john hawks weblog

paleoanthropology, genetics and evolution

Zhoukoudian

  • When anthropological and geological facts collide

    Mon, 2011-11-28 01:56 -- John Hawks

    This passage is the first paragraph of the introduction to Franz Weidenreich's monograph, The Skull of Sinanthropus pekinensis [1].

    In my earlier contributions to the study of Early Man I pointed out repeatedly the danger of confusing anthropological facts with geological facts. In determining the character of a given fossil form and its special place in the line of human evolution, only its morphological features should be made the basis of decision; neither the location of the site where it was recovered nor the geological nature of the layer in which it was imbedded [sic] are important. Discrepancies cannot be smoothed out by bringing morphological facts and opposing geological data into closer harmony with artful interpretations or by touching-up reconstructions. It is a generally accepted conception that Man has developed in the course of time by gradual transformation from an ape-like type to the type he presents today. Viewed from this fundamental standpoint, it is logical to assume that the more a form resembles the supposed ancestor the more ancient it will be, or that the more ancient it is the more "primitive" it should be.

    I am concerned with this passage today because of a re-emerging mismatch of evidence from the morphology of Middle Pleistocene humans and the genetics of Neandertals. Some paleoanthropologists have asserted that Europeans of the Middle Pleistocene were the exclusive ancestors of Neandertals. I have in the past written that Middle Pleistocene Europeans were among the ancestors of Neandertals, with sustained gene flow from other populations including Africa [2]. The Sima de los Huesos people, maybe 600,000 years old, resembled the (much) later Neandertals in several aspects of their anatomy, as did other Middle Pleistocene Europeans.

    The genetic differences between living people and the ancient Neandertal genomes appear consistent with the emergence of distinct African and Neandertal populations only within the last 400,000 years or less [3], [4].

    Such a recent date seems a poor match for the morphological evidence of Neandertal ancestry in Europe. I can think of several ways to make these morphological and genetic comparisons concordant with each other, all of which balance some shift in one body of inference against the other. As long as we can't pin down the human mutation rate within a factor of two ("What is the human mutation rate?"), there's a lot of room to make different population models consistent with the genetic data.

    This is, in today's language, Weidenreich's point. Morphological data must be interpreted in accordance with evolutionary principles, and if it doesn't fit a temporal scheme, it doesn't fit. Likewise, genetic similarities must be explained in their own evolutionary framework. These two sources of evidence must in the end be consistent with a single history. We will find that consistency not by shoehorning the evidence together, but by interpreting each with the strongest possible skepticism concerning assumptions and models.

    Weidenreich's introduction illustrates two cases. The simpler, from our point of view today, was Piltdown. Many establishment anthropologists, particularly in Britain, had maintained that Piltdown was a morphologically advanced ancestor of modern humans, which had lived early in the geological record of human evolution. Weidenreich had been an early and prominent critic of this idea, because he was convinced that the specimen simply did not fit together with its supposed geological context.

    I cannot believe, even making very liberal allowances for these uncertainties, that such incongruity between morphology and chronology as is found in the case of Piltdown can be completely brought into accord. The only hope of solution in this case would lie in assuming that the human bones were not contemporaneous with the layer in whih they were found but were deposited there later. Otherwise, modern man must be much more ancient than we ever imagined, or else Western European man did not pass through evolutionary stages as did the hymans of other regions of the earth.

    We now know, of course, that Weidenreich was entirely correct. The apparent geological facts were false, and the "advanced" characters of the specimen were simple reflections of the fact that the skull is a modern human skull.

    The other problem Weidenreich discussed in some detail was the phylogenetic position of the Steinheim skull. Like Piltdown, this specimen had been placed in a Presapiens context by other workers. Steinheim lacks most of the derived characteristics of later Neandertal specimens. Weidenreich, along with many of his contemporaries, accepted its lack of Neandertal features as evidence for affinity with modern humans. In Weidenreich's view, this similarity with modern humans was "anachronistic". Even so, the case did not challenge an evolutionary interpretation, only the assumption that features could evolve from "primitive" to "modern" along a single line. If we admit that Neandertal features were not in all cases "primitive", even if they may resemble superficially the characteristics of some apes, we can accommodate specimens like Steinheim within a population model where both moderns and Neandertals may have derived (and in some cases, secondarily derived) characters that appeared afterward.

    This scenario requires us to straighten out the analysis of the characters themselves, a process for which larger fossil samples are essential. It was to that end that Weidenreich supposed the Sinanthropus sample to be of such great utility. The subtext of the introduction was to illuminate the kinds of evolutionary problems that could be further illuminated by a full description of fossil variation. Finding variation in fossil humans did not repudiate the concept that modern humans had evolved in stages from primitive ancestors, but helps to clarify cases where the evolution has not been a simple linear progression. In many cases, features that are superficially "primitive" may actually have been secondarily derived in recent humans compared to earlier hominins.

    Along similar lines, I ran across this old post: "Dobzhansky on Weidenreich's species concept", in which Dobzhansky predicts:

    Some modern populations may carry genes that were present in the Neanderthaloids, and other moderns may not carry such genes.

    References

    1. Weidenreich F. The Skull of Sinanthropus pekinensis: A Comparative Study on a Primitive Hominid Skull. Pehpei: Geological Survey of China; 1943.
    2. Hawks J, Wolpoff MH. The Accretion Model of {Neandertal} Evolution. Evolution. 2001;55:1474–1485.
    3. Green RE, Krause J, Briggs AW, Maricic T, Stenzel U, Kircher M, Patterson N, Li H, Zhai W, Fritz MH, et al. A Draft Sequence of the Neandertal Genome. Science [Internet]. 2010;328:710–722. Available from: http://dx.doi.org/10.1126/science.1188021
    4. Reich D, Green RE, Kircher M, Krause J, Patterson N, Durand EY, Viola B, Briggs AW, Stenzel U, Johnson PLF, et al. Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature [Internet]. 2010;468:1053–1060. Available from: http://dx.doi.org/10.1038/nature09710
    Tags: 
    Franz Weidenreich
    history of anthropology
    Neandertal DNA
    Neandertals
    Zhoukoudian
    Middle Pleistocene
    Steinheim
    Piltdown
    morphology
    phylogenetics
    Synopsis: 
    Weidenreich's introduction to the Sinanthropus cranial monograph illuminates some issues I'm facing with ancient genomes.

  • Asian Homo erectus

    Mon, 2011-11-07 23:59 -- John Hawks
    Synopsis: 
    Examining a sample of crania from the Early and Middle Pleistocene of Asia and Indonesia

    Homo erectus entered Asia as early as 1.8 million years ago. One of the earliest specimens of the species is the Modjokerto skull, from Java. The spread of this species across the tropical Old World was a major event in our evolution. After Homo erectus reached East and Southeast Asia, it had a long history — up to 200,000 years ago or even more recently.

    This station has several representatives of this Asian dispersal of early humans.

    • Trinil 2, Java, 1.2 million years old.
    • Sangiran 2, Java, 1.0 million years old.
    • Zhoukoudian L2, China, 700,000 years old.
    • Zhoukoudian L1, China, 700,000 years old.
    • Ngandong 10, Java, 200,000 years old.
    • Ngandong 8, Java, 200,000 years old.
    • Nganding 4, Java, 200,000 years old.

    What to do: Overall, these fossils are very similar. However, they come from a wide range of times. Make an attempt to seriate the fossils by cranial size. List the results of your seriation. Does it correlate with time?

    Try seriating the skulls according to the form of their frontal bone or supraorbital torus. This feature differs between fossil specimens from Java and China. Does your seriation indicate this difference in geography?

    Study terms: 
    Zhoukoudian
    Sangiran
    Ngandong
    Trinil
    seriation
    Tags: 
    Anthropology 105
    laboratory
    Homo erectus
    Asia
    China
    Indonesia
    Sangiran
    Zhoukoudian

  • Open every box

    Thu, 2011-05-26 02:26 -- John Hawks

    Fascinating: "Unique Canine Tooth from 'Peking Man' Found in Swedish Museum Collection"

    Swedish paleontologists were the first scientists to go to China in the early 20th century, and they carried out a series of expeditions in collaboration with Chinese colleagues. They found large numbers of fossils of dinosaurs and other vertebrates. The material was sent to Sweden and the well-known paleontologist Carl Wiman, who identified and described the fossils. But when the direction of research changed after Wiman's death, 40 cartons were left unopened and forgotten -- until know. In recent weeks, they have been opened by Per Ahlberg, his colleague Martin Kundrát, and Museum Director Jan Ove Ebbestad, who had drawn attention to the cartons in the storeroom at the Museum of Evolution.

    You know, this is why open science is so important. When you have a small group of people working a collection, the information goes when they die. I hear about cases like this all the time. And we're talking about hominins in relatively well curated collections. The number of unique specimens of other fossil organisms sitting in boxes must be enormous.

    The more eyes you have on your collection, the more it is worth.

    Tags: 
    history of archaeology
    China
    open science
    history of paleontology
    Zhoukoudian
    metascience

  • Quote: Boyd on New World pigmentation clines

    Tue, 2010-09-28 16:44 -- John Hawks

    I'm using some statistics out of William Boyd's 1956 printing of Genetics and the Races of Man[1]. It gives a good accounting of blood group data known more than fifty years ago, which I'm using to illustrate my intro lectures. Meanwhile, there are some interesting passages, from the standpoint of today's knowledge of the human genome and its variation.

    On skin pigmentation -- this is the earliest statement I've run across of the argument that the New World pigmentation cline is shallower than the Old World cline because of the relative recency of occupation (pp. 178-180):

    The aborigines of the New World, though not by any means identical, agree in having on the whole considerable skin pigmentation. If pigmentation is adaptive, and conforms to climate, why are not the Eskimo and the inhabitants of Tierra del Fuego as light as Europeans? This looks like a considerable difficulty, but the solution is probably comparatively simple. The aborigines of the New World have not been here for more than about 25,000 years, or about 1000 generations. They are by origin Asiatic, and in Asia skin pigmentation is fairly heavy. Unless the selection of light skin as opposed to dark were fairly intense, the time elapsed has simply not been enough to allow for much adaptation to occur (12). As a matter of fact, the populations which might have been expected to become lighter, namely the Fuegans and the Eskimo, have probably had a shorter time in which to achieve this end than other American aborigines, for it is reasonable to suppose that the Fuegians did not reach their present home until long after their northern neighbors were well installed. And all students of the Eskimo agree in recognizing them as probably the most recent (aside of course from the whites) arrivals in America. It could well be that there has just not been enough time for selection to bleach the skins of the American aborigines.

    Reference 12 is Haddon's Races of Man, which I have requested from the library.

    I'm following up, because skin pigmentation is one of the traits most clearly subject to recent rapid selection. The new mutations that lighten skin tone in Europe and Asia are only partially shared between those populations. Many alleles are very common in one population, but nearly absent in the other. So far, the estimates of dates for these new variants are all within the last 20,000 years, but many remain undated. So we can't specify the level of pigmentation of people 15,000-20,000 years ago, yet, but it would have been substantially darker than those populations today.

    Which leaves us with the same question, but from the opposite perspective. We now know that pigmentation evolved rapidly in Eurasia, the strong gradient of pigmentation having increased greatly within the last 20,000 years. We also know that the occupation of temperate South America began quite early, with people having been there longer than 10,000 years. So why did the New World end up with a more gradual cline -- darker pigmentation in the temperate and Arctic regions, lighter in the tropics than in the Old World? Was selection less intense? Can we attribute the difference to demography? Or chance?

    Boyd next alluded to a demographic explanation -- low population density:

    In any case, the pre-Columbian population was so sparse compared with that of Asia and India that on a statistical basis alone we should be justified in asserting that skin pigmentation conforms to climate.

    Them's some tricky statistics.

    We would of course today recognize that the sheer number of people is not especially relevant; much more powerful is the independent occurrence of a similar response in two long-separated populations. But Boyd was concerned with a different issue: Some had been claiming pigmentation as a neutral trait, making it more useful as a race marker:

    This has been denied chiefly by those who were concerned to prove skin color a non-adaptive character, so that it might safely be used in the classification of races (12). Since the more up-to-date students of anthropology have given up the idea of relying on non-adaptive characters, or even believing that any such exist (13), there is no longer much dispute about the probable adaptive value of skin color (emphasis added).

    Well, makes me glad to be an "up-to-date" student! There in fact has been an ongoing debate about "non-adaptive characters" as concerns the relationship of Pleistocene people. Many geneticists were surprised to discover the persistence of Neandertal genes, but in fact the skeletons of Upper Paleolithic Europeans clearly bear Neandertal traits. The debate for the last 30 years hasn't been chiefly about the presence of these traits, but instead about whether they were adaptive. Some argued that adaptive traits were not suitable evidence for a relationship, because they could emerge by parallelism in distinct populations.

    Others observed that adaptive traits were more likely to be shared among populations linked by gene flow.

    Now, of course, we have remaining unanswered questions about these shared traits. The shared traits are clearest between Upper Paleolithic Europeans and European Neandertals. We don't have genetic information yet telling us about the extent of Neandertal gene sharing with these early Europeans. Was it more than elsewhere? The traits would argue for it.

    What about the Neandertal genes in populations far from Europe? One might expect Neandertal-like morphology to show up at some low level. Of course morphological features are polygenic, so that phenotypic resemblance falls much faster than genic identity. And Holocene populations have continued to evolve. Maybe early Asian skeletal remains like the Upper Cave skulls (ca. 11,000-20,000 years old) actually reflect that Neandertal heritage to a greater extent than recent samples.

    Then there is the likelihood of other contributions, more local ones, to later populations.

    Returning to the topic of pigmentation, many of us used to assume that the light skin of Europeans in part reflects Neandertal ancestry. That is, just as Boyd suggested, it would have taken a lot longer than 25,000 years to get the current strong cline of skin pigmentation in the Old World. If you could have longer, getting lighter pigmentation from earlier inhabitants of Europe, for example, you could explain a stronger cline with the same strength of selection.

    I no longer think this is necessary. It's still possible that we got some pigmentation variants from Neandertals, but we haven't found any yet. And we've been looking. It does seem that Neandertals had some of their own pigmentation variants. Maybe we'll find many more of those, maybe not.

    References

    1. Boyd WC. Genetics and the Races of Man. Boston: Little, Brown and Company; 1956.
    Tags: 
    quotes
    introgression
    Zhoukoudian
    America
    pigmentation
    recent selection
    Neandertal DNA
    China
    American Indians

  • The changing height of Homo erectus

    Wed, 2010-01-13 14:00 -- John Hawks

    Gretchen picked up a partial set of Time-Life volumes, from 1973, part of the series "The Emergence of Man". She found them at a garage sale. There's a lot of fun stuff in them, and some very useful illustrations.

    For example, I'm looking through the volume titled, The First Men, which is basically about Homo erectus. The meat of the book is a series of descriptions of fossil and archaeological finds -- Dubois on Java, Terra Amata, Torralba and Ambrona. No surprise, each of these has a very different theme than we would give them today!

    Here's a fun comparison:

    Body proportions of fossil hominins, from 1973

    This is a two-page spread in the book; really a fine illustration by Roger Hane.

    Homo erectus, in the middle, is reconstructed with a stature pretty much right in between Australopithecus and Homo sapiens.

    If you open up most recent textbooks, you'll find Homo erectus illustrated as the same height or taller than us. This is mostly due to the KNM-WT 15000 (Nariokotome) skeleton, discovered in 1984. This skeleton was estimated to have a moderately tall adult stature -- around 185 cm (6 feet 1 inch). There are three or four other femora from the Lower Pleistocene that also correspond to stature estimates up around 180 cm, in particular KNM-ER 736 and KNM-ER 1808.

    Now one might reasonably wonder, what's the big deal about 185 cm? The Nariokotome skeleton hardly represents a giant -- at 6'1" he would have been an inch shorter than me, for goodness' sake! And with only one fossil specimen within 2 inches, it shouldn't be churlish to point out that the Nariokotome estimate is not based on a real femur length -- it's an estimate based on an estimate. Most Lower Pleistocene fossil femora were much shorter, and yield stature estimates well under 180 cm. So why did anthropologists so eagerly cling to the tallest estimates for Homo erectus?

    Few Lower Pleistocene postcranial bones are associated with skulls, so it's difficult or impossible to assign smaller bones to a species. How do we know whether a short femur belongs to Homo erectus or Homo habilis -- which we know from OH 62 is much smaller in body size? Or A. boisei, which it would appear from KNM-ER 1500 is also smaller? We don't really know -- so the bones that correspond to mid-range stature estimates, say around 160 cm, might belong to any of the above. But the tall ones -- well, we know that those must represent the largest-bodied hominin. So there was a tendency to assume that the tall specimens were near the average for Homo erectus.

    It was a hypothesis. It has turned out to be false.

    The illustration in the Time-Life book is based on entirely different fossils. The Turkana fossils were unearthed during and after the early 1970's. Before that, Homo erectus stature could be estimated from the Trinil (Java) and Zhoukoudian (China) femora. These are later than the early African Lower Pleistocene sample. The Zhoukoudian femora in particular give stature estimates at or under 5 feet (152 cm). We might read it as a decline, and some people did as recently as 5 years ago. But the Dmanisi postcrania are also short, a bit shorter than the Zhoukoudian femora. And they're earlier than Nariokotome. And we now know of smaller crania of Homo erectus in the East African Lower Pleistocene. So the tallest statures aren't the average; they're the tallest.

    It ain't rocket science, I know. But this is progress.

    Today, I think it's fair to say that the variation of stature in Homo erectus was more or less like the variation within living people. There are short and tall populations today, varied in ecology and latitude. The average stature of young men in the Netherlands today is 184 cm. Adult women in the Philippines average only 150 cm. So the best way to compare statures is to illustrate the range.

    That being said, I don't think we know how stature has evolved over time. We do have some data points -- the Neandertals were shorter than Upper Paleolithic Europeans, for example, but seem to have been around the same height as Mesolithic people (and a shade taller than Neolithic Europeans). The Dmanisi people were on the short end of the human range, but not unusually so. The variability within Lower Pleistocene East Africans seems high, but I'd want to see a serious test compared to human populations.

    It's a case in the fossil record where discovering more seems to have resulted in us knowing less. But that's just because we can now reject several categorial statements that people used to accept uncritically.

    Related articles:

    "News flash: Dmanisi hominids were not short"

    "Body size in Holocene South Africa"

    Body mass in ancient humans and high-latitude populations

    Tags: 
    Zhoukoudian
    Dmanisi
    Nariokotome
    Turkana
    Homo erectus
    stature
    Synopsis: 
    The re-evaluation of the stature of KNM-WT 15000 provokes a "blast from the past", looking at the statures of other Homo erectus specimens.

  • Zhoukoudian cave site in danger of collapse

    Tue, 2009-06-30 17:05 -- John Hawks

    From China Daily:

    Reinforcement has begun at the Peking Man site to prevent one of its walls from collapsing.

    The protective excavation, which started Wednesday, focuses on the west section of the cave where the first Peking Man skull, hundreds of thousands of years old, was found in Zhoukoudian, 46 kms from downtown Beijing.

    The west section is the only part that has remained untouched since the cave's discovery.

    "Repair work cannot be done without a comprehensive excavation," Gao Xing, deputy director and research fellow of the Institute of Vertebrate Paleontology and Palaeoanthropology of the Chinese Academy of Sciences, said at a press conference Wednesday.

    Xinhua news agency has a short history of excavations at the site, although it omits Weidenreich's role entirely, and misses the details of the 1960's excavations beyond a mention.

    Tags: 
    Middle Pleistocene
    Zhoukoudian
    China
    site conservation

  • A Mongolian hominid

    Thu, 2008-03-13 23:39 -- John Hawks

    Yves Coppens and colleagues have found a frontal bone, and a bit more, in Mongolia. They do not report a date for the specimen beyond Late Pleistocene; it comes from a pit dug for gold mining. The site is north of Zhoukoudian and other northern Chinese sites by several hundred kilometers, and is approximately the same latitude (though further east) as Okladnikov Cave (discussed in my interview with Mica Glantz). The place is called Salkhit.

    They describe the anatomy of the specimen: it has a complete supraorbital torus, thicker in the superciliary area than laterally; a slight frontal keel, and an overall sloping profile. In other words, it looks to be generally archaic in morphology. Their metrical comparisons put it generally with Middle Pleistocene crania like Zhoukoudian, Steinheim, and Petralona.

    (via Paleoanthro)

    References:

    Coppens Y, Tseveendorj D, Demeter F, Turbat T, Giscard P-H. 2008. Discovery of an archaic Homo sapiens skullcap in Northeast Mongolia. Compte Rendus Palévol (in press) doi:10.1016/j.crpv.2007.12.004

    Tags: 
    Late Pleistocene
    Middle Pleistocene
    Zhoukoudian

  • A new Middle Pleistocene hominid from Turkey

    Sun, 2007-12-09 15:12 -- John Hawks

    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

    Tags: 
    Homo erectus
    Zhoukoudian
    Neandertals
    Middle Pleistocene
    Synopsis: 
    The announcement of a cranium from Kacabas, Turkey, a Middle Pleistocene specimen that suffered from tuberculosis.

  • Casts don't substitute for fossils

    Thu, 2007-09-20 12:15 -- John Hawks

    Slate's Scott Solomon asked some people, and presents a nice, short explanation of why original fossils are important to paleoanthropology:

    You can't rely on bone-length measurements taken from a cast, either, since the replicas tend to deteriorate or deform faster than original fossil material. Casts made from plaster or plastic -- such as most casts of Lucy -- often become misshapen in unpredictable ways, especially when they are repeatedly handled or moved. The problem becomes worse when new casts are made from old casts, since a copy of a copy is less likely to be accurate.

    And then there's this:

    The [Piltdown] forgery was not exposed until 1953, in part because many paleontologists were allowed to carefully examine only casts but not the original "fossils," which contained clues about the subterfuge.

    The same goes for reconstructing fragmentary specimens: it is often not possible to tell which details of a cast may be determined entirely by reconstruction, and which are genuine features of the preserved anatomy.

    UPDATE(2007/09/19): Wow, a lot of people didn't like this post! Don't read so much into it; the linked article is about why you don't want to lose original fossils. Casts do become misshapen in unpredictable ways. Later methods of analysis can reveal new facts about them. There are lost originals we'd like to have back; like those from Mladeč and Zhoukoudian, all lost or destroyed during World War II. I guess an even better example is Vindija 33.19, which shows that some fossils can be very unique in their preservation in ways that later become very, very important. I've posted some new thoughts related to the correspondence I've gotten.

    Tags: 
    metascience
    Zhoukoudian

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