archaeology

Simon Armitage and colleagues [1] 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 [2], 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.

References

UPDATE (2011-09-06) Note: The conclusions of the research were later critiqued, I posted on that criticism after this post.

Shannon McPherron, Zeresenay Alemseged and colleagues working at the Dikika field site in Ethiopia have found evidence of stone tool use 3.39 million years ago [1]. That's 800,000 years earlier than the previous first-known tool use, and occurs during the existence of Australopithecus afarensis.

The evidence is a series of cutmarks and one percussion mark on two bovid bones. One is a piece of rib from a large "cow-sized" animal, the other a femur fragment from a smaller "goat-sized" bovid. The analysis goes through several microscopic comparisons to rule out alternative causes for the cutmarks, such as trampling. The key paragraph of the results:

The cut marks demonstrate hominin use of sharp-edged stone to remove flesh from the femur and rib. The location and density of the marks on the femur indicate that flesh was rather widely spread on the surface, although it is possible that there could have been isolated patches of flesh. The percussion marks on the femur demonstrate hominin use of a blunt stone to strike the bone, probably to gain access to the marrow. The external surfaces of ribs have thin sheaths of flesh, so the scraping marks on the fossil rib suggest stripping off of these sheaths.

I have some lingering doubts, none of which are very serious, but that point out the need to look harder at other sites. It sure would have been nice if they'd found an anomalous sharp-edged rock nearby.

The two bones are compelling, but the study does not give much indication of how representative they are. How many similar-sized bone fragments were left at the site? How many were collected? What fraction of "cutmarked" bones does that make? What fraction show signs of trampling and various kinds of post-depositional damage?

Those questions are essential to answer the "green car" problem. If you don't know this one, it's fairly simple -- a witness reports a green car leaving the scene, and green cars are very rare -- the police think this is a great lead. But blue cars are very common in the city, and there is a small chance that the witness mistook a blue one for a green one. Whether it actually was a green car depends on the actual proportion of green to blue cars, and the actual probability that the witness was wrong.

In this case, I think there is a very small chance that the marks on these bones could have been produced by processes other than deliberate cutting by a stone tool. But in a sample of hundreds or thousands of bone fragments, a small chance might well happen a couple of times. It's very difficult to quantify this, because archaeologists don't collect every bone fragment. The only real way to address the problem is to find more cutmarks and do other statistics on them -- do they occur where flesh is attached to bone, etc.

It does seem odd that nobody's identified clear stone tools, which are in later sites a lot more common than cutmarked bones. A tool-user will make many artifacts during her life. (Why "her"? Well, in chimpanzees, it's the females who dominate technology transmission...) We have a lot of australopithecine bones. If this was a long-lasting tradition, we should have found a lot of stone tools by now.

Maybe it wasn't a long-lasting tradition. Chimpanzee technology is significantly clustered geographically, some of the most interesting behaviors have been observed only at a single field site. If Australopithecus had a similar pattern of cultural diversity, tool use may have been innovated many times without "catching on" over a wide geographic or temporal extent. Here's what McPherron and colleagues conclude along similar lines:

Whether A. afarensis also produced stone tools remains to be demonstrated, but the DIK-55 finds may fit with the view that stone tool production pre-dates the earliest known archaeological sites and was initially of low intensity (one-to-a-few flakes removed per nodule) and distributed in extremely low density scatters across the landscape such that its archaeological visibility is quite low (16).

Or maybe we just haven't noticed. Fluvial contexts may have been bad places for Australopithecus to hang out. McPherron and colleagues allude to this explanation for the local absence of tools at Dikika:

However, stone tool production and consequently archaeological accumulations are not expected at this locality given the sedimentary environment characterized by the palaeo-Awash River emptying into a nearby lake (3, 4). In this relatively low-energy depositional environment, clasts suitable for stone tool production are not present (few particles larger than fine gravel, 8 mm diameter). Within the exposed SH Member, the distance from DIK-55 to cobble-sized raw materials (>64 mm) is ~6 km (at Gorgore; Fig. 1). Thus, in this instance the absence of evidence for stone tool production in the immediate vicinity of the cut-marked bones may reflect landscape-level raw material constraints.

The research article is accompanied by an essay by David Braun reviewing the find [2]. He stretches a bit, but I think the interpretations he suggests are worth airing. One -- why are there cutmarked bones 6 km from any good source of stone raw material?

The meat and marrow of large animals must have been a valued resource, because McPherron et al. conclude that the tool users incurred the cost of transporting stones 6 kilometres from where they occurred naturally to the site where the butchery took place. Further costs that were associated with the consumption of carrion, and were apparently worth the risk, include exposure to parasites and competition with large carnivores.

Two -- what about the "meat-brain" connection?

This provides exciting evidence of how A. afarensis behaved. At one time, the species was considered to be a relatively primitive hominin, but this perception is being redefined. For example, it now seems that Lucy's kin had body proportions that were more similar to those of humans than of apes (6). Analyses of the hand of A. afarensis show that it had relatively short fingers that would allow the kind of fine-scale manipulation necessary for tool use (7). A recently discovered skeleton from the Woranso–Mille area of Ethiopia suggests that A. afarensis did not have the ape-like, 'funnel-shaped' thorax usually associated with a large digestive tract and low-quality diet (8). Perhaps the findings that these hominins used tools and had a carnivorous component to their diet should not have been so unexpected.

A 2.6-million-year-old butchery tradition should already have refuted the hypothesis that meat-eating caused the expansion of brain size in Homo. But it was still possible to maintain that the initial Oldowan was insufficiently dedicated, or that the anatomical specializations (e.g., small guts) allowing brain expansion took time to develop, or that as-yet-undiscovered large-brained hominins would be found. Any of these are still possible, but the observations Braun points out pretty much demolish the 15-year-old story of "expensive tissue." Australopithecus seems to have had a small gut, and a bigger brain than chimpanzees. If there was a tradeoff, A. afarensis had already made it.

Braun didn't mention A. sediba, which adds another wrinkle. A late species of Australopithecus with human-sized teeth. Or (as some prefer), a pre-habilis species of Homo with an Australopithecus-sized brain. What was its diet like? I have a feeling we'll know before too long.

Meanwhile, I'll be floating for the rest of the year, since I included this as the far-out "bonus" entry in my 2010 New Year predictions! You know, the one that's so bizarre that it seems like it'll never happen. Heh.

UPDATE (2010-08-11): John Noble Wilford got ahold of some skeptics for his NY Times story on the discovery:

Still, the discoverers are already being pressed to defend their interpretation that the cut marks on the bones are evidence of stone-tool butchery. Tim D. White of the University of California, Berkeley, one of the foremost investigators of early human origins, said flatly that their “claims greatly outstrip the evidence,” and noted, “We have been working sites in this area for 40 years, and not a single stone tool has been found in deposits of this antiquity.”

Sileshi Semaw, a paleoanthropologist at Indiana University who was a discoverer of the oldest confirmed stone tools, from 2.6 million years ago, noted in an e-mail message from Ethiopia that researchers had often been misled by bone markings left by trampling animals and other natural causes. “I am not convinced of the new discovery,” he said.

UPDATE (2010-08-12): Maybe some are looking for more about australopithecine diets. My post from 2005, "Chemistry and early hominid diets" has a good compilation of stable isotope observations and what may explain them. With the cutmark evidence, you can read through the discussion of C₄ plant contributions, and think about the grazers that A. africanus may have been eating.

UPDATE (2010-08-16): Science Friday with Ira Flatow covered this story last week, including commentary by Alemseged and David DeGusta, who suggests that the marks may be crocodile bite marks. Doesn't look like it to me, but as I wrote above, I'd like to see statistics on a few hundred damaged bones to see the probability that an arbitrary one will look like stone cutmarks.

References