john hawks weblog

paleoanthropology, genetics and evolution

A. boisei

  • Putting together australopithecine diets

    Sat, 2011-11-26 18:06 -- John Hawks

    Peter Ungar and Matt Sponheimer earlier this fall [1] reviewed the evidence for diet in early hominins, from both microwear studies (Ungar's specialty) and stable isotopes (Sponheimer's forté). I wanted to point to this article because it is a very useful short review that illuminates the cases in which these two sources of evidence lead to a single interpretation.

    [T]he isotope data also suggest enormous and unanticipated differences between contemporaneous taxa with strong morphological similarities, notably the “robust” australopiths P. robustus and P. boisei. Despite their attribution to the same genus, there is no overlap in their carbon isotope compositions (41), which is a rarity for congeners among extant mammals.

    Maybe this should give pause to those who insist that A. robustus and A. boisei are sister species. Ungar and Sponheimer here reiterate the observation that microwear is very similar between A. boisei and A. afarensis:

    The apparent continuity of microwear pattern through the putative lineage Au. anamensis–Au. afarensis–P. boisei could even suggest that morphological changes reflect increasing efficiency for grinding large quantities of tough food. Although living primates that eat tough items typically have sharp shearing crests, eastern African australopiths and especially P. boisei may have evolved a different solution for processing such foods, given the flattened, thickly enameled teeth of their close ancestors (23). Natural selection must work with the raw materials available to it. Thus, the present-day ecomorphological diversity within the primates may not be sufficient for making some paleoecological inferences, which is not surprising given that the vast majority of all primates, especially apes, that have ever lived are now extinct.

    This idea was raised earlier, for example in the context of the stable isotope findings on A. boisei ("'Nutcracker Man' debunked"). Until we have more stable isotope results from the known sample of A. afarensis or A. anamensis, we won't be able to test this "tough C4 food" hypothesis. "Ecomorphological diversity" refers to the match between food types and the topological properties of tooth crowns among living primates. Generally speaking, primates with high crowns and high cusp relief with shearing crests are thereby well-suited for eating tough foods like leaves and stems. That's the common ground between gorillas and colobines, for example. A. afarensis and especially A. boisei have exactly the opposite morphology from what would seem to be the "tough foods" pattern. So why do these species seem to be acting like grazers? Very peculiar.

    My own attitude is that if we can't clearly make sense of the anatomy of A. boisei, then we won't be able to untangle the diets of the other species. Early hominins evolved along a distinctive trajectory toward larger molars, smaller canines, and bigger jaw musculature within a common body plan. A. boisei represents the extreme of this trend. So if A. boisei is the logical morphological extreme, why does it seem to have such a different dietary strategy than every other hominin with stable isotope evidence?

    Meanwhile, if Ungar and Sponheimer are correct in asserting a common dietary strategy in the East African species, then it seems pretty clear that early Homo shares a dietary commonality with the South African species, not the East African ones. One might argue that Homo differentiated from other hominins within East Africa by adopting a fundamentally South African dietary strategy. But I would be more inclined to suppose a South African-derived hominin made incursions into East Africa, possibly repeated ones, as Homo was emerging. Ungar and Sponheimer are correct that natural selection works with the materials available. Population growth and migration are vastly more rapid than in situ evolution. What if the apparent "early Homo" record actually represents a series of successive dead-end migrations from southern Africa?

    References

    1. Ungar PS, Sponheimer M. The diets of early hominins. Science (New York, N.Y.). 2011;334(6053):190-3.
    Tags: 
    diet
    A. boisei
    A. afarensis
    A. anamensis
    stable isotopes
    microwear
    Synopsis: 
    A review of microwear and stable isotope evidence of diet prompts questions about early hominin relationships.

  • Meet Australopithecus boisei

    Tue, 2011-10-11 08:25 -- John Hawks
    Synopsis: 
    Compare and contrast A. boisei and A. robustus, with a discussion of their ages and locations.

    The robust australopithecines existed between 2.5 and 1.5 million years ago. At this station are skeletal remains from two kinds of robust australopithecine. You have already met Australopithecus robustus earlier in the semester. The new species for you here is Australopithecus boisei. This species had the largest molar and premolar teeth of any hominin ever to have existed.

    A. boisei comes from East Africa, with remains found in Ethiopia, Kenya, and Tanzania. The most famous fossil is OH 5, from Olduvai Gorge, Tanzania, around 1.7 million years old. Other significant specimens here include KNM-ER 739, KNM-ER 732 and KNM-ER 406, from Koobi Fora, Kenya, around the same age.

    The specimens of Australopithecus robustus here will be familiar to you. All are from South Africa, and they include SK 48 and SK 12, from Swartkrans, South Africa, around 1.7 million years old, and TM 1517 from Kromdraai, South Africa, around 1.8 million years old.

    These species may be closely related, but there are some differences between them. Examine them closely with the following questions:

    1. The defining features of the robust australopithecines are the large postcanine dentition and large jaw musculature. How do these two groups of fossils compare on those features?

    2. Robust australopithecines also have a very reduced anterior dentition (incisors and canines). Which fossils show that morphology?

    3. The premolars in these species have enlarged, at the extreme they become more like molars in their morphology. Which fossils have the most molar-like premolars? Is the trend the same in the upper and lower dentitions?

    3. With such great robusticity of the jaws and teeth, there are potentially great differences between males and females. Are the differences here consistent with sexual dimorphism? Which fossils are male, and which are female?

    Study terms: 
    Australopithecus
    Australopithecus boisei
    Australopithecus robustus
    robust australopithecine
    Tags: 
    Anthropology 105
    laboratory
    anatomy
    A. boisei
    A. robustus

  • No brain expansion in Australopithecus boisei

    Sun, 2011-08-21 11:07 -- John Hawks
    Research authors: 
    John Hawks
    Publication information: 

    This is an archived pre-publication manuscript of the article published in the American Journal of Physical Anthropology, doi:10.1002/ajpa.21420 (citation information)

    Work status: 

    This is a completed manuscript that represents the work before final peer review, posted here in accordance with the copyright agreement with the American Journal of Physical Anthropology. Citations and references to the paper should direct readers to the final published version.

    Abstract: 

    The endocranial volumes of robust australopithecine fossils appear to have increased in size over time. Most evidence with temporal resolution is concentrated in East African Australopithecus boisei. Including the KNM-WT 17000 cranium, this sample comprises 11 endocranial volume estimates ranging in date from 2.5 million to 1.4 million years ago. But the sample presents several difficulties to a test of trend, including substantial estimation error for some specimens and an unusually low variance. This study reevaluates the evidence, using randomization methods and a related test employing an explicit model of variability. None of these tests applied to the A. boisei endocranial volume sample find significant evidence for a trend in that species, whether or not the early KNM-WT 17000 specimen is included.

    Tags: 
    my research
    A. boisei
    brain

    The endocranial volumes estimated for late Australopithecus boisei specimens (e.g., after 1.8 Ma) are larger than those of earlier specimens. Elton et al (2001) [1] found that this trend is statistically significant, arguing for the evolution of larger brains over time. Such a trend bears on the ecology and social behavior of A. boisei, and lends some doubt to the idea that brain size evolution in early Homo was exceptional [1].

    But the A. boisei sample has some unusual aspects that may complicate the test of a trend. One question is whether the early KNM-WT 17000 specimen represents A. boisei or another species (possibly, Australopithecus aethiopicus). Another question arises from the very small variation of estimated endocranial volumes in the A. boisei sample. Even including the small KNM-WT 17000 volume estimate, the coefficient of variation in the sample examined by Elton et al (2001) [1] is only 8.2 percent. Excluding KNM-WT 17000, the within-sample CV is 6.8 percent. By comparison, Tobias (1971) [2] reported data on endocranial volumes of hominoids. Great ape values include chimpanzees with 9.7 percent, orangutans at 10.9 percent, and gorillas with a CV of 13.1 percent. According to these estimates A. boisei had less variation than any living hominoids, even though its craniodental variation was as great as gorillas or orangutans [3].

    There are several possible interpretations for the low variation of the A. boisei sample: (1) A. boisei actually had very low size dimorphism; (2) its endocranial variation has been greatly undersampled, or (3) the sample has been biased by estimation error. Other characters of the A. boisei sample show extensive variability compared to extant hominoids [3], so that monomorphism for this species seems unlikely. Low sample variance is a special concern because estimation error might lead to false positive results in a test of trend.

    Here, I conduct three new tests of the null hypothesis of stasis of endocranial volume in A. boisei. These tests explore the effect of estimation error on the appearance of a trend in the sample, as well as the effect of low sample variation and small sample size. None of these tests find a statistically significant trend in the sample.

    Materials and methods

    Fossil specimens

    Estimating endocranial volume can be challenging even for relatively complete specimens, considering the subtle distortion exhibited by many fossils. For more fragmentary cranial remains, the estimation of endocranial volume requires not only the correction of distortions but also the reconstruction of missing portions.

    A. boisei endocranial volume estimates plotted against time

    Endocranial volume estimates for specimens of A. boisei against time. The sample is that used in this study, excluding Omo 323.

    The eleven cranial specimens of Australopithecus boisei listed below vary in their completeness and preservation of relevant anatomy. There is no explicit way of statistically controlling for error in the estimation of endocranial volume, considering the diversity of methods of reconstruction. In several cases, different workers have provided competing estimates. For less complete specimens, choosing one estimate above another must involve a close critique of anatomical details. The following list reviews the anatomical condition of each of these specimens. It is not an exhaustive list of volume estimates, but focuses on the range between credible extremes for the more disputed specimens. This gives an impression of the boundary conditions for measurement accuracy for each specimen.

    1. KNM-WT 17000 is a well preserved skull with relatively small vault fragments missing. Walker et al. (1986) [4] estimated the volume as 410 ml.
    2. Omo L338-y6 is a juvenile cranium of uncertain age. Holloway (1981) [5] estimated its volume at 427 ml. Elton et al (2001) [1] estimated an adult volume 4% higher, or 444 ml.
    3. The Omo 323-1976-896 cranial remains are exceedingly fragmentary. One side of the posterior cranial base is preserved, allowing a relatively good estimate of the posterior endocast breadth. The preserved frontal and parietal elements do not join with each other or the temporal; their small size and unknown positions do not allow an accurate estimate of endocast volume. [6] reported an estimate of ``about 490'' based on similarity with the 491 ml KNM-ER 23000. Falk et al (2000) [7] considered it too fragmentary for an accurate estimate. I concur; the available estimate cannot be considered independent of other endocasts on which it may have been based.
    4. KNM-WT 17400 preserves only the anterior third of the endocast, consisting mainly of the frontal lobes. Brown et al. (1993) [6] gave an estimate of 500 ml by modeling missing portions after the more complete KNM-ER 23000, but Holloway (1988) [8] put the volume between 390 and 400 ml, and Falk et al. (2000) [7] adopted an estimate of 390 ml.
    5. OH 5 has good preservation of the endocast, but an uncertain join between the anterior and posterior portions of the vault. This discontinuity has caused a disparity in estimates of its volume, including a low 500 ml estimate by Falk et al (2000) [7] and a high 530 ml estimate by Tobias (1963) [9]. The range of estimates on this well-preserved specimen covers nearly a quarter of the range of variation cited for A. boisei as a whole.
    6. KNM-ER 13750 preserves only the superior vault, accounting for under half of the total endocranial contour. The range of estimates provided by Falk et al (2000) [7], from 450 to 480 ml, again covers roughly a quarter of the range attributable to the species. Brown (1993) [6] reported a higher estimate of 500 ml.
    7. KNM-ER 23000 is a nearly complete vault missing the midline cranial base. Its endocranial volume of 491 ml [6] may be the most accurate assigned to A. boisei.
    8. KNM-ER 406 is also well-preserved [10]. Its volume estimate of 525 ml is uncontroversial [11].
    9. KNM-ER 407 is missing several vault sections including those enclosing the frontal lobe. Holloway (1988) [11] estimated the volume at 510 ml; Falk et al (2000) [7] prepared a new reconstruction with a volume estimate of 438 ml. The difference between these two estimates covers nearly 50 percent of the total range of the sample.
    10. KNM-ER 732 has good preservation of the left side of the vault, but is not complete across the rear of the cranium or basicranium, making a mirror reconstruction problematic. Holloway (1988) [8] estimated the endocast volume at 500 ml; Falk et al (2000) [7] at 466 ml.
    11. KGA 10-525 lacks most of the frontal and anterior cranial base. Suwa et al. (1997) [12] estimated its volume at 545 ml.

    The damaged or missing frontals of many specimens have added to ambiguity about their reconstructed volume. Robust endocasts that preserve this region, such as KNM-WT 17400, differ in their anatomy from other taxa, especially early Homo. Falk et al (2000) [7] reconstructed specimens with missing or incomplete frontal endocasts using more complete robust australopithecine endocasts as models; this resulted in substantially smaller endocranial estimates for OH 5, KNM-ER 732 and KNM-ER 407.

    Tests of temporal trends

    Most A. boisei specimens with EV estimates date to the approximate center of the species' temporal span. The reason for the appearance of a trend is quite clear: there is little variation in the center of the species' temporal range; the latest two specimens are also the two largest; the earliest two specimens include two of the three smallest (Figure 1).

    A test of a temporal trend might be conducted in several ways. A simple linear regression of endocranial volumes against time will test for a trend, but may be confounded by small numbers of specimens at early and late temporal extremes. Testing for a difference in means among temporal subsamples may address this problem. Comparing each specimen as a temporal subsample results in Spearman's rank-order correlation (ρ), which [1] reported as significant for their sample of A. boisei EV estimates.

    Also, following Leigh (1992) [13] and Konigsberg (1990) [14], Elton et al (2001) [1] applied the "Hubert test" [15], sometimes simply called the "Gamma" (Γ) test [16] [17]. This test is a randomization test of association of one continuous and one ranked variable, involving four steps:

    1. The age of each specimen is converted to a rank within the sample. For a two-tailed significance test, ranks are standardized with a mean of zero.
    2. The endocranial volume of each specimen is multiplied by its temporal rank, and all the values thus obtained are summed. This is equivalent to calculating the dot product of a vector of endocranial volumes with a vector of ranks.
    3. The sample is reordered at random an arbitrarily large number of times, each time obtaining the dot product of endocranial volume and rank vectors.
    4. The statistic Γ is estimated to be (M+1)/(N+1), where M is the number of permutations with dot products greater than or equal to that of the observed sample, and N is the number of permutations examined. A Γ ≤ 0.05 is taken as a significant rejection of the null hypothesis of no trend.

    It is perhaps of interest that although the Hubert test uses the dot product of the two vectors, the use of the product-moment correlation yields precisely the same Γ (shown in Appendix). Samples for which the dot product shows a significant trend are samples that have significant correlations between EV and temporal ranks. This suggests a weakness of the test, since a correlation is a measure not of change over time, but of fit to a linear model. A sample may have a significant correlation with very little change, if its variance is also very low. Hence, the interpretation of the test depends on whether the variance is biologically realistic. Since A. boisei appears to be relatively invariant in endocranial volume compared to sexually dimorphic hominoids, the test might be confounded by error in the sample of EV estimates.

    The Hubert test has been applied in the anthropological literature in two partially incompatible ways. As applied by Konigsberg (1990) [14], following Hubert (1985) [15], the vector of temporal ranks is centered on zero (i.e., the values are ... -2, -1, 0, 1, 2 ...). But as applied by Leigh (1992) [13] and Elton et al (2001) [1], the temporal ranks are simple ordinal ranks (i.e., 1, 2, 3, ...). These two alternatives are mathematically equivalent for performing a one-tailed test. But while the first alternative (zero-centered ranks) readily admits a two-tailed test, the second alternative requires a bit more algorithmic complexity for a two-tailed test. Elton et al (2001) [1] and Leigh (1992) [13] did not report whether their tests are one- or two-tailed; following the procedures they described will result in a one-tailed test. Wood et al. (1994) [17] also applied the Hubert test to test for trends in dental characters of A. boisei, citing Leigh (1992) [13]; these authors also did not specify whether they performed one-tailed or two-tailed tests. Lockwood (2000) [16] employed the Hubert test (there called the Γ statistic), and explicitly described a two-tailed approach. One-tailed tests ignore the strength of any negative associations in the permuted samples, and therefore lead to incorrect assessments of statistical significance. The current study applies only two-tailed tests of the null hypothesis of no trend.

    Test 1: Lower estimate for KNM-WT 17400

    Falk et al (2000) [7] argued that smaller estimates are more accurate for several robust australopithecine specimens, and the smaller estimates were generally used by Elton et al (2001) [1]. One exception is KNM-WT 17400, for which Elton et al (2001) [1] used the highest estimate of 500 ml [6], even though both Holloway (1988) [8] and Falk et al (2000) [7] adopted much lower estimates, between 390 and 400 ml. This smaller estimate would make KNM-WT 17400 the smallest member of the sample. A small size for this specimen at the center of the species' time range increases overall sample variability and decreases the relative contribution of early specimens to that variability. This makes KNM-WT 17400 very important to any test of a trend.

    As a preliminary step, I recalculated Spearman's ρ and the Hubert test statistic Γ for the sample of Elton et al (2001) [1], using the smaller 390 ml estimate for KNM-WT 17400. This replicates the methods of that study, except for the change in size of the single KNM-WT 17400 specimen.

    Test 2: Model-based simulation values

    A difficulty of the A. boisei sample is the non-independence of estimates. Less complete specimens have been reconstructed using explicit information from more complete endocasts, chiefly Sts 5 and OH 5. The sample should therefore have reduced variation compared to a sample of intact crania. A reduced variance may increase the chance that a null hypothesis of stasis will be falsely rejected. This is a context in which randomization tests are potentially invalid: they do not assume a statistical distribution, but they do assume independence.

    An additional aspect of the problem is that the state of preservation of fossils may be autocorrelated with time. In the present sample, the early and late specimens are relatively complete, while the middle of the time range is dominated by incomplete specimens. This situation arises frequently in paleontology, because species abundance is often highest at the center of a species' temporal range. Early and late specimens will be more likely attributed to a species if their anatomy is unambiguous --- which is more likely if they are more complete. Early or late specimens may be represented at different fossil localities than the majority of specimens, again requiring more complete specimens for confident assignment. In a Holocene context, specimens are likely to be more fragmentary and rarer earlier in time. These situations present the possibility of finding spurious trends due to differential preservation.

    To attempt to correct for these issues, it is necessary to employ tests that rely on an explicit model of sample variability, instead of randomization of the sample values themselves. A simple model-based test replaces the sample EV estimates with new random deviates from a normal distribution. A normal distribution takes two parameters: the population mean and standard deviation. Deviates drawn from this distribution are independent; an arbitrary number of simulated samples may be obtained by repeatedly drawing new values to replace the sample values.

    Here, the model-based sampling technique was used to generate samples with the same temporal ranks as the observed data, but with new EV values. In cases where the observed sample has two specimens of the same date, two specimens in all simulated samples were assigned the same temporal rank. The observed A. boisei sample has two such pairs of specimens. As in the Hubert test, the computer generated an arbitrarily large number of simulated samples (in this study, 100,000). The dot product of EV and temporal rank vectors in each simulated sample is compared to the dot product of the observed sample. The significance measure is taken as (M+1)/(N+1), where N is the number of simulated samples, and M is the number of those samples in which the absolute value of the dot product is more extreme than the observed value. This is a two-tailed test of the null hypothesis of no trend. I refer to the test below as the ``model-based Hubert test.''

    This test was applied to the A. boisei sample described above, including KNM-WT 17000, excluding the extremely fragmentary Omo 323-1976-896, and employing an estimate of 390 ml for KNM-WT 17400. Simulated samples were generated using the observed sample mean (468 ml) and standard deviation (49.1).

    Test 3: Arbitrary variation

    The model-based Hubert test described above is not limited to the observed sample variation. It can also be applied using a different value for the population standard deviation.

    This option is relevant to the A. boisei endocranial volume sample, because the sample of estimates may have lower variation than the population from which the specimens were drawn. Even with the lower estimate of 390 ml for KNM-WT 17400, the CV of the observed A. boisei sample is still only 10.3 percent --- between chimpanzees (9.7) and orangutans (10.9). This value might be uncharacteristic of the A. boisei population, if its sexual dimorphism or temporal variability are undersampled by available EV estimates. Since the test described here derives its simulated EV estimates from a model distribution, it is easy to apply a more variable model --- for example, matching the CV of gorillas at 13.1 percent [2]. As a further example, I varied the population CV parameter of the model-based test, covering the entire range between 4 percent to 15 percent This range encompasses the CVs of all extant hominoids. In all cases I assumed a mean equal to the A. boisei sample mean (468 ml). Using this procedure, it is possible to evaluate whether possible underestimation of variability in the observed sample may affect the significance of the test of no trend.

    Results

    Test 1: Lower estimate for KNM-WT 17400

    The first tests performed were on the A. boisei sensu lato sample of Elton et al (2001) [1], with the exception of a lower estimate of 390 ml for KNM-WT 17400. With this estimate, the nonparametric Spearman's correlation ρ = 0.52, which is nonsignificant (p>0.10, two-tailed). For the two-tailed Hubert test on the sample, p=0.10. For both tests, the lower estimate for KNM-WT 17400 causes the significance of a temporal trend in A. boisei to completely disappear. This low estimate currently appears to be a consensus for the specimen, although it must be treated cautiously since the endocast is less than 50 percent complete. This single specimen illustrates well the importance of accurate estimates.

    Sample Test p-value
    Including Omo 323 Spearman's ρ p>0.10 (ns)
    Hubert test p=0.10 (ns)
    This study (no Omo 323) Spearman's ρ p>0.05 (ns)
    Hubert test p=0.07 (ns)
    model-based test p=0.07$ (ns)

    Results of Tests 1 and 2.

    Test 2: Model-based simulated values

    The removal from the sample of the 490 ml estimate for Omo 323-1976-896 actually enhances the appearance of a trend. This is reflected by the Hubert test result, with p=0.07 (compared to p=0.10 when Omo 323 is included). Spearman's nonparametric correlation for the sample was 0.58, again nonsignificant (p>0.05, two-tailed). The model-based test described in this paper came to a very similar result on this sample, with p=0.07. Both these tests failed to reject the null hypothesis of no trend for the A. boisei sample.

    Further examination of the simulated samples gave some indication of the relationship between sample variability and the appearance of a trend. One hypothesis might be that the sizes of early KNM-WT 17000 specimen is actually relatively extremely small, and the late KGA 10-525 specimen is actually relatively extremely big, resulting in the apperance of a steady expansion from smallest to biggest through the sample. The simulated samples, in which specimens are drawn from a population with equal standard deviation (49.1) to the A. boisei sample, rejected this hypothesis. Forty-four percent of the simulated samples had at least one specimen smaller than 390 ml, the smallest in the observed sample. Forty-six percent had at least one specimen larger than 545 ml, and 19 percent of simulated samples had specimens more extreme than both the largest and smallest of the observed sample.

    Test 3: Arbitrary variation

    Result of test 3

    Result of Test 3, testing the significance of a trend in A. boisei with a range of models for population CV. Each point represents 100,000 simulated samples of equal mean to the A. boisei sample and CV given as on the x-axis. The greater the assumed variation in the underlying population, the greater the chance that an increase over time equal or greater than that in the A. boisei sample will be observed. There is no significant trend for any model of variation within the range of living great apes and humans.

    An alternative hypothesis is that the appearance of a trend is due to low sample variability, increasing the correlation of EV and temporal rank. The result of the model-based test applied to a range of model CV between 4% and 15% shows the close relationship of significance of the A. boisei trend and population variation. Briefly, the greater the variation in the population, the more likely each simulated sample will present a trend at least as great as that in the observed sample. If the A. boisei sample was drawn from a population with greater EV variability, then the level of correlation of EV with time is less surprising. If the A. boisei population was as variable in endocranial volume as extant gorillas, then 15.1% of randomly drawn samples would exhibit an apparent trend as strong or stronger than the observed sample. With the extant sample, it is not possible to confirm this hypothesis of underrepresentation --- in particular, body size dimorphism does not necessarily follow from variability in cranial and masticatory variability.

    Discussion

    The problem with testing a trend in any early hominid species is similar in form to the problems discussed by Holloway (1970) [18]. All reconstructions are based on relevant knowledge of the anatomy of other specimens. Whether reconstructions are done on crania, endocasts, or CT data, they all rely on knowledge of more complete specimens — for A. boisei endocasts, these models include OH 5 and KNM-ER 23000, and the well-known \emph{A. africanus} endocast Sts 5. When we test hypotheses using samples of reconstructions, we are to some extent including multiple instances of these well-known specimens, spread through many semi-independent reconstructions. There is no ready statistical model to incorporate the effects of estimation error from fragmentary specimens. These estimates are likely to be biased by the use of more complete specimens as models, the more frequent preservation of some parts of the cranial surface as opposed to others, or unrecognized sex differences in fossil individuals. In other words, one effect of estimation error is to reduce the variation within the fossil sample.

    Estimation error may also tend to elevate the between-species differences among early hominins. Presently, samples assigned to different early hominid species exhibit some anatomical differences. For example, These differences may result from differing neuroanatomical adaptations in these different species. If so, then it would be anatomically misleading to use a specimen of A. africanus like Sts 5 as a model for the reconstruction of an incomplete A. boisei specimen. On the other hand, differences are observed between very small samples, and may be idiosyncratic rather than systematic. Instead of distinctive adaptations, they may represent only chance differences between small samples. In this case, the use of only other A. boisei specimens as models for incomplete A. boisei reconstructions would tend to artificially inflate the differences between A.boisei and A. africanus, as well as artificially reducing variation within A. boisei. The smaller the sample, the more likely that between-species differences will be inflated by reconstruction and within-species differences minimized.

    Even with a CV of 10.3%, the variation in A. boisei is likely undersampled. The extant sample is apparently male-biased, with only 3 presumed females (KNM-ER 732, KNM-WT 17400, and KNM-ER 407). Incomplete specimens have been reconstructed by modeling after more complete crania, reducing variation from anatomical differences. Beyond this, temporal fluctuations should tend to inflate variability with or without a directional trend.

    All of these factors also must affect the samples currently assigned to Homo habilis (including KNM-ER 1470), which taken together have an endocranial volume CV of 12.6%. Endocranial volume has a disproportionately important role in differentiating between smaller and larger Plio-Pleistocene Homo morphs, and this may bias the consideration of evolutionary trends in early Homo.

    The only solution for these problems is the discovery of more specimens. But in the meantime, it would be appropriate to exercise caution in the interpretation of variability within and among species. Significant differences among species are tested with reference to within-species variation. For estimated characters like endocast volume, within-species variation is potentially biased by estimation error. This bias may often tend to inflate between-species differences and reduce within-species variation attributed to fossil samples.

    Appendix

    The dot product is commonly used in vector transformations, but interpreting it in the context of a temporal trend may not be intuitive. The dot product of two vectors is the sum of the products of their respective elements:

    equation

    This product is a measure of the projection of one vector onto the other; it increases as the angle between the vectors (taken from the origin) decreases. The dot product of two perpendicular vectors is zero.

    The product-moment correlation between two vectors is:

    equation

    where zxi and zyi are standardized values of xi and yi, respectively. Thus, the product-moment correlation is the dot product of two standardized vectors divided by their rank ( - 1).

    In a randomization test, the different values of x and y are scrambled with respect to each other. However, the sample means ¯x and ¯y and the sample standard deviations sx and sy are constant in all of these randomized samples, because each includes exactly the same specimens. Thus, within any random set of permutations of a sample, the product-moment correlation can be obtained by a simple linear transformation from the dot product:

    equation

    References

    1. Elton S, Bishop LC, Wood B. Comparative Context of {Plio-Pleistocene} Hominin Brain Evolution. Journal of Human Evolution. 2001;41:1–27.
    2. Tobias PV. The Brain in Hominid Evolution. Columbia: New York; 1971.
    3. Silverman N, Richmond B, Wood B. Testing the Taxonomic Integrity of \\emph{Paranthropus boisei sensu stricto}. American Journal of Physical Anthropology [Internet]. 2001;115:167–178. Available from: http://dx.doi.org/10.1002/ajpa.1066
    4. Walker AC, Leakey RE, Harris JM, Brown FH. {2.5–Myr} \\emph{Australopithecus boisei} From West of {Lake Turkana}, {Kenya}. Nature. 1986;322:517–522.
    5. Holloway RL. The Endocast of the {Omo} Juvenile {L338y–6} Hominid Specimen. American Journal of Physical Anthropology. 1981;54:109–118.
    6. Brown B, Walker AC, Ward CV, Leakey RE. A new \\emph{Australopithecus boisei} cranium from east Turkana, Kenya. American Journal of Physical Anthropology. 1993;91:137–159.
    7. Falk D, Redmond, Guyer J, Conroy G, Recheis W, Weber GW, Seidler H. Early hominid brain evolution: a new look at old endocasts. Journal of Human Evolution. 2000;38:695–717.
    8. Holloway RL. ``{Robust}'' australopithecine brain endocasts: some preliminary observations. In: Grine FE Evolutionary History of the ``Robust'' Australopithecines. Evolutionary History of the ``Robust'' Australopithecines. Aldine de Gruyter: New York; 1988. pp. 97–105.
    9. Tobias PV. Cranial Capacity of \\emph{Zinjanthropus} and Other Australopithecines. Nature. 1963;197:743–746.
    10. Wood BA. The Cranial Remains from {Koobi Fora}, {Kenya}. Oxford: Clarendon Press; 1991.
    11. Holloway RL. Brain. In: Tattersall I, Delson E, Couvering VJ Encyclopedia of Human Evolution and Prehistory. Encyclopedia of Human Evolution and Prehistory. New York: Garland; 1988. pp. 98–105.
    12. Suwa G, Asfaw B, Beyene Y, White TD, Katoh S, Nagaoka S, Nakaya N, Uzaha K, Renne P, WoldeGabriel G. The first skull of Australopithecus boisei. Nature. 1997;389:489–492.
    13. Leigh SR. Cranial capacity evolution in \\emph{Homo erectus} and early \\emph{Homo sapiens}. American Journal of Physical Anthropology. 1992;87:1–14.
    14. Konigsberg LW. Temporal Aspects of Biological Distance: Serial Correlation and Trend in a Prehistoric Skeletal Lineage. American Journal of Physical Anthropology [Internet]. 1990;82:45–52. Available from: http://dx.doi.org/10.1002/ajpa.1330820106
    15. Hubert LJ, Golledge RG, Costanzo CM, Gale N. Tests of Randomness: Unidimensional and Multidimensional. Environment and Planning. 1985;A17:373–385.
    16. Lockwood CA, Kimbel WH, Johanson DC. Temporal Trends and Metric Variation in the Mandibles and Dentition of \\emph{Australopithecus afarensis}. Journal of Human Evolution. 2000;39:23–55.
    17. Wood B, Wood C, Konigsberg L. \\emph{Paranthropus boisei}: An Example of Evolutionary Stasis?. American Journal of Physical Anthropology [Internet]. 1994;95:117–136. Available from: http://dx.doi.org/10.1002/ajpa.1330950202
    18. Holloway RL. New Endocranial Volumes for the Australopithecines. Nature [Internet]. 1970;227:199–200. Available from: http://dx.doi.org/10.1038/227199a0

  • "Nutcracker Man" debunked

    Tue, 2011-05-03 00:44 -- John Hawks

    This week, Thure Cerling and colleagues report in PNAS [1] carbon stable isotope data from 24 specimens of Australopithecus boisei. This is a huge sample as fossil hominins go, and they give a very consistent picture about the diet of this most robust of the australopithecines. These 24 individuals got between 61 and 91 percent of their carbon from grasses.

    My 2005 explainer on stable isotope chemistry and early hominin diets fills in the details about carbon-12, carbon-13 and their relationship to 3- and 4-carbon photosynthetic cycles. The salient aspect of the comparisons involving A. boisei here is that C4 plants, mostly grasses, incorporate relatively more carbon-13 than do other plants, and herbivores assimilate this carbon-13 into their bones and teeth.

    The high ratio of grass-derived carbon in A. boisei is fundamentally different from all living and fossil apes, and it is far higher than the values found for other early hominins. The only other primate that comes close is the fossil giant gelada Theropithecus oswaldi, a savanna-living species.

    What were these extinct species really eating? Was grass the food? For living geladas, grass consumption includes seeds -- a fact that led Clifford Jolly to suggest that early hominins might also have specialized on seeds [2]. Of course, humans today also specialize on grass seeds. We call them grains, eat them in bread and drink them in soda. And beer.

    But what about A. boisei? The large, thick-enameled premolars and molars, with their low cusps, seem well suited to grinding small hard objects and resisting the resulting wear. But Cerling and colleagues devote a good chunk of their discussion to the description of molar wear in A. boisei and other early hominins. Their argument is that the teeth of A. boisei show no signs of "hard object" feeding:

    Of perhaps greater moment than its potential specific simila- rities, the microwear of P. boisei molars, which shows remarkable uniformity over time from about 2.3 Ma to about < 1.4 Ma (9, 24), stands in stark contrast to the wear fabrics exhibited by primate hard-object consumers. Indeed, there is no evidence beyond the anecdotal [e.g., the broken left first permanent molar crown in the KNM-ER 729 P. boisei mandible (8) and the observation that a couple of P. boisei molars show antemortem enamel chipping (25)] that these food items were hard.

    These observations are not new, but putting them together with the evidence of grass consumption makes it pretty clear that seed eating was not a predominant source of dietary carbon. The "Nutcracker Man" sobriquet, applied to A. boisei because of its powerful jaw mechanics, must be false. No significant hard object feeding, very low dietary carbon from trees and non-grassy (or sedgy) plants.

    Instead, Cerling and colleagues propose that both A. boisei and other early hominins wore their teeth on the, well, grassy parts of grass.

    P. boisei cheek teeth display notable gradients of gross wear, resulting in large, deeply excavated dentine exposures, and in this regard, they are similar to other australopith species (e.g., A. afarensis and A. africanus) that also possess low tooth cusps with thick enamel. Thus, like other australopiths, P. boisei undoubtedly had a diet that consisted of foods with abrasive qualities—the gross wear is as likely due to repetitive loading of phytolith-rich tough foods as exogenous grit. Thus, either grass or sedge consumption and/or exogenous grit might well have contributed to P. boisei’s notable wear gradient.

    And:

    Recent dental microwear studies suggest that the mechanical properties of A. afarensis (and A. anamensis) diets were nearly identical to those of P. boisei (9, 24, 40–42). If this is so, could it be that the australopith masticatory package represents an adaptation to C4 resources such as grasses or sedges? The similarity in dental microwear fabrics among the eastern African australopiths, all of which lack any evidence for hard-object food consumption (9, 24, 40–42), is consistent with the notion that their craniodental morphology could reflect “repetitive loading” rather than hard-object consumption (7, 8, 43).

    Grit might get in from eating underground parts like rhizomes. Phytoliths are small, hard silicate structures in the green parts of plants, including the stems and leaves of grass.

    Last year I wrote about carbon isotope analysis of two specimens of Australopithecus boisei, the famous OH 5 "Zinj" specimen, and the Peninj mandible. Both specimens show evidence of a high consumption of grass-derived carbon -- estimated at 77% and 81% grass-derived carbon, respectively. Those levels are characteristic of grazing animals. Cerling and colleagues show that these values are right in the middle of the range among specimens of A. boisei that cover a half million years in Kenya and Tanzania.

    In the paper reporting the carbon stable isotopes of OH 5 and Peninj, van der Merwe and colleagues [3] suggested that A. boisei may have relied on papyrus as a staple. The culms and rhizomes of papyrus both have substantial nutritional content but are very fibrous and require much chewing and spitting out fiber at intervals. The hypothesis would imply that A. boisei relied on these foodstuffs for the majority of its calories.

    Cerling and colleagues do not mention papyrus, and take a much more direct approach on grass-eating. But they do report data on oxygen stable isotopes from the specimens that may be relevant to the ecological context of grass (or sedge) consumption. Oxygen isotopes in bone and teeth reflect the pattern of water consumption by an animal. Oxygen-16 evaporates and transpires preferentially from leaves, so an animal living in an arid environment that gets most of its water from plants will be relatively enriched for the heavier oxygen-18. An animal that depends on drinking water from lakes or rivers will tend to have lower oxygen-18. A. boisei is almost as low in oxygen-18 composition as hippopotamus, suggesting they were strongly dependent on water sources.

    A highly water-dependent grass-eating A. boisei is a very different picture of the biology of this robust species. The South African robust species, A robustus, is very different in this regard. These two species are often lumped together, but this is unfair in many ways to their distinctive anatomical patterns. Knowing that their dietary adaptations were very distinct, we should be more inclined to focus on the details where they differ.

    Bottom line: A. boisei represents a highly distinctive dietary pattern, not present in any living ape, that no longer exists. At least the giant gelada, T. oswaldi, may also have exploited similar resources. Some grass resources, including papyrus corms and rhizomes, have high caloric and nutritional value, but require adaptations to deal with the fibrous content.

    References

    1. Cerling TE, Mbua E, Kirera FM, Manthi FK, Grine FE, Leakey MG, Sponheimer M, Uno KT. Diet of Paranthropus boisei in the early Pleistocene of East Africa. Proceedings of the National Academy of Sciences [Internet]. 2011;108:9337–9341. Available from: http://dx.doi.org/10.1073/pnas.1104627108
    2. Jolly CJ. The seed eaters: a new model of hominid differentiation based on a baboon analogy. Man. 1970;5:5–26.
    3. van der Merwe NJ, Masao FT, Bamford MK. Isotopic evidence for contrasting diets of early hominins Homo habilis and Australopithecus boisei of Tanzania. South African Journal of Science [Internet]. 2008;104:153–155. Available from: http://www.scielo.org.za/scielo.php?pid=S0038-23532008000200016&\#38;script=sci\_arttext&\#38;tlng=en
    Tags: 
    Early Pleistocene
    Koobi Fora
    Tanzania
    A. boisei
    diet
    Kenya
    Olduvai Gorge
    A. robustus

  • Papyrus and A. boisei

    Fri, 2010-06-11 17:20 -- John Hawks

    I've had on my stack for quite a long time, a short paper by Nicholas van der Merwe and colleagues, assessing the stable carbon isotope ratios in several specimens from Tanzania. These include the Homo habilis specimens OH7, OH62 and OH65, and the A. boisei specimens OH5 and the Peninj mandible.

    The ratio of stable carbon-13 and carbon-12 enable an assessment of the amount of C4 versus C3 plants in the diet. I discussed the basic ideas in a longer post from 2005.

    The results on the Homo specimens are not too surprising. All three specimens overlap with South African A. africanus. OH7 and OH62 in particular have values around 20% C4, which is right near the mean observed for South African Homo and A. robustus from Swartkrans. OH65 has a higher C4 percentage than the other two, but within the range observed for Sterkfontein Member 4 A. africanus, which was significantly higher than Makapansgat or the other South African samples. So it would appear that the diet of Homo habilis did not differ from earlier hominins in terms of the ultimate origin of carbon in grasses versus non-grass plants.

    What is more surprising is the extremely high amount of C4-derived carbon in OH5 and Peninj. They score 77% and 81% C4, respectively. These are the only two specimens of A. boisei for which these stable isotopes are known, and they are very far from the observed range in the South African A. robustus.

    The authors suggest an interesting source for this high C4 proportion -- papyrus. They described a tasting tour of the wild plants of the Okavango:

    Bamford and van der Merwe investigated (and ate) the edible plants of the Okavango Delta in Botswana during the dry season (July 2003), assisted by Ezaya Karesaza, a tourist guide who grew up in this extensive wetland. Among the C3 plants that are traditionally eaten raw in this region are a variety of fruits and seeds, as well as plants of which the leaves and rhizomes are eaten. The latter include Aeschynomene fluitans, a floating legumi- nous plant, of which the leaves taste like lettuce; Typha capensis, which grows in thick stands along the water’s edge, of which the rhizomes have a pleasant taste; and Schoenoplectus corymbosus, a big water sedge, of which the stem is succulent at the bottom end. Among C4 plants, the rhizomes and culms of three other species of sedges are edible. These include Cyperus denudatus and C. dives, which grow in the grasslands of the floodplains. Unlike the grasses, they are green year-round, although not particularly prolific. The most common C4 sedge, by far, is Cyperus papyrus, which grows in dense thickets along the water edge. This species has culms as high as 4 m, of which the lowermost 0.5 m is frequently chewed by local people. It has a soft, white rind about 0.5 cm thick; the interior, about 2 to 3 cm in diameter, is more fibrous. It is chewy and pleasant tasting. The thick rhizome of papyrus is more fibrous and starchy than the culm, somewhat astringent, and requires considerable chewing effort. It produces a bolus in the mouth that has to be spat out at intervals.

    They then reported the results of a nutritional analysis of the papyrus culm and rhizome, which have roughly the nutritional and caloric value of domestic potatos, although would require a significant gut flora to deal with the cellulosic content.

    All in all, it's very curious that A. boisei is so different in these isotopic values compared to other early hominins. The theme was picked up last year in a paper by Richard Wrangham and colleagues, who focused on the idea of "fallback foods" -- the kinds of foods that an animal does not prefer, but eats when other more highly preferred foods are not available. Considering the very high C4 proportion indicated by the OH5 and Natron isotope values, it doesn't seem likely that this reflects a fallback strategy, but possibly an initial exploitation of such resources as fallbacks facilitated a later, more developed adaptation to them.

    Related posts:

    "Chemistry and early hominid diets"

    "Robust australopithecine diet ablated"

    "Average diet versus extreme diet in robust australopithecines"

    References:

    van der Merwe NJ, Masao FT, Bamford MK. 2008. Isotopic evidence for contrasting diets of early hominins Homo habilis and Australopithecus boisei of Tanzania. S Afr J Sci 104:153-155.

    Wrangham R, Cheney D, Seyfarth R, Sarmiento E. 2009. Shallow-water habitats as sources of fallback foods for hominins. Am J Phys Anthropol 140:630-642. doi:10.1002/ajpa.21122

    Tags: 
    A. boisei
    diet
    Olduvai Gorge
    Australopithecus
    nutrition
    Tanzania

  • Shrinking erectus

    Tue, 2010-04-27 10:02 -- John Hawks

    Ann Gibbons reports on the AAPA meetings with a story about all the Homo erectus pelvis and stature papers ("Human ancestor caught in the midst of a makeover," subscription required). Research on the proportions of early Homo was the main event of the meetings, and Gibbons really caught the highlights of the story.

    I wrote about body size in Homo erectus a few months ago, and much of the story follows from the basics I outlined there ("The changing height of Homo erectus"). But there I emphasized that the estimated adult height of KNM-WT 15000 was an outlier in a relatively small body size distribution.

    What I didn't anticipate is that some interesting work might come along to question the tall adult stature estimate for that skeleton. Gibbons describes the work of Ronda Graves and colleagues, presented at the meetings:

    Using intermediate growth rates, graduate student Ronda Graves of Stony Brook University in New York state calculated that Nariokotome Boy would have had less time than originally predicted to reach his adult height when he died. She estimated at the meeting that he would have reached 163 cm in height and 56 kg in weight as an adult—"shorter and wider" than previously thought.

    This seems very short, at least when I first saw it. On reflection, Ohman and colleagues (2002) had provided a stature estimate at death of KNM-WT 15000, as only 147 cm, and they suggested it might have been as short as 141 cm. That's an awful lot shorter than had previously been estimated on the basis of regressions.

    If Graves and colleagues are right about the lack of a human-like growth spurt, an additional 20 cm (8 inches) wouldn't be unusually small for an adult stature. Those stature estimates would put KNM-WT 15000 between the 50th and 90th percentiles for American 10-year-old boys, or between the 25th and 75th percentiles for 11-year-olds. By contrast, an adult stature of 163 would be around the 3rd percentile for adult American men. The assumptions about growth totally determine the outcome for adult height.

    The credibility of the growth assumptions can only be tested by looking at other adult and juvenile remains. There is much more to say on this topic, but I'll point out one relevant comparison: The estimated stature of the adult skeleton from Dmanisi, including the complete D4167 femur and D3901 tibia, is between 145 and 166 cm. Graves' KNM-WT 15000 stature estimate is right within this range.

    Meanwhile, there was a lot of disagreement about hips.

    [Scott] Simpson and Linda Spurlock of the Cleveland Museum of Natural History realigned the pieces of Nariokotome Boy's pelvis, guided by a female H. erectus pelvis from Gona, Ethiopia, that Simpson reported 2 years ago (Science, 14 November 2008, p. 1089). They found that the widest measure from side to side on the boy's pelvis is 255 to 260 millimeters rather than 225 to 230 mm. This would give the boy an adult hip breadth of 295 to 301 mm rather than the 266 mm originally proposed, and would match those of the short, wide-hipped female from Gona, whose pelvic breadth was 288 mm. "H. erectus was not simply a small-brained modern human," says Simpson.

    Simpson's reconstruction seemed reasonable, and it's actually not that big a difference -- roughly an inch and a half (3 cm) in bi-iliac breadth. The main differences were in the overall shape of the pelvis, being shorter with a more flaring iliac blade.

    Gibbons describes the disputation that happened after Chris Ruff's presentation. Ruff has suggested that the Gona pelvis may not represent Homo -- that its broad proportions and small acetabula (hip sockets) suggest it may have belonged to an australopithecine (presumably, A. boisei).

    Much of the disagreement comes down to the estimation of femur head diameter from acetabulum breadth -- Ruff (2010) gave an estimate of 32.6 mm, Simpson and colleagues estimated between 35 and 36 mm, based on a different method. What you would want is enough acetabula of both genera to be able to examine their variation directly. We don't have such a sample; what we have are a few acetabula and several femur heads. We have the additional problem that living people seem to have a different relation of femur head and acetabulum diameters than in other anthropoids, and it's not obvious which should be applied to early hominins.

    I guess (in the relative absence of data) that this acetabulum diameter of the Gona pelvis was in the zone of overlap between Homo and Australopithecus. There's no question that later Homo -- say after 1 million years ago -- is substantially larger in acetabulum diameter, from every specimen so far described. But there are occasional small specimens of Homo even in the Middle Pleistocene. At 1.15 million years old, the Gona specimen is more than 300,000 years later than the last known occurrence of Australopithecus. The femur head that would fit the Gona acetabulum would be smaller than KNM-ER 1472 or D4167 from Dmanisi, both around 40 mm. At least one australopithecine femur head (AL 333-3) is that large, so the femur head diameter distributions do overlap. The STW 431 acetabulum diameter is a sliver larger than that of the Gona pelvis (Ruff 2010 makes it 3 mm bigger, but other workers have given a smaller estimate). SK 3155 may well be Homo and has a smaller acetabulum.

    Of course, if we go as far as SK 3155, we have to consider the topic of the Malapa innominate. Can we tell small-bodied Homo from Australopithecus on the basis of pelvic morphology? Several people writing about the Gona pelvis have made it sound like a bigger version of Lucy's. But that's not really true. The australopithecine-like appearance comes from its breadth and consequent features, including the long pubes and flaring anterior ilia. The rest? Maybe there's something here for a clever anatomist.

    UPDATE (2010-04-27): I have some e-mail about the last occurrence of A. boisei, which I wrote above was more than 300,000 years older than the Gona pelvis.

    The most potent counterargument is Swartkrans Member 1, which has uranium-lead dates around 830,000 years ago, and has been placed by many workers around a million years ago. I actually hadn't been thinking of South Africa. But it is relevant, as the East African record between 1.4 and a million years ago may not be strong enough to argue that the last occurrence of A. boisei is really very close to the extinction time.

    Meanwhile, there is OH 36, an ulna from Olduvai Gorge that may represent A. boisei. Since it's (obviously) not cranial, and is quite large and robust compared to postcranial remains that are associated with A. boisei, I've always been very skeptical of that assessment. If there's one feature of the ulna that actually has some phylogenetic importance in the Early Pleistocene, I figure it's size.

    But given the current question about body size, that reason for skepticism may have receded in importance. On the other hand, OH 36 seems to represent a substantially bigger individual than the Gona pelvis, so maybe introducing robust australopithecines into the mix doesn't help anything.

    Several things puzzle me. Even into Member 1 times, Swartkrans is dominated by A. robustus, with very little Homo. In East Africa, A. boisei is never quite so predominant in the hominin assemblage as the case in South Africa, but was nevertheless very common up to 1.5 million years ago. Did it persist much later? Was it cryptic from the point of view of the fossil record? Are the Swartkrans dates older than we think?

    References:

    Gibbons A. 2010. Human ancestor caught in the midst of a makeover. Science 328:413. doi:10.1126/science.328.5977.413

    Ohman JC, Wood C, Wood B, Crompton RH, Günther MM, Yu L, Savage R, Wang W. 2002. Stature-at-death of KNM-WT 15000. Hum Evol 17:129-141. doi:10.1007/BF02436366

    Ruff C. 2010. Body size and body shape in early hominins -- implications of the Gona pelvis. J Hum Evol (in press) doi:10.1016/j.jhevol.20 09.10.0 03

    Tags: 
    body size
    stature
    East Africa
    Homo erectus
    Malapa
    A. robustus
    pelvis
    Early Pleistocene
    Swartkrans
    Gona
    KNM-WT 15000
    A. boisei
    meetings
    Synopsis: 
    The 2010 AAPA meetings featured a fight about the Nariokotome and Gona pelves.

  • Average diet versus extreme diet in robust australopithecines

    Wed, 2008-05-07 00:38 -- John Hawks

    I've followed the literature on early hominid diets from the beginning of the weblog. In 2005 I discussed Peter Ungar's analyses of dental occlusal morphology in A. afarensis versus Homo, concluding:

    The contrast between Homo and A. afarensis is in the same direction as the contrast in occlusal morphology between primarily meat-eating carnivores like felids and canids as opposed to more omnivorous carnivores like bears. Another observation is that meat is a major food resource of chimpanzees, although this is hardly a fallback resource. Indeed, if meat eating was indeed an important component of the behavioral repertoire of early Homo, it probably is not fair to assert that the difference in diet between Homo and Australopithecus was primarily a difference in fallback resources. It may be true that australopithecines and early Homo overlapped in their food resources, particularly in plant species consumed. But considering the likely effectiveness of early humans as predators, I think it likely that the fallback foods of early humans--when hunting was ineffective--may well have been the preferred foods of australopithecines. And when australopithecines were forced to abandon their preferred foods by early humans, they were forced to fall back upon resources that either were common or were difficult for early Homo to exploit. The disappearance of early small-bodied Homo by around 1.6 million years ago, and the ultimate extinction of the robust australopithecines after a progressive increase in their molar sizes (Wood and Lieberman 2001) indicate that this fallback strategy could not be maintained in the face of increased hunting effectiveness by large-bodied Homo.

    The concept of "fallback foods" has captured a large mindshare in explaining early hominid diets. The idea is that a species may depend on preferred, staple foods for most of the year, but adopt less preferred, "fallback" foods when their staple is not available -- for instance, during the dry season.

    What can fallback foods explain about early hominids? For one thing, they could explain the difference between robust and non-robust australopithecines. We know from isotope data (reviewed in this 2005 post about Matt Sponheimer's work) that A. africanus and A. robustus had similar fractions of C3 and C4 plant source foods in their diets. Across the year, they may have eaten roughly the same mix of foods. A 2005 paper by Greg Laden and Richard Wrangham (discussed here) explored the idea of underground storage organs of plants, or tubers, as fallback foods for australopithecines. Later studies of isotope data using laser ablation of small segments of the enamel (discussed here) showed that diet proportions may have substantially varied across the time that teeth were developing -- possibly concordant with the idea of seasonal or longer-period fallback foods. An earlier analysis of dental microwear in the two hominids by Scott and colleagues (discussed here) came to a similar result: there was great variability in wear properties, especially within A. robustus, although the average in the two species showed a possibly greater fraction of brittle, hard foods consumed by the robust australopithecines.

    So I've written about the topic a lot, and followed it closely.

    Now, Peter Ungar, Frederick Grine and Mark Teaford have examined the wear properties of the molars of Australopithecus (Paranthropus) boisei. They find that -- unlike A. robustus -- none of the seven specimens showed any evidence of having eaten hard or brittle foods:

    Comparisons with the extant baseline series suggest that none of the Paranthropus boisei individuals examined consumed extremely hard or extremely tough foods in the days before death. All of these specimens lacked the extremes of Asfc evinced by Lophocebus albigena and especially Cebus apella, both known to consume hard, brittle foods. Paranthropus boisei molars also lacked the extremes of epLsar seen in Trachypithecus cristata and Alouatta palliata, both known to consume tough leaves and stems. The P. boisei individuals examined evidently avoided such metabolically challenging foods, at least in the days before death. This is notably consistent with Walker's [23] early assertion that P. boisei microwear patterns resemble those of living frugivores, and differ from those of living grazers, leaf browsers, and bone feeders.

    Comparisons with the South African hominins suggest that while Paranthropus boisei may have consumed foods with similar ranges of toughness as those eaten by Australopithecus africanus, the eastern African "robust" hominin did not eat harder and brittler foods than the South African "gracile" form. Further, the patterns for P. boisei and P. robustus are very different. Paranthropus robustus likely ate foods that were on average much harder and less tough than P. boisei. The differences in both central tendencies and ranges of variation suggest different feeding strategies, and by implication, that the two species of Paranthropus probably had markedly different diets or foraging strategies (Ungar et al. 2008, italics lost).

    That is very interesting that A. robustus and A. boisei are so different in their microwear patterns. It makes me wonder whether there may have been substantial habitat variation in the use of hard foods -- maybe the extant A. robustus sample, mainly drawn from a small area of South Africa, had access to some food items that were rare or absent across the larger East African range of A. boisei. But if some A. boisei populations had also depended on such hard resources some of the time, you might expect that we would have found one, or at least a bit more variability. Yet the sampled specimens, drawn from a distance from Ethiopia to Tanzania and well over a half million years of time, are pretty uniform in their microwear, showing some variability in the anisotropy dimension (here, high values have mostly parallel striations, attributed to fibrous food consumption).

    So we can return to the question: the major hominid competitor of A. boisei was Homo. Both lineages appeared in the period around 2.5 million years ago, and remained sympatric throughout the next million years. Some of the dynamics of that interaction must have involved diet (considering the different dietary adaptations of the two). We can speculate that A. boisei didn't get much meat, which would then be an important difference. But what else was A. boisei eating?

    Meanwhile, the data are still consistent with the idea of fallback foods in A. robustus as a driver of dental morphology, but the story for A. boisei now seems less clear. With only seven specimens, there is almost certainly not enough data to test the hypothesis -- which after all predicts that the use of hard brittle foods may be rare. But that's not positive evidence either. Is there some other food that might explain the hyperrobust craniodental morphology?

    References:

    Ungar PS, Grine FE, Teaford MF (2008) Dental Microwear and Diet of the Plio-Pleistocene Hominin Paranthropus boisei. PLoS ONE 3(4): e2044. doi:10.1371/journal.pone.0002044

    Tags: 
    A. boisei
    diet
    A. afarensis
    A. africanus
    A. robustus

  • An interview with Michelle Drapeau

    Sat, 2008-01-12 20:17 -- John Hawks

    I've been trying to spread the interviews across the field in various directions. I (virtually) talked with Mica Glantz about Neandertals, Adam Van Arsdale about early Homo, and Anne Weaver about human brain evolution, all the australopithephiles in the readership are probably feeling neglected.

    So I wrote to Michelle Drapeau, who was very generous in answering questions about her work on the anatomy of early hominids and her recent field work in Ethiopia. Michelle is on the faculty of the Université de Montréal, in the Department of Anthropology. She serves as co-director of field operations in the Bala Paleoanthropological Research Area of southern Ethiopia.

    Hawks: I will start out by asking about your dissertation work, which centered on the new partial skeleton from Hadar, A.L. 438-1. How did you get involved in that analysis?

    Drapeau: It's a case of being at the right place at the right time. Bill Kimbel and Don Johanson had asked my advisor at the time, Carol Ward, to describe all the postcranial material recovered from the field in Hadar since 1990. Among those specimens was the partial skeleton of A.L. 438-1 which included associated fragments of the humerus, clavicle, radius, right ulna, mandible, and frontal as well as a complete left ulna, right and left second metacarpals and left third metacarpal. Considering the relatively numerous body parts from one individual, Carol thought the specimen deserved a more detailed analysis. I was Carol's Ph.D. student at the time and the 438-skeleton (as we started to call it) appeared like an ideal subject.

    Hawks: What did you have to learn to be able to undertake the work?

    Drapeau: I had to learn a lot! My master's thesis was in the history of science field, so all the functional anatomy, including the descriptive and comparative aspects were completely new to me. It was something I really wanted to do, however, so I really enjoyed immersing myself into it.

    Hawks: A.L. 438-1 exhibits more curvature across its length than A.L. 288-1, an issue that you discussed in your analysis of the fossil. I have always been puzzled by the problem of ulna curvature -- mainly because I've always been puzzled by the comparison of later, larger, and more curved fossils like Omo L40-19 and OH 36 -- and then, of course, KNM-WT 15000 is a lot more like most recent humans. Do you have any insights about these contrasting morphologies?

    Drapeau: Forearm bone curvature is an intriguing issue. Intuitively, it makes sense to assume that curvature reflects arboreality since the curvature of both the ulna and radius give greater area on the interosseous membrane for attachment of forearm muscle important for arboreal locomotion such as the finger flexors. However, orangutans and gibbons do not have the most curved forearm bones. It is an honor that goes to gorillas, definitely not the most arboreal animal of the bunch. If the area of muscle attachment is the variable that interests us, then it is important to take into account forearm length as well. When that is done, species generally sort by locomotor preference, with the most arboreal having the greater ‘area' for muscle attachment relative to body size and humans having the smallest (at least, when measured on the ulna). So gorillas appear to have very curved forearm bones because they also have relatively short forearms when compared to other apes.

    The differences between A.L. 438-1 and A.L. 288-1 are fairly minor and probably reflect normal within-species variation. Neither is very curved and they may belong to a population with slightly more curved ulnae than modern humans but definitely less curved than any extant apes.

    The KNM-WT 15000 specimen is pretty much what you would expect an ulna belonging to a completely terrestrial biped to look like, i.e., it is not particularly curved. Since it is a juvenile, it is difficult to compare it to other fossils, but there is nothing really surprising about it.

    That said, what about the intriguing Omo L40-19 and OH 36? These specimens present combination of morphologies that are difficult to underscore in quantitative analyses. The former had a human-like proximal morphology but a really long and curved (ape-like) diaphysis. The latter, OH 36, has a general ape-like morphology with a pronounced curvature, but is unique in a few characters. The whole bone (proximal articulation and diaphysis) is very constricted medio-laterally, more comparable what is observed in monkeys (and it is not the result of distorsion). Despite its general ape-like morphology, it has an olecranon process that projects proximally like no other ape of its size. It is definitely much more human-like for that trait and it is generally agreed that it is a hominin. McHenry and colleagues argue in a recent article (AJPA, 134: 209-218) that these two fossils are very different and can hardly be accommodated into the same genus (Paranthropus) as it is usually done (probably by default). McHenry and colleagues argue that it may indicate Paranthropus is in fact a polyphyletic taxon. They also conclude, as I stated above, that OH 36 is unlike anything living today.

    So, if curvature of the ulna reflects arboreality, does it mean that these fairly recent fossils were much more arboreal than A. afarensis? Remember that they are big ulnae, particularly L40-19, likely belonging to large individuals.... Maybe the Paranthropus clade (if indeed it is a clade) is more arboreal than A. afarensis? This would imply either reversal of behavior or that A. afarensis is not ancestral to Paranthropus. Or, alternatively, could the curvature in these individuals reflect forelimb muscularity but not necessarily related to arboreality? As you can see, I have many more questions than answers. All this variability suggests that the behaviors of fossil hominin species were much more variable than what we have been used to think and may have been (very?) different from the behaviors of extent species.

    Hawks: Of course, the big debate about forelimb proportions is the idea that they may have been very different (and more apelike) in A. africanus compared to A. afarensis. (reviewed by Green, Gordon, and Richmond 2007) What do you think about the issue?

    Drapeau: That idea first met with some resistance because it involved a reversal of proportions from A. afarensis to A. africanus and implied a more arboreal behavior in the latter than the former. Given that Homo habilis is often described has having more ape-like proportions than A. afarensis, it also implied that A. afarensis may not be the ancestor of the Homo lineage (an idea more recently suggested by Yoel Rak and colleagues based on mandibular data). Since I remain unconvinced of the primitive proportion of H. habilis, I am not so certain that the 'derived' proportions of A. afarensis exclude it from being an ancestor to the Homo lineage.

    Back to the differences between the two australopithecine species. Despite original skepticism, the data appears to be robust and the differences in joint size between A. afarensis and A. africanus appear to be real. As observed in the previous question, this variability may reflect locomotor differences possibly related to differences in the environment. If A. afarensis was still occasionally arboreal, is it too hard to imagine that, if the environment is changed (more wooded, greater predator pressure, more resources found in trees, etc.), the percentage of arboreal behavior would increase and that the proportions would revert to being more chimp-like in A. africanus? Again, there is no reason to assume that all early hominins, because they were bipedal, were identical in their locomotor behaviors.

    I want to underscore that these differences are in joint SIZE, not in limb length, and reflect relative loading of the limbs. Usually, the major source of loading of the limbs is related to locomotion, but it is an assumption that cannot be verified in early hominins. If, as stated above, OH 36 is unlike anything living today, maybe it did things that have no modern equivalent. And the same can be said of other hominin species including A. africanus with its 'apparent' primitive proportions.

    Hawks: You have recently been involved in field research in the Bala-Weyto region of southern Ethiopia. Can you describe the site, and your role?

    Drapeau: The Bala–Weyto basin is part of a series of small parallel rifts that link the northern limit of the East African Rift to the southern limit of the Main Ethiopian rift. These small rifts constitute today a string of many small basins. The Bala-Weyto basin is located east of the Omo river basin. It is a region more difficult to survey when compared to dryer region because of the vegetation coverage that limits exposures visibility and access. However, it is little-explored paleoanthropologically speaking. Work in the Konso, another small basin a few kilometers away, but at a higher altitude, has a fauna with a certain degree of endemism and an A. (P.) boisei specimen with unique morphological variations. Among other things, we want to know if this variation and the faunal endemism are due to the relative isolation of the basin or to its particular environment. These answers may be found in contiguous basins that vary in their physical characteristics, such as the Bala-Weyto basin.

    I am co-director of that project with Elizabeth Harmon of the City University of New York. At this stage of the project, being co-director involves organizing the whole expedition, securing funding, and coordinating the work of other team members. I would say that the most time consuming aspect is coming up with money and getting everything moving in the field. As a director, I am responsible for the team's well-being and it is a pressure that can sometimes weigh heavily on my shoulders. It is nice to be able to share the burden with a co-director.

    Hawks: Do you involve students in your work?

    Drapeau: My funding is limited and field work in Ethiopia is not particularly cheap. However, I plan to bring one student in the field this summer. I look forward to share this experience with a highly motivated student!

    Hawks: Many of us have heard about the difficulties of field research, particularly in East Africa. What are some of your biggest challenges?

    Drapeau: Doing field work in Ethiopia can be a challenge for many reasons. As can be expected, there are numerous permissions, letters, official documents, etc., that are required and the bureaucracy is somewhat heavy. However, I find Ethiopians very helpful and professional and, usually, the quest for documents goes smoothly, particularly once you know what to do and in what order.

    A second difficulty is the access to the sites. Ethiopia did not have one highway until relatively recently and road traveling remains an experience that can be frightening. A lot of work is being done on the roads, however, and I believe that things will keep improving. Access to the research area involves off-road traveling as well, with all the difficulties that it entails. When you leave for the field, you have to be a self-sufficient unit, relying on the local environment as little as possible. It is still necessary to get gasoline on a regular basis, but except that, we try to be as autonomous as possible. It is particularly important when you go to a new area and don't know what (if anything) will be available to you.

    A third aspect of field work, particularly in Ethiopia, is the politics, the paleoanthropological politics that is. Although most scientists are polite and civilized to each other, I really feel that we had to walk on eggs when we were researching an area in which to conduct field work.

    A final difficulty (and certainly not the least) in our situation, is to find an area that has fossiliferous exposures of a time period that interests us and in which we can work at least a few years. The numerous discoveries that are made in East Africa give the impression that finding hominin fossils is something easy to do, but it usually involves many years of surveying. We are still at the exploratory phase of our project, i.e., we are still actively looking for an area that could sustain scientific work for a few years. Hard work (and perhaps a little luck) is essential.

    Hawks: You had a lot of field experience before going to Ethiopia. How did you get your start?

    Drapeau: At the end of my undergraduate degree, I had the chance of getting a couple of paying jobs in prehistoric archaeology. It was the beginning of a series of jobs in field archaeology conducted in parallel to my studies. I used to think (and still do) that these were the best summer jobs an anthropology student could have. The pay check was very descent and it usually came with room and board. These jobs allowed me to see many regions of Quebec and Canada that I would otherwise have never visited and to do things I would probably have never done otherwise. I have flown in helicopters for hours (and even survived a major crash), piloted a hydroplane (just for a few minutes, but still!), hear wolves howl into the night while trying to sleep in a tent hundreds of miles from any road or civilization, dipped my foot in the arctic ocean (too chicken to swim), seen the midnight sun, and I could go on. This fieldwork experience, and a stint in the Caune de l'Arago in Tautavel, France, opened another door: to be invited to do field work in Hadar in 2000.

    Hawks: Any interesting stories?

    Drapeau: I have an anecdote that I find amusing, but mostly informative on the nature of humans. When we were doing field work in the Bala basin, our camp was set up about a 2-hour drive off the road. It was clear that the local people had seen very few foreign workers. For the whole time we were there, we had a constant group of people just sitting in the shade observing us like zoo animals, watching our every move, laughing when we did things unexpected, etc. We were quite the entertainment. The occupation of the local Mali people appeared to be tending their few sorghum fields, but mostly to take their sometime large herds of cows, goats and sheep a few miles down to the river for a drink every day. Even though it was not that hot, the men walk around wearing only colorful underwear (the Speedo-type) and it was sometimes literally falling apart. From our western perspective, they really seem to have almost nothing. Anyhow, after a few days in the field, some crew members were starting to crave fresh meat. We agreed to allow the cook to purchase one goat from a local herder. We didn't think it would be a problem given the large quantities of these animals around and our willingness to pay a fair price for it. It came as quite a surprise that no one was willing to sell us any! It turned out that goats, sheep and cows were not herded to be eaten or even milked, but were really just status items. One man from the village nearby apparently owned more than a hundred head of livestock but was still unwilling to sell. We were all quite shocked of the apparent frivolity of it all, particularly considering that food (for humans and beasts) did not appear to be particularly abundant in the region. But then, we couldn't miss seeing the connection to what we can observe in the western world: huge houses for one or two people, oversized and overpriced cars. These are just to show off. The same frivolities, although expressed slightly differently, can be found anywhere. I guess it really is in the human nature. We were finally able to convince someone to sell us a goat, but we paid a really high price.

    Hawks: Congratulations! You seem to be a very busy person right now, both professionally and personally. What's next for you?

    Drapeau: I just started one of the most challenging projects of my life, a project that will keep me busy for the rest of my life. His name is Henri and he is almost 8 months old. Professionally speaking, I am investigating manipulatory adaptations in the early hominin hands and the morphology of muscle markings. However, one of my main objectives in the next two years is to settle on a specific field research area with good scientific potential.

    Tags: 
    A. africanus
    Hadar
    A. boisei
    Homo habilis
    A. afarensis
    Neandertals
    Omo

  • An interview with Adam Van Arsdale

    Sat, 2007-12-01 20:50 -- John Hawks

    After my Q and A with paleoanthropologist Mica Glantz, I got a lot of great response -- people really liked reading about work in the field from somebody other than me!

    So, I'm going to make these interviews a regular feature. When I was in Michigan last week, I got a chance to talk with Adam Van Arsdale, who graciously agreed to answer some questions about his work.

    UPDATE(11/29/2007): After posting, I heard from a reader who reminded me that I omitted Adam's affiliation and info! Adam is a lecturer in anthropology at the University of Michigan. You can find out more about his interests on his webpage.

    Hawks: You were lucky enough to work at one of today's most exciting paleoanthropological sites, Dmanisi. What can you tell us about your experience there?

    Van Arsdale: Dmanisi is a wonderful place and I can't say enough positive things about the site and all the people I have worked with through the project. To begin, the site itself is just a nirvana for anyone with an interest in history or prehistory. The primary excavation area is in the middle of a ruined medieval citadel complex which rose to prominence as a trading town along the silk road; down from the promontory are the tombs of Mongols who sacked the city in the 12th century; further down are early Christian burials, and along the river are the remains of bath houses for travelers along the Silk Road. It is a literally a place where time seeps out of the ground.

    Leaving the setting aside, the people associated with the project have been wonderful to work with. The size of the excavation team would vary but there would be times when, at the end of a long excavation day, I would find myself sitting at a long dinner table surrounded by 40 people speaking more than half a dozen languages. In the years I worked there as a graduate student I think we had students and researchers from 15 different countries (and I'm probably missing a few). Everyone who works at the site, including the local residents of Patara Dmanisi, adds their own character to the project. As a graduate student, my summers at Dmanisi served as something of a Paleoanthropology bootcamp, with regular discussions and debates between all of us with very different training and different theoretical perspectives on the issues of human evolution.

    And then on top of all of this there are, of course, a remarkable set of fossils and archaeological materials.

    Hawks: Do you want to give a shout-out to anybody in Georgia?

    Van Arsdale: There are too many to name, but certainly David Lordkipanidze, who first invited me to Dmanisi in 2001, deserves recognition. I'll also add Gocha Kiladze, Teona Shelia and Dato Zhvania, who began working at Dmanisi in 1991 as students and who continue to play a significant role in the operation of the site today. One of the great things about the site is that it has served as a tremendous springboard for Georgian students interested in paleoanthropology. I think it is a safe bet we will be hearing a lot from our Georgian colleagues in the years ahead.

    Hawks: Your dissertation work focused on the Dmanisi mandibles. I know that you still have publications coming out on these, so feel free to keep quiet about anything you're saving for print. What can you tell us about the sample?

    Van Arsdale: The Dmanisi mandibles are a remarkable sample. They show a huge amount of morphological variation in a set of fossils derived from a temporally and geographically constrained set of deposits. One of the mandibles is in many characters the largest mandible assigned to the genus Homo. Two of the others are quite small, with variably large and small teeth. And the fourth specimen is one of the earliest edentulous mandibles in the hominid record. Given the current season, it is perhaps appropriate to describe the sample as a real cornucopia of variation. And the location and date of the site itself is surprising. Dated to 1.8 million years and about 2000 miles from the outlet of the rift valley in northeast Africa, the site is a long way from the contemporaneous and well-known deposits from the Turkana Basin in Kenya and Olduvai Gorge in Tanzania.

    So how do we account for all this variation? That was basically the question of my dissertation. I sought to answer this question by testing a series of hypotheses focused first on sources of intraspecific variation, particularly age and sexual dimorphism, then secondarily on hypotheses of interspecific differentiation (i.e. multiple species). I then evaluated the results of these quantitative tests in the context of the comparative anatomy of the Dmanisi sample. Sparing you all the details, I think there are strong reasons to consider the Dmanisi hominid sample as that of a single species, but one displaying considerable amount of variation associated with age and possibly elevated levels of sexual dimorphism relative to what we observe in contemporary and recent human populations.

    Hawks: Of course, your work required a lot of comparisons with other samples, and mandibles are among the most common skeletal elements represented in the fossil record. How did you handle your comparative work?

    Van Arsdale: Paleoanthropology is at its root a comparative discipline. It is difficult to interpret any set of fossils outside of some comparative model. My work is no different. In asking questions about variation associated with age and sex, my dissertation is really asking how strange (or not strange) does the variation in the Dmanisi sample look if we treat it like a mixed age and sex sample of humans? Of chimpanzees? Of gorillas? Each of these species possess somewhat differing patterns of variation so that our final understanding of the Dmanisi specimens is based on a combination of similarities and differences with these different comparative models.

    You can also try to understand the sample from the perspective of other fossils. These comparisons are more challenging because we have less certainty regarding the things we think we know about fossils. For example, in my dissertation I also make a series of comparisons between the Dmanisi mandibles and a sample of Australopithecus boisei mandibles from East Africa. It is much more difficult to say for certain whether any given fossil specimen is male or female, and in the absence of well preserved teeth, young or old. That uncertainty limits the power of the hypothesis tests we can bring to the question by limiting the amount of information we have to work with.

    One of the exciting aspects of Paleoanthropology's comparative perspective is that new fossils give us new ways of looking at old fossils. Possibly the most exciting aspect of the Dmanisi fossils is that they provide us a tremendous platform from which to look back at these large samples from East and Southern Africa that we have known about for a long time and reexamine questions which had either previously been unanswerable or whose accepted answers no longer seem so clear.

    Hawks: Any stories you can share about your travels?

    Van Arsdale: One of the more unique experiences from my travels occurred while I was tagging along with a graduate student from Yale on her project involving 4.5 million year old fossil exposures in the Tugen Hills of the Central Rift Valley, Kenya. I was off on my own one day, walking along one of the exposures when I came across what appeared to be part of a fossilized crocodile skull just barely sticking out of the ground. I sat down and began very carefully exposing its boundaries so that it could be properly prepared and taken out. After about 20 minutes of this, a young Tugen boy came out of the bushes and sat down next me and began watching me work. I tried to say a few words of greeting in my very rudimentary Kiswahili, but either my pronunciation was too terrible to be understand (quite likely) or he was too young to have yet learned Kiswahili (he looked like he was between 8 and 10). After a few more minutes the boy, who had been carrying a small bow and set of arrows, took out one of his arrows and began using its steel tip as a mini-trowel. I would have discouraged him out of fear he might damage the fossil or go on trying to dig up other fossils in the area, but as I watched him he was exceedingly careful and seemed completely enraptured by the work. It was just one of those moments where, while the event was going on, I recognized how amazingly unique it was. Here we were, a graduate student from the University of Michigan with twenty plus years of formal education and a young Tugen boy with at most a few years of schooling, sitting side by side on a hillside in the middle of Kenya carefully exposing a 4.5 million year old fossil. The only common language between us was the action of my Marshalltown trowel and his handmade arrow point and a basic curiosity in this fossil.

    Hawks: It's a story you hear from students a lot: teeth and mandibles are "bor-ing". But of course, they're the best representatives of variation we have through much of human evolution -- if you want to study evolution, you'll be studying jaws and teeth. What keeps these questions exciting for you?

    Van Arsdale: One of the reasons I enjoy looking at mandibles and teeth are that they can potentially provide a window into numerous aspects of human evolution. As you point out, they are the most abundant element in the fossil record and therefore provide a large set of data with which to address questions of evolutionary relationships and evolutionary change. They can also tell you something about the ecology and diet of the individual specimen. Finally, they tell us something about how an organism develops throughout life and ages.

    This also means that questions regarding variation in jaws and teeth can be difficult to answer because many different processes might account for the observed variations. When testing hypotheses about mandibular variation it is important to keep this in mind. It is always striking to me how many hominid type specimens are or have served at some time as type specimens for a new species. This is in part a reflection of their relative abundance, but I think it also reflects how difficult it is to adequately address all the potential sources of variation in mandibles. If you accept the conclusions of my research, the Dmanisi mandibles serve as a cautionary tale in this regard.

    Hawks: Some readers may know that you and I share the same graduate advisor, Milford Wolpoff, who has certainly been a strong influence on the way I approach evolutionary questions. But I also find myself going back to other people who influenced my training. Who/what really got you interested in the field, or shaped the way you think about evolution?

    Van Arsdale: I initially entered anthropology by happy circumstance. Entering college (Emory University) I was interested in majoring in both English Literature and Evolutionary Biology. My first year two things happened; I realized Emory's biology department was primarily focused on microbiology and full of pre-med students (something I was not interested in) and I took my first Anthropology course to fulfill a distribution requirement. I was immediately hooked. Here I could have the best of both worlds... an integrative approach towards understanding what it means to be human and a careful examination of the evolutionary processes which have shaped the pattern of human evolution. I owe a huge part of my perspective to Milford and the other faculty and students I worked with as a graduate student, but I don't think I fully realized the influence my undergraduate teachers had on my perspective till the AAA meetings last year when I was able to attend a session honoring the graduate advisor (Jack Kelso) of my undergraduate advisor (George Armelagos). I listened to talks by people I had never met, but with whom I share some of my academic phylogeny, and what I heard were familiar themes on the interaction of human biological and cultural processes. This bio-cultural perspective is something I carry with me from Emory and is evident in the approach I take towards questions of Pleistocene human evolution, where changes in human skeletal form cannot be understood outside of the context of our ever-expanding brains and the increasingly complex ways in which we interact with the people and environments around us. Now that I am teaching, it is something I am aware of when I am in front of the undergraduates in my own classes.

    Hawks:Some of your current research involves a lot of genetic modeling. How did you get into this area? Can you tell us about some of your thoughts?

    Van Arsdale: My interest in genetic modeling first began as an undergraduate. In part it reflects my status as an admitted math nerd. I like numbers, I like using computationally intense models and simulations to address specific hypotheses, and I like understanding how evolutionary and cultural processes interact in dynamic ways. But when I was an undergrad my interest in genetic models stemmed out of my interest in modern human origins and the belief that any really good model should be able to simultaneously explain the pattern of fossil, archaeological, and genetic evidence. At the time there was quite a bit of discussion not just about how the increasing amount of genetic data related to previously held understandings of the fossil and archaeological record, but also how compatible data from different genetic systems were with each other. In particular, data from non-recombinant genetic systems (mtDNA and parts of the Y-chromosome) seemed to provide a different picture of human evolution than data from recombinant genetic systems. My attempt to understand these differences is what really drew me into aspects of genetic modeling.

    Since that time my interest genetic modeling has really developed out of what I consider an anthropological approach towards understanding genetic systems. I like to quote one of the take-away messages from the dissertation defense of another Michigan graduate, Keith Hunley, who modeled genetic aspects of South American population structure in his dissertation. As Keith said in his defense, what people do matters. Most genetic models are dependent on a variety of demographic parameters (population size, structure, etc.), all those things that people do. And yet most geneticists do not, or simply cannot directly address these demographic parameters with the data available to them. As a paleoanthropologist, one role my research serves is to provide better understandings of what people did and the ways in which they interacted in the past so as to better inform such genetic models.

    On a more theoretical level I am very much interested in exploring how the unique ways in which humans shape and interact with our evolutionary landscape serves to structure genetic variation and the evolutionary forces which shape it.

    Hawks: What's the next step for you? Where do you go from here with your research?

    Van Arsdale: Most of the questions I am working on now reflect my current thinking that the basic pattern which characterizes Pleistocene human evolution; the complex interaction between increasing cultural complexity, expanding ecological niches, and basic anatomical changes (encephalization, dental reduction); establishes itself early in the Pleistocene if not prior than that. Essentially, that sometime around 2-2.5 million years ago a group of hominids stopped acting like bipedal apes (the Australopithecines) and started acting human. This basic human pattern then continued to develop and characterize Pleistocene hominids until about 10-20,000 years ago when we stopped acting like humans and started acting like domesticated humans.

    By understanding how this pattern manifests itself early in the Pleistocene, for example, by considering how, why and with what changes human populations expanded into places like Southern Georgia as early as 1.8 million years ago, you can develop broader understandings of the Pleistocene as a whole. I am just finishing up two projects related to this broad topic, one examining the Habiline-Erectine transition in the Lower Pleistocene and another attempting to characterize broad demographic changes within the Pleistocene.

    I also want to continue my involvement in paleoanthropological field work and would like to continue examining Plio-Pleistocene deposits in Western and Central Asia. Dmanisi is an incredible site and has provided a great amount of detailed evidence to address questions of human evolution from this time period. But the detailed picture it provides encompasses only a narrow range of time and space...the more we can expand that window the better we can understand the broad patterns of change which characterize humans in the Plio-Pleistocene.

    Tags: 
    Olduvai Gorge
    A. boisei
    Dmanisi

  • Big arms, small sacrum

    Sun, 2007-08-19 10:52 -- John Hawks

    In case you're following the debate about Homo habilis limb proportions, there's a new contribution by Martin Haeusler and Henry McHenry in the JHE holding pen. They examined the partial KNM-ER 3735 skeleton.

    KNM-ER 3735 is often assigned to Homo habilis, but it's not exactly an easy diagnosis. There are a few pieces of the skull preserving anatomy, including the cheek, frontal and temporal. Here's what Bernard Wood (1991) had to say about the skeleton:

    The form of the mandibular fossa and malar region virtually preclude this specimen from being attributed to A. boisei. Its general affinities are with Homo. Some features (e.g. vault thickness) ally it with a Homo erectus-like hominid, but in other areas (e.g. the frontal) it is more like crania such as KNM-ER 1813, a conclusion endorsed by Walker (1987) and by Leakey et al. (1989). Tobias (1989) includes KNM-ER 3735 within H. habilis.

    Provisional taxonomic assessment: Homo sp. indet.

    Well, that's not exactly a rousing endorsment. You can see the problem --- and it's a common tale for hominid fossils. It has a smaller brain than early H. erectus (that would be the "frontal looks like KNM-ER 1813 bit). But its cranial bones are thick. The most complete of the bones in the skeleton is a radius, but it's not complete. The best bone for estimating joint surface area is the sacrum; a femur shaft is there, but it falls short of the midshaft length.

    And there's a problem: the radius seems pretty big, but the sacrum is little. If it were a human, the radius looks like it came from a body twice the size of the sacrum. There's something going on here. Previous work has assumed that the sacrum is more likely reflective of the size of the body, and the radius is therefore big compared to a small body mass. Maybe that means more climbing, leading to a greater role in weight support for the arms. Or maybe it means a retention of more apelike proportions.

    This is a frustrating literature to follow, because pretty much every other early specimen except Lucy (AL 288-1) and the Nariokotome skeleton (KNM-WT 15000) present exactly the same problem. You can't estimate limb sizes very accurately from small pieces of bone. And you can't estimate proportions accurately at all without estimates of size. Plus, it's not clear that you can interpret limb proportions without a decent estimate of body mass. Two years ago, there was a huge go-around about the limb proportions of OH 62. Like KNM-ER 3735, it looks to have a relatively large arm compared to its body. Or maybe the legs are short. Or maybe the estimates are bad. You get the picture. So everybody has a different clever statistical transformation to try to make these fossils comparable to each other. I have no argument with any of the work; but it seems like the error involved in these assessments of proportions is pretty large relative to the information content of the bones.

    Here's some of the conclusion from Haeusler and McHenry:

    Our analyses suggest that the idea that KNM-ER 3735 had more primitive body proportions than A.L. 288-1 (e.g., Leakey et al., 1989) needs to be refined. We found a unique but distinct mosaic of modern and ape-like limb proportions in the two early hominid species. H. habilis shares a gracile humerus and radius and a small base of the hand phalanges with the earlier A.L. 288-1 and modern humans. In addition, other characteristics, including the relatively small size of the sacrum and a robust midshaft of the phalanges, are common to both early hominids and extant great apes. Surprisingly, however, those upper limb proportions that differ between the two fossil species, such as a robust scapula, a long radial neck, and a long forearm, are all more ape-like in H. habilis.

    In KNM-ER 3735, the shoulder muscles that originate on the scapula (trapezius, deltoid, supraspinatus, and infraspinatus) as well as the biceps brachii were, therefore, probably not only more powerful than in modern humans, but also stronger than in A.L. 288-1. On the other hand, the extraordinarily short lever arm of A.L. 288-1's biceps muscle, the minute elbow size, and the small radial head may indicate a weaker arboreal component in its behavioral repertoire than in H. habilis. However, in the absence of modern correlates, caution is needed with respect to possible behavioral implications of the different forearm proportions in the two species.

    They also note the Homo-like anatomy of the femur shaft, including a marked pilaster.

    Seth Dobson (2005) claimed that that the sacrum of STW 431 (A. africanus) is also small -- it certainly yields a small mass estimate compared to other elements of the skeleton. Heck, all of the early hominid sacra yield small mass estimates. Well, you can see it's confusing.

    References:

    Haeusler M, McHenry HM. 2007. Evolutionary reversals of limb proportions in early hominids? Evidence from KNM-ER 3735. J Hum Evol (in press) doi:10.1016/j.jhevol.2007.06.001

    Dobson SD. 2005. Are the differences between Stw 431 (Australopithecus africanus) and A.L. 288-1 (A. afarensis) significant? J Hum Evol 49:143-154. doi:10.1016/j.jhevol.2005.04.001

    Tags: 
    A. africanus
    Homo erectus
    A. afarensis
    Homo habilis
    A. boisei

Pages

Subscribe to A. boisei

Neandertals

For years, I've worked on their bones. Now I'm working on their genes. Read more about the science studying these ancient people.

Denisova

From a finger bone of an ancient human came the record of a completely unexpected population. My lab is working on the science of the Denisova genome.

Acceleration

The advent of agriculture caused natural selection to speed up greatly in humans. We're uncovering some of the ways that populations have rapidly changed during the last 10,000 years.

Malapa

Just outside Johannesburg, the Malapa site is producing some of the most exciting finds in human evolution. This site is the headquarters of the Malapa Soft Tissue Project.