Jebel Faya and early-stage reduction

Simon Armitage and colleagues Armitage:2011 describe archaeological remains from Jebel Faya, in the United Arab Emirates. The assemblages come from a rock shelter in the mountain, which is around 100 km south of the Straits of Hormuz, entry to the Persian Gulf. Below Bronze Age and later remains, are three Paleolithic units. The oldest (assemblage C in the paper) is dated by OSL to the last interglacial, around 125,000 years ago. My comments here are more note-like than usual; this topic opens a window into some work we've been recently doing.

The authors' main conclusion is that the oldest assemblage displays technical similarities to East African archaeological assemblages, which are not present in the archaeology of the Levant either before or after this time. We have to dig into the supplementary material to the paper to get a good account of the technical similarities:

Technologically, this assemblage has general links to East Africa (S3 S4) while showing none of the technological traits characteristic in the contemporaneous Levantine Mousterian (S5). As in the early Middle Stone Age (MSA) of East Africa, Assemblage C exhibits three profoundly different reduction strategies: bifacial, volumetric blade, and radial Levallois. This combination is unknown in the Levant after about 200 ka, where there is no bifacial reduction and the Levallois method is largely limited to unidirectional converging. The latter produced large numbers of Levallois points, which are absent from Assemblage C.

For a layman's description of the result from coauthor Anthony Marks, I can recommend Katherine Harmon's account at Scientific American's website.

I like the observation, but I think we should be cautious about it. The basic idea is that African assemblages display three different strategies early in the reduction sequence, none of which are evident in Levantine assemblages of equivalent age.

Reduction sequences and conservatism

Yesterday I talked over this concept with my graduate student Marc Kissel. I find it very interesting that the authors focused on initial reduction stages as elements of technical similarity. They thereby assume much about the cultural transmission of the reduction sequence.

It seems reasonable that the initial steps of a reduction sequence -- from quarrying through early core shaping -- should be conservative. Early stages necessarily constrain the later steps toward finished tool production, so that a skilled toolmaker who wants to carry out the later stages of a reduction sequence has first to get the early steps right.

Paradoxically a naive learner may be ill-equipped to attend to the importance of these first steps, compared to later steps where the preform is more readily identifiable by its physical configuration. Within a social group, the early steps of reduction may well be carried out by other people, including less-skilled artificers. The best toolmaker may go to the quarry himself, but often he may call on someone less skilled to carry out the initial reduction, or may be forced to work with partially exhausted cores from earlier attempts.

I'm willing to hazard a guess that the social learning that enables tool manufacture would exert a bias toward low error rates early in the reduction sequence. We can consider a biological analogy -- early embryonic development is more strongly conserved across taxa (and phyla) than later development. Changing something early in a developmental sequence may make later events impossible. If I'm right, the argument by Armitage and colleagues should have some force -- finding that the early stages of the reduction sequence are shared among sites should be a better indicator of relationship than most archaeological indicators.

But Armitage and colleagues' conclusion has force just to the extent that we accept two proposals: (1) that we understand the technical variation in the Levant, and (2) that independent development of the early-stage reduction strategies in the Jebel Faya assemblage is unlikely.

These proposals hang together. The Levant is richly documented across the period before and after the last interglacial, moreso after OIS 6 (around 130,000 years ago) than before. These assemblages were directed toward convergent removal of Levallois points. I'm not immediately in a position to discuss the variation within these assemblages, but the question strikes me as crucial. Although the archaeological record from this area is relatively dense, like all places it samples only a small fraction of the actual groups that must have existed at the time -- to use a genetic comparison, the record has high coverage over a very small fraction of the regional behaviorome.

Was independent invention of these early-stage reduction strategies likely? The answer depends on whether a particular early-stage reduction strategy is merely rare in the large Levantine sample, or entirely absent. If such a strategy (in this case, foliate reduction) occurs at all, we can infer that its invention was possible, if not likely. With assemblage C at Jebel Faya, we are considering the cultural tradition represented by 500 artifacts. If we treated these as a random sample of the Levantine record, they are exceedingly unusual, no doubt. But random sampling across an entire record isn't the correct comparison; we want some equivalent sampling of the cultural information in terms of time and space.

The paper's conclusion that Jebel Faya represents an incursion of African-derived technical traditions into the Arabian peninsula depends on these assertions. I don't have strong feelings about them, but I think we should work to get a better statistical understanding about the issue. I am singularly unimpressed when archaeologists assert that one assemblage "resembles" another on purely typological grounds. Typological similarities may result from many constraints other than cultural information, and rare appearances actually carry a lot of information about them.

Out of Africa early

Now, what about this "southern route" business? I say it's a year behind the times. The entire reason for the "southern route" hypothesis was to explain how Africans could have left Africa 70,000 years ago without being stopped by Neandertals in the Levant. Sail them around the southern coast of Asia, and you can get them early into SE Asia and Australia without mixing with those darned Neandertals.

We obviously don't need to rule out Neandertal interbreeding anymore. We know it happened, most likely in West Asia. Putting Africans into the Levant during the last interglacial isn't a bug, it's a feature. We need contact between moderns and Neandertals in this area to explain the genetic data.

The dates may seem like more of a stumbling block. If we accept that a major out-of-Africa movement was underway by 70,000 years ago, we are going to have a hard time explaining why the Levant seems to have been entirely uninfluenced by it.

But a 70,000-year-long chronology, based on estimates of mtDNA haplogroup divergences, is already out of kilter with the majority of evidence. Nuclear DNA suggests a substantially longer timescale, which would derive non-African and sub-Saharan populations from common ancestors before 140,000 years ago. Depending on the amount of mixture among these populations and the mutation rate we adopt, these populations may have begun to differentiate very early in the Middle Stone Age.

It's hard to account for the diversity of people outside of Africa with a short migration timescale. People outside Africa are around 20 percent more inbred than sub-Saharan Africans, but they don't look like they underwent any sudden severe bottleneck. Even accounting for the mixture with archaic people like Neandertals and Denisovans, much of the variation of Middle Pleistocene humans (still present in Africa) just didn't get into non-Africans.

I would propose a movement of MSA Africans into West Asia before the last interglacial as a model that provides a good fit to these data. An early movement followed by long interactions in this limited area would explain so much of the population structure and morphological variation of MSA Africans wasn't represented in the people who peopled Eurasia. A substantial delay between the initial entrance into West Asia and the dispersal to Europe and the rest of Asia would explain why the later archaeological transitions in those regions have no sign of immediate technical or cultural links to the MSA. It would also explain why the initial "modern" humans outside Africa share few if any derived morphological features with Africans after 100,000 years ago.

The anatomy of the Skhul and Qafzeh samples suggests that an African incursion into the Near East did occur before 100,000 years ago. Many paleoanthropologists have supposed that this early incursion did not persist, even locally. The later Levantine sample includes individuals with more Neandertal resemblances, chiefly Amud and Kebara. But each of the later specimens shares several traits with early modern humans from Skhul or Qafzeh. Indeed there is no clear constellation of derived traits that sorts the Skhul-Qafzeh sample cleanly from Tabun 1 and the later Levantine specimens. I just don't think this skeletal record poses any problem for the idea of a long interaction of populations in this area -- especially if we extend the focus from the Levant into the Arabian peninsula and Persian Gulf region.

The strongest reason to suppose that an African incursion was extinguished is not the skeletal record but instead the mtDNA timescale. I can refer readers to the paper by Endicott and colleagues Endicott:2009, which discusses a range of mutation rate estimates and their effects on the origin of macrohaplogroups M and N, the key ancestral non-African lineages. Current estimates unanimously suggest that these clades originated within the last 75,000 years. By itself, this would suggest that the mtDNA common ancestors of non-Africans and sub-Saharan African populations diverged shortly before that time.

I keep coming back to this, because the mtDNA just seems so out of line with the autosomal and X-chromosome picture. I regard this as a serious sticking point and hesitate to just wave it away. As I suggested to Charles Choi, the resolution may involve a time of isolation outside Africa during which the ancestors of non-Africans lost heterozygosity (and became enriched for the later mtDNA clades M and N). Or maybe we just have the mtDNA clock wrong -- the large revisions of the Neandertal-human mtDNA divergence in the light of developing evidence don't inspire confidence about the timing of internal nodes to the human mtDNA tree.

The early archaeological assemblage from Jebel Faya strikes me as consistent with a model of early dispersal from Africa, but not especially good evidence for it. The outstanding question is whether the early reduction strategy is a behavioral trait that provides good evidence about biological relationships. I see the logic but think that it is tenuous.

The model obviously is relevant to the question of an early presence of African-derived modern humans in India. If we combine the presence of an African-derived population in eastern Arabia with the large exposed Persian Gulf region during the last interglacial, this begins to look like a large habitable region with easy land connections to the Indus River valley. But the Indian subcontinent would potentially have been home to a very large population of ancient humans. I doubt that an occupation across the large area of West Asia plus the Indian subcontinent would have enabled the substantial reduction of heterozygosity that we see in present-day non-Africans.