John Kappelman was kind enough to send me a preprint of the report on the new Turkish Middle Pleistocene specimen. The specimen consists of frontal and parietal fragments from a single skull, and comes from a travertine quarry outside the town of Kacabaş, western Turkey. The skull was found in the course of quarrying activities, and may be the first hominid specimen to have been "reduced to a standard rough-cut tile thickness of about 35 mm."

The paper reports a date estimate of 490 ± 0.05 to 510 ± 0.05 kyr, based on thermoluminescence of travertine. The paper contains some pictures of the specimen, description of its anatomy, and evidence for the presence of tuberculosis.

I review these elements below.

Is it Homo erectus?

At 500,000 years old, the specimen is just the right age to be in the middle of a taxonomic mess. Many agree that the roughly contemporary Zhoukoudian sample should be referred to H. erectus. But there are other alternatives. Some people contribute a number of penecontemporary samples to "Homo heidelbergensis", or alternatively to "archaic Homo sapiens". Among these, the Bodo skull is earlier at 600,000 years old, and arguably so is the Sima de los Huesos sample (Bischoff et al. 2003, 2007). So in terms of time, this specimen might qualify for any number of names.

Kappelman and colleagues include this paragraph, which I think is an admirable discussion of the problem:

The most conservative approach is to provisionally attribute the Kocabaş specimen to H. erectus but the combination of features in the Kocabas hominin highlights the ongoing controversy about whether Pleistocene hominins attributed to the genus Homo represent a polytypic species with an Old World range (Asfaw et al., 2002) or instead provide evidence for multiple lineages with more limited temporal and geographical distributions (Schwartz, 2004). A single specimen clearly cannot answer this question, and it seems likely that the ongoing controversy will extend past the sample traditionally referred to H. erectus and on into that referred to H. heidelbergensis.

The problem of taxonomy is confounded with geography. Ultimately, whether this specimen looks like Homo erectus depends on whether it looks more like contemporary East Asians (e.g., Zhoukoudian, or possibly Sambungmachan), or whether instead it looks like contemporary Europeans or Africans (e.g., Bodo).

But this is to some extent complicated by variability within the western sample. Consider, the African Middle Pleistocene includes large skulls like Bodo and Kabwe but also small ones like Ndutu and Salé. Europe has Petralona, but also Steinheim. With this specimen, only the top of the skullcap is preserved, so the comparisons will be heavily dependent on size: browridge size, bone thickness and robusticity, and measured frontal breadth and length. The essential question is whether it is more like the contemporary Asian or African/European samples.

The frontal fragment includes the lateral two-thirds of the supraorbital torus, which is relatively uniform in thickness to its lateralmost extent. The browridge is very thick -- nearly as thick as Kabwe, and substantially thicker than any of the Zhoukoudian specimens. The frontal squama slopes posteriorly -- with less of a frontal boss than most of the Chinese specimens. This makes the supratoral sulcus relatively slight.

The minimum frontal breadth as reconstructed (by mirroring) is fairly narrow -- it is comparable to OH 9 or Sangiran 17. Kappelman and colleagues give a nice graph of this dimension compared to estimated cranial breadth and supraorbital torus thickness. With a vault thickness of 10 mm at bregma, the skull is relatively thick but inside the ranges of both contemporary Asian and African samples.

So, is the Kacabaş skull more like Asians or Africans? Metrically, Kappelman et al. show that the skull is a lot like the Zhoukoudian L3 skull, OH 9, and Sangiran 17. I could see more comparisons (and I am a bit curious about the way the orbital breadth compares to the other samples) but Kappelman et al. have done a nice job of presenting the skull and their description of the supraorbital region is quite detailed. The overall morphology of the specimen and its size are very comparable to Zhoukoudian, so it is fair to refer the skull to Homo erectus.

Compared to most of the Middle Pleistocene Africans and Europeans it is small -- but then, the sample did not include some very small crania like Ndutu or Salé.

On the other hand, there are those who would assign these small African crania to H. erectus. In other words, both taxonomy and geography are confounded by size!

In terms of this specimen, the morphology is more similar to Zhoukoudian, but it is not so very different from Kabwe or Bodo, other than its smaller size. If we approach the skull with the hypothesis that it is part of a West Asian population intermediate between Middle Pleistocene East Asians, Africans, and Europeans, I don't see anything to disprove that hypothesis.

Tuberculosis

I think the absolute coolest part of the paper is its demonstration of cranial lesions attributable to tuberculosis. They provide microscopic views of the lesions and detail why they are consistent with the disease. I'm no paleopathologist, but it looks like a solid case to me.

The discussion of the paper considers why the specimen might have faced an unusual risk of TB, focusing on the possibility of vitamin D deficiency. The authors suggest that a dark-skinned, presumably African population of H. erectus may have moved north into temperate latitudes, where lower UV radiation levels caused vitamin D deficiency. In theory, low vitamin D levels may have an immunosuppressive effect, possibly increasing the risk or severity of TB. The paper discusses the increased incidence of TB in dark-skinned immigrants to Europe as support for this hypothesis.

Personally, I think this section was unnecessary. The fact stands on its own: the individual suffered from TB, by no means an uncommon disease in historic human populations. There has been no shortage of light-skinned TB sufferers. While it is possible that dark-skinned people have an higher intrinsic risk, there are other factors such as access to health care that may contribute to this risk as well. Most important, there is really no reason to suppose that a 500,000-year-old Turkish hominid need have been dark-skinned. Indeed, the presence of at least one light pigmentation variant in Neandertals plainly shows that we can assume nothing about earlier hominids.

The idea that tuberculosis is a very old hominid disease has gained currency in the last few years. Older molecular analyses made this hypothesis seem unlikely -- most pathogenic strains of Mycobacterium tuberculosis stem from a common ancestor sometime during the last 20,000-35,000 years. This passage from Gutierrez et al. (2005) describes the history:

Members of the Mycobacterium tuberculosis complex (MTBC), the agents responsible for tuberculosis, are among the most successful human pathogens. The MTBC as defined here comprises the so-called M. tuberculosis, M. bovis, M. microti, M. africanum, M. pinnipedii, and M. caprae species. Although the members of the MTBC display different phenotypic characteristics and mammalian host ranges, they represent one of the most extreme examples of genetic homogeneity, with about 0.01%-0.03% synonymous nucleotide variation and no significant trace of genetic exchange among them. Therefore, it is believed that the members of the MTBC are the clonal progeny of a single successful ancestor, resulting from a recent evolutionary bottleneck that occurred 20,000 to 35,000 y ago.

However, Gutierrez and colleagues studied several strains of "smooth" bacilli from East Africans, finding that these were distant relatives of the MTBC strains, with a divergence around 3 million years ago. The usual hypothesis had been that TB was a zoonotic pathogen, possibly from cattle, which entered humans recently. But the presence of TB-related bacilli in humans suggests instead that TB is an old hominid pathogen, and that a particularly virulent strain may have spread recently through the human population, spreading from people into cattle and other domesticates. That may seem surprising (how exactly did it spread from humans into seals, one may ask?), but all of the identified animal forms are pathogenic in humans under some circumstances, and the majority of human cases come from three different bacterial species -- M. tuberculosis, M. bovis, and M. africanum.

The hypothesis of an old hominid-specific strain of TB has been challenged (Smith 2006), mainly because working out divergence times and relationships among reticulating bacterial strains is complicated. Gutierrez and colleagues defended their hypothesis in a reply, but this problem clearly is not yet settled. Actually finding the disease in such an ancient specimen pretty much brings closure to the initial question of whether TB is old, but doesn't yet tell us whether the particular phylogenetic scenario really happened.

I think tuberculosis is especially likely to be an old human pathogen, because it has a number of characteristics that would facilitate its survival in small-scale hunter-gatherer societies. Infections last for many years and are active (and therefore transmissible) for a large fraction of that time in some infected people. The primary mode of infection is respiratory, and the bacteria are easily spread. Present-day tuberculosis is predominantly a crowd disease -- spreading quickly in crowded cities. But if Gutierrez and colleagues are correct about the smooth bacilli being an ancestral form of the bacterium, this earlier form may not have been as dependent on population density for its spread.

Other pathogens with these characteristics are also candidates for old Pleistocene human diseases. My favorite at the moment is pertussis (whooping cough), which Diavatopoulos and colleagues (2005) showed may have diverged from an old, human-specific strain of Bordetella bronchiseptica. It has nothing to do with this specimen, but the common thread of diseases suitable for spread in Pleistocene humans may give us some things to look for in terms of ancient genetic adaptations to disease.

References:

Bischoff JL, Shamp DD, Aramburu A, Arsuaga JL, Carbonell E, Bermudez de Castro JM. 2003. The Sima de los Huesos hominids date to beyond U/Th equilibrium (>350 kyr) and perhaps to 400-500 kyr: new radiometric dates. J Archaeol Sci 30:275-280. doi:10.1006/jasc.2002.0834

Bischoff JL, Williams RW, Rosenbauer RJ, Aramburu A, Arsuaga JL, Garcia N, Cuenca-Bescós G. 2007. High-resolution U-series dates from the Sima de los Huesos hominids yields 600 kyrs: implications for the evolution of the early Neanderthal lineage. J Archaeol Sci 34:763-770. doi:10.1016/j.jas.2006.08.003

Diavatopoulos DA, Cummings CA, Schouls LM, Brinig MM, Relman DA, Mooi FR. 2005. Bordetella pertussis, the Causative Agent of Whooping Cough, Evolved from a Distinct, Human-Associated Lineage of B. bronchiseptica. PLoS Pathog 1: e45. doi:10.1371/journal.ppat.0010045

Kappelman J, Alçiçek MC, Kazancı N, Schultz M, Özkul M, Şen Ş. 2007. First Homo erectus from Turkey and implications for migrations into temperate Eurasia. Am J Phys Anthropol (in press) doi:10.1002/ajpa.20739

Gutierrez MC, Brisse S, Brosch R, Fabre M, Omaïs B, Marmiesse M, Supply P, Vincent V. 2005. Ancient Origin and Gene Mosaicism of the Progenitor of Mycobacterium tuberculosis. PLoS Pathogens 1:e5. doi:10.1371/journal.ppat.0010005

Smith NH. 2006. A re-evaluation of M. prototuberculosis. PLoS Pathogens 2:e98. doi:10.1371/journal.ppat.0020098