middle Stone Age Industry in Southern Arabia,” PLoS ONE 6 (11), 2011) observe that the only example of technological continuity between Africa and the adjacent area of southern Arabia, namely the late Nubian Complex, is dated at 128,000-74,000 YBP, while “coalescence ages for non-African mtDNA lineages range from 70 to 45 ka, depending on the use of different mutation rates, calibration methods, and statistical models, placing these mtDNA studies at odds with the archaeological picture beginning to emerge from Arabia.”
Groucutt & Petraglia (“The Prehistory of the Arabian Peninsula: Deserts, Dispersals and Demography,” Evolutionary Anthropology 21, 3, 2012) are confident that there is “a general lack of connections between Africa and Arabia after MIS 5 [84,000 YBP. – G.D.], the last interglacial,” that evidence for Upper Paleolithic blade and microlith technologies in Arabia is “ephemeral” and that “the Arabian record seems to lack specific features relating to lithic technology and other features of the African Late Stone Age.” In plain language, it means that out-of-Africa, as formulated by geneticists, does not receive support from the archaeology of Arabia.
In addition to being at odds with each other, archaeological, paleobiological and genetic evidence is crippled by multiple uncertainties, multiple interpretative possibilities, mismatches and missing pieces.
1. The earlier, Mid-Pleistocene candidates for an expansion out-of-Africa such as the late Nubian Complex lack associated human remains, so it’s impossible to say which hominin species created them.
2. There is no ancient DNA from Africa or Near East and because of poor preservation conditions it’s unlikely that it will ever be retrieved. Hence, it’s impossible to map modern DNA studies onto the Mid-Pleistocene populations in Africa and the Near East.
3. In the Levant, Mousterian technologies are found in association with the remains of both anatomically modern humans and Neandertals (Dennell, Robin, & Michael Petraglia, “The Dispersal of Homo sapiens across Southern Asia: How Early, How Often, How Complex?” Quaternary Science Reviews 47, 16). This could indicate reverse acculturation of modern humans by Neandertals corresponding by the hypothetical genetic admixture between the two hominin species but, throughout Asia, lithic technologies remained unchanged (simple flakes and cores) from the times of Homo erectus all the way into the Late Pleistocene and even occasionally into the Holocene. Similarly, in East and North Africa, early H. sapiens used the same lithic toolkits as their predecessors. The conclusion one must draw, therefore, is that stone tools in general are not diagnostic of hominin species and can shift up and down on the putative evolutionary scale in response to local environmental pressures.
4. Signs of advanced lithic technologies and “symbolic” behavior are observed in both modern human and Neandertal contexts. Neandertal technologies were apparently evolving and modernizing along their own developmental path toward a “blade stage.” This seems to be the case with the Lincombian-Ranisian-Jerzmanowician technocomplex in northern Europe (from Wales to Poland) dated at 38,000 BP (43,000 cal BP) where a switch to “laminar blanks” marked the in-situ transition from Mousterian to Upper Paleolithic technologies (Flas, Damien. “The Middle to Upper Paleolithic Transition in Northern Europe: the Lincombian-Ranisian-Jerzmanowician and the Issue of Acculturation of the Last Neanderthals,” World Archaeology 43 (4), 2011: 605-627). While the Neandertal baldes tend to be lighter and more curved than the Aurignacian ones, it’s increasingly hard to define, in archaeological terms, what constitutes a modern human behavioral package. As Flas (p. 616) writes, “The LRJ…shows both an ‘Upper Paleolithic’ technology and typology and similarities with the Late Middle Paleolithic industries of the regions where it evolved.” Technological convergence and environmental adaptation, rather than tradition and acculturation, seem to be better drivers of type in Paleolithic lithic technologies, hence their use as a source of information about population movements is questionable. Even the famous Upper Paleolithic art is now being re-dated (Pike, A. W. G. et al. (2012). “U-Series Dating of Paleolithic Art in 11 Caves in Spain,” Science 336 (6086): 1409-1413) to the time dangerously close to the period of Neandertals’ supreme reign over Europe. It’s hard to believe that Neandertals were the ones who were painting the caves, and the obtained dates technically fall narrowly within the time frame of modern human emergence in Europe. But these dates are “minimum ages.” Since it’s also claimed that Aurignacian may not be the earliest technocomplex attributable to modern humans (Ulluzian was found to be associated with the teeth of a modern human infant) and that European Paleolithic art may have been an invention that occurred in-situ among just one group of modern humans originally occupying Swabian Jura and nowhere else in Europe (Higham, Thomas, et al. (2012) “Τesting Models for the Beginnings of the Aurignacian and the Advent of Figurative Art and Music: The Radiocarbon Chronology of Geißenklösterle,” Journal of Human Evolution 62 (6): 664-676), it’s just as likely that this breakthrough occurred among a geographically restricted band of Neandertals.
5. Biology and technology don’t seem to map well onto each other. As Flas (p. 619) writes, again with respect to Lincombian-Ranisian-Jerzmanowician, “This shift from Mousterian technology to the LRJ is a further example of the absence of a link between an ‘Upper Paleolithic’ lithic industry and the dispersal of AMH, giving credence to the idea of a separation between the cultural and the biological issues involved in the ‘Middle to Upper Paleolithic transition’.” It’s, therefore, premature to postulate the migration of anatomically modern humans from the Near East to Europe on the basis on the technological similarities between Boker-Tachtit (layers 1-2) at 47,000 BP and the Bohunician complex in Moravia (Eastern Europe) at 43,000 BP. In the absence of associated human remains in either Negev or Moravia and with no intermediary cultures in between, it’s just as likely that the shared traits between Bohunician and Boker-Tachtit were product of cultural diffusion or independent invention from separate Mid-Pleistocene antecedents.
6. It’s a well-known fact in population genetics that gene trees and population trees don’t necessarily coincide. Now, Dennell & Petraglia (2012, p. 17) add another complexity to the historical interpretation of molecular data. They observe that a population’s genetic history can be at odds with its residential history. They use Andamanese as an example: Andamanese have been portrayed by geneticists as relics of the founding coastal migration out of Africa. But, at least according to archaeology, the residential history in the Andaman islands is very shallow. The islands have been occupied for just 2,000 years, and there’s no evidence to date suggesting Pleistocene dates for human presence in the Andamans. According to Dennell & Petraglia,
“This might indicate that either the Pleistocene (and early Holocene) occupation of the islands has yet to be detected, or that the modern islanders were until recently living on the mainland, and not even necessarily on the coast for much of their history. ”
Dennell & Petraglia overlooked two recent mtDNA studies of the Andamanese (Barik et al. (2008) “Detailed mtDNA genotypes permit a reassessment of the settlement and population structure of the Andaman Islands,” American Journal of Physical Anthropology 136 (1): 119-127; Wang et al. (2011). “Mitochondrial DNA Evidence Supports Northeast Indian Origin of the Aboriginal Andamanese in the Late Paleolithic,” Journal of Genetics and Genomics 38 (3): 117-122) that, too, argued for a more recent settlement on the Andaman islands and not from a coastal area. These studies confirm their suspicion that genetic studies tend to generate unreliable dates for population movements derived from the confusion between genetic and residential histories of populations, but it also shows that the situation is bound to improve.
7. A population of “anatomically modern humans” (Skhul and Qafzeh) expanded out of Africa into the Levant around 100,000 YA only to be replaced by Neandertals at 65,000 YA. There’s nothing in the archaeologically visible facets of anatomically modern human behavior that foreshadows the pattern of adaptation increasingly observed among the ancestors of living human populations in the Late Pleistocene and Holocene. Over the past 40,000 years (which is an instant compared to the duration of the Mid-Pleistocene), modern humans have managed to replace all pre-existing hominin species globally, expand to previously unknown frontiers and spearhead a series of cognitive, social and economic revolutions to reach the level of technological sophistication far superior to anything observed in the Mid-Pleistocene archaeological record. This pattern of expansion, innovation and growth sets us definitively apart from Homo erectus, Neandertals and the so-called “anatomically modern humans.” Behavioral difference being that vast, it’s difficult to maintain the notion that “anatomically modern humans” are the ancestors of living human populations.
8. There are hints of presence of anatomically modern humans in India, East Asia, Southeast Asia and the Sahul prior to 50,000 years. But the data is so fragmentary, unsystematic and the dates are so uncertain that evidence admits to multiple interpretations. For example, Dennell & Petraglia (2012, 19) discuss the Zhirendong mandible from South China in the following terms:
“More recently, a mandible from Zhirendong, South China, has been attributed to H. sapiens and dated by U-series dating to ca 125 ka (Liu et al., 2010). Interestingly, because the mandible shows a mixture of characteristics associated with both H. sapiens and H. erectus, it is interpreted as showing inter- breeding between the two species at an early date, rather than purely in situ evolution (An alternative interpretation is that it indicates a gracile late H. erectus; see Dennell, 2010).”
One might add that Zhirendong is an isolated find separated by tens of thousands of years from the more obvious Niah cranium from Borneo (43,000 YBP).
9. Archaeological and paleobiological finds are slow in coming (it took scientists decades to discover the Zhirendong mandible, which may potentially deepen the timing of modern human arrival to Asia by a whopping 60,000 years) and every bit of new evidence may lead to an utterly unexpected conclusion (without the ancient DNA from the Denisova pinkie we would have never know about a new hominin species).
It is unclear how geneticists, archaeologists and paleobiologists are planning to demonstrate the dispersal of anatomically and behaviorally modern humans from Africa either before or after 50,000 YBP without contradicting each other. If the skeletal data is so fragmentary, if the criteria used to differentiate modern human behavior from the behavior of earlier hominins are so fuzzy, if correspondences between species and technology are poor, if technological convergence is commonplace, if behavioral modernity is divorced from anatomical modernity, if there’s very little chance that DNA can survive in the latitudes of Africa, South India, Southeast Asia and the Sahul, if the molecular clock is questionable at the core, what remains is the generic observation that there are modern-looking skulls in Africa as early as 195,000 YBP and that African populations are most diverse and African lineages are most divergent among human groups. These observations come blatantly short of coalescing into a scientific theory but instead can spawn a myth of human origins, as we are missing such key pieces as 1) proof that anatomical modernity is possible without behavioral modernity (all living human groups are both anatomically and behaviorally modern); 2) proof that the divergent African lineages are not the result of archaic admixture in Africa and that genetic diversity indicates population age; 3) a clear archaeological definition of technological and behavioral modernity that will work across geographies and time periods and can contain an evolutionary rationale for the continuing success of modern human adaptations and the demise of all earlier hominins.
Without such a definition archaeology fails as a credible source for modern human prehistory. One can hardly agree with Dennell & Petraglia (2012, 20) that “early H. sapiens populations were emphatically not “modern” in the same senses as Holocene or Late Pleistocene ones (Klein, 2008), and the expansion of H. sapiens across southern Asia (and other regions) before 50 ka may have stemmed as much from ecological opportunism as from behavioural superiority over other hominin species.” It’s only because archaeology is intrinsically unable to cast light on such fundamentals of human behavior as language, cooperative breeding, kinship systems, folklore and music that we can’t see beyond ecological opportunism. Our essential difference from the earlier hominins may lie in our “software,” in the way we think, talk, raise children, tell stories and sing songs and not in lithic hardware.