French Connection to China Syndrome, dentally

I've read through the new paper by Martinón-Torres et al., on Eurasian continuity in the Middle Pleistocene. They've put out an interesting hypothesis, with some support from previous work, but ultimately I think their methods are too weak to test it.

The press coverage of the paper so far (e.g., this AP article) has been a little confusing, because it misses this point: this paper is not about modern human origins, it's about much earlier evolutionary relationships. National Geographic News resorts to the always-safe:

The finding suggests that the hominid family tree could be much more complex than previously thought.

Ah, so that's what it means! More complex than previously thought! Why isn't there ever a story that makes things simpler than previously thought? I mean, isn't it a sign of a failed science if you have to add complexity to your hypothesis every time you make a new observation? It's like Ptolemaic paleoanthropology!

Anyway, enough of that rant. Let's look at what the paper really says, which is much more interesting than the press! Here's the abstract:

A common assumption in the evolutionary scenario of the first Eurasian hominin populations is that they all had an African origin. This assumption also seems to apply for the Early and Middle Pleistocene populations, whose presence in Europe has been largely explained by a discontinuous flow of African emigrant waves. Only recently, some voices have speculated about the possibility of Asia being a center of speciation. However, no hard evidence has been presented to support this hypothesis. We present evidence from the most complete and up-to-date analysis of the hominin permanent dentition from Africa and Eurasia. The results show important morphological differences between the hominins found in both continents during the Pleistocene, suggesting that their evolutionary courses were relatively independent. We propose that the genetic impact of Asia in the colonization of Europe during the Early and Middle Pleistocene was stronger than that of Africa.

OK, so this is about the initial colonization of Europe and the subsequent evolutionary trends in Europe, Asia, and Africa. The observation is that European teeth show a continued similarity to Asians during the Middle Pleistocene, and there is no evidence that European teeth evolved in the direction of Africans during that time period.

Why is that interesting? Two reasons:

1. The hypothesis directly conflicts with the idea that Middle Pleistocene Europeans were linked to Africans. A large number of anthropologists have been pushing the European-African link, under the old hypothesis that these ancient people belonged to a species that was distinct from East Asians. The European-African clade in this hypothesis is often called Homo heidelbergensis; the Asian clade remains Homo erectus.

2. The hypothesis also seems to conflict with genetic data, which suggest that the relationship of European and African hominids is more recent than the early Middle Pleistocene. In particular, the genetic divergence time between human and Neandertal genomes appears to date to more recently than 700,000 years ago (Green et al. 2006, Noonan et al. 2006), which means that the population divergence must be still more recent. Also, Alan Templeton's papers (e.g., 2002, 2006) claim evidence for migrations from Africa into Europe and Asia during the Middle Pleistocene. Those claims are consistent with the Neandertal genome data, as far as we know it, and they suggest gene flow from Africa into Eurasia.

So, the authors ought to deal with these issues. They do so in their discussion, which in this short paper is one long paragraph. I'm quoting it here in full to comment on the details:

If the population of the Eurasian continent during the Early and
Middle Pleistocene was mainly the result of several out-of-Africa incursions, we should have found African influences in the morphology of the Eurasian populations. However, the continuity of the "Eurasian dental pattern" from the Early Pleistocene until the appearance of the Upper Pleistocene Neanderthals suggests that the evolutionary courses of the Eurasian and the African continents were relatively independent for a long period and that the impact of Asia in the colonization of Europe was stronger than that of Africa.

That is the conclusion of the analysis, in brief. The strength of the conclusion depends on the power of the analytical methods to detect gene flow based on morphological similarities. More on that below.

This finding does not necessarily imply that there was not genetic flow between continents, but emphasizes that this interchange could have been both ways (25, 26).

This seems a little misleading. They have no particular evidence of gene flow from Eurasia to Africa (that would be the "both ways"). Nor do they have evidence in their analysis of gene flow from Africa to Eurasia, after the initial colonization. So they don't have any evidence for gene flow at all. So the finding doesn't emphasize anything about gene flow, other than that the teeth don't show obvious evidence for it.

Around 1 Ma, hominins appear to have dispersed into temperate latitudes as far north as 40 - 45° N (27-29), not only from Africa, but also within Eurasia (29 - 31). These populations were probably descendants of an ancient out-of-Africa exodus, rather than a later one at the end of the Early Pleistocene (30).

This is an important assertion. Other workers have emphasized the similarities of some African fossils to East Asian fossils (mainly from Java, plus Gongwangling in China) in the late Early Pleistocene. That has always been the case with OH 9, and it influenced the description of the Daka and Buia crania as well. The question is how early Asian populations became morphologically distinctive. Here, the authors argue that it was very early, without substantial signs for later interaction, which in the context of the cranial comparisons is now an extreme claim.

In addition, a recent study on the European Lower Pleistocene hominin populations has revealed a possible Eurasian origin for these groups (32).

This refers to the description of the ATD6-96 mandible, which contains an earlier assertion about Asian-European connections. I return to this below.

Furthermore, it has been pointed out that during the Middle Pleistocene there was hardly any faunal exchange bet ween East Africa and the Levant (33) and that the desert between the Sahara and Arabia was an important barrier at that time (26), therefore contributing to the isolation of both continents.

This is an important argument in support of their hypothesis. If movement between Africa and Eurasia was difficult during this time span, that reinforces their claim, and makes it less plausible that there were large-scale dispersals out of Africa during the Middle Pleistocene. That leaves us with a mention of a major exception to their proposed pattern: the evolution of humans in the Late Pleistocene:

With the exception of the SAP [i.e., H. sapiens] out-of Africa dispersion based mainly on genetic data (2), the history of human populations in Eurasia may not have been the result of a few high-impact replacement waves of dispersals from Africa, but a much more complex puzzle of dispersals and contacts among populations within and outside continents. In the light of these results, we propose that Asia has played an important role in the colonization of Europe, and that future studies on this issue are obliged to pay serious attention to the "unknown" continent (Martinón-Torres et al. 2007:3).

The citation of the ATD6-96 mandible leads us to a passage from that earlier paper (Carbonell et al. 2005), which also describes the hypothesis that the founding population of Europe was Asian. Remember that this research group calls the Gran Dolina sample, Homo antecessor, and they initially had written that this species probably colonized Europe from Africa in the late Lower Pleistocene. Here's the relevant paragraph from the cited paper (Carbonell et al. 2005):

The differences in dimensions and robustness between the TD6 mandibles and the East and North African mandibles cast doubt on the African origin of H. antecessor. In contrast, our comparative analysis suggests looking toward the Asian continent. In this respect, it is relevant to mention some data that remained unpublished in 1997, when the new species was named (10), and that are relevant to this discussion. The partial cranium Nanjing I, recovered in 1993-1994 from the Hulu Cave (Tangshan Hill, eastern central China), shows clear modern midfacial traits similar to those observed in the specimen ATD6-69 (19). Wang and Tobias (20) also found similarities between Nanjing I and the Zhoukoudian hominins. Geochronological dates, combined with ecological and paleoclimatic evidence, indicate that the Nanjing skull is ~600 thousand years old (21). Furthermore, the Locality 1 levels at Zhoukoudian, which yielded most hominin specimens, are now considered at least 800 thousand years old (22). Thus, these Chinese hominins may be contemporaneous with or slightly younger than the TD6 hominins. If the Gran Dolina and Chinese populations are phylogenetically related, they should share a common ancestor that also had a modern midfacial pattern and a gracile mandible. In the cranium, this hypothetical common ancestor would have had a low and flat temporal squama, and an unfused styloid process. These traits would have been retained in the Asian hominins but lost in the TD6 hominins, who exhibit a fused styloid process, a convex temporal squama, and probably a significant increase in cranial capacity (19). The Ceprano calvaria (Italy), which has been tentatively assigned to H. antecessor (23), exhibits a convex temporal squama and a cranial capacity of about 1,057 ml (24). Interestingly, TD6 and Zhoukoudian are the only hominins that have a zygomaxillary tubercle before the Upper Pleistocene (19).

So that provides cranial and mandibular evidence of an Asia-Europe connection, supporting the dental evidence provided in the current paper. Still, that evidence is from the initial founding of Europe in the Early Pleistocene and doesn't necessarily apply to the trends during the Middle Pleistocene.

After working through the data supplements for the paper, I think that the analysis is much weaker in statistical power than it could be. In their analysis, they disregard much of the variation within these ancient samples and focus on the differences between samples according to their scoring methods. This may reveal the broad relationships among samples -- if we disregard the possibility of selected parallelisms -- but it does not say anything about the possibility of gene flow among the samples.

Indeed, the result of their analysis (a dendrogram, or branching tree) is quite incapable of showing genetic exchanges at all. It can only show branching events, which means that the result will show either an exclusive relationship between Europeans and Asians, or an exclusive relationship between Europeans and Africans, but never a mixed relationship.

The only result in the paper that indicates a European-Asian relationship is from their cladistic analysis of a subset of the data. And it isn't especially strong evidence, since the Middle Pleistocene Africans are limited to the relatively early sites of Rabat and Tighenif (Ternifine). Granted, the later sample is also small in number, but this isn't really a test of relationships; it's more of a suggestion.

The phenogram inexplicably omits Middle and Lower Pleistocene Africans entirely, and considers only australopithecines and habilines as the African sample.

So, at the moment I consider this to be a very interesting hypothesis in search of a good test. There is no test of gene flow here, just an assertion. Yet, the cranial comparisons give the assertion some plausibility -- and remember, another idea out there is the hypothesis that early Homo originated in Asia and migrated to Africa later.

I think that these topics together constitute the important problem in early human relationships right now, so I'll be writing some more about them. There are many additional interesting facts to consider...

References:

Martinón-Torres M, Bermúdez de Castro JM, Gómez-Robles A, Arsuaga JL, Carbonell E, Lordkipanidze D, Manzi, G, Margvelashvili A. 2007. Dental evidence on the hominin dispersals during the Pleistocene. Proc Nat Acad Sci USA (early) doi:10.1073/pnas.0706152104

Stringer C. 2002. Modern human origins: progress and prospects. Phil Trans Roy Soc Lond B 357:563-579. doi:10.1098/rstb.2001.1057

Rightmire GP. 1998. Human evolution in the Middle Pleistocene: the role of Homo heidelbergensis. Evol Anthropol 6:218-227. doi:10.1002/(SICI)1520-6505(1998)6:6<218::AID-EVAN4>3.0.CO;2-6

Carbonell E and 19 others. 2005. An Early Pleistocene hominin mandible from Atapuerca-TD6, Spain. Proc Nat Acad Sci USA 102:5674-5678. doi:10.1073/pnas.0501841102

Bruner E, Manzi G. 2005. CT-based description and phyletic evaluation of the archaic human calvarium from Ceprano, Italy. Anat Rec A 285A:643-657. doi:10.1002/ar.a.20205