Variation in the Habiline Crania – Must it be Taxonomic?

The problem of whether the hominid fossil sample of habiline specimens is comprised of more than one species has received much attention in paleoanthropology. The core of this debate has significant implications about when and how variation must be explained by taxonomy. In this paper, we examine the problem of whether the observed variation in habiline sample must be interpreted to reflect species differences. We test the null hypothesis of no difference by examining the degree of variability in habiline sample in comparison with other single-species early hominid fossil samples from Sterkfontein and Swartkrans (Sterkfontein is earlier than the habiline sample; Swartkrans may be within the habiline time span). We use the standard error test for this analysis, a sampling statistic based on the standard error of the slope of regressions between pairs of specimens that relates all of the homologous measurements each pair shares. We show that the null hypothesis for the habiline sample cannot be rejected. The similarities of specimen pairs within the habiline sample are not more than those observed between the specimens in the two australopithecine samples we analyzed.     

Early hominid enamel hypoplasia.

Enamel hypoplasia in the South African Plio-Pleistocene fossil hominid sample is examined. The Swartkrans hominids are shown to have a higher incidence of hypoplasia than the Sterkfontein hominids. Within the Swartkrans sample, individuals with hypoplasia of the first upper molar have lower-than-expected ages of death. Possible taphonomic explanations for these observations are discussed.     

Brief communication: Is variation in the cranial capacity of the Dmanisi sample too high to be from a single species?

This study uses data resampling to test the null hypothesis that the degree of variation in the cranial capacity of the Dmanisi hominid sample is within the range variation of a single species. The statistical significance of the variation in the Dmanisi sample is examined using simulated distributions based on comparative samples of modern humans, chimpanzees, and gorillas. Results show that it is unlikely to find the maximum difference observed in the Dmanisi sample in distributions of female-female pairs from comparative single-species samples. Given that two sexes are represented, the difference in the Dmanisi sample is not enough to reject the null hypothesis of a single species. Results of this study suggest no compelling reason to invoke multiple taxa to explain variation in the cranial capacity of the Dmanisi hominids.     

Role of carnivores in the accumulation of the Sterkfontein Member 4 hominid assemblage: a taphonomic reassessment of the complete hominid fossil sample (1936-1999)

New taphonomic data on the Sterkfontein Member 4 (South Africa) fossil hominid assemblage are presented. The previous estimate of hominid individuals represented in the deposit (45) is increased to 87. New minimum numbers of hominid skeletal elements are provided, and incidences of bone surface damage inflicted by prehistoric biological agents are summarized. The hominid sample from Member 4 is composed predominately of gnathic remains and has a paucity of postcrania. This dearth of postcrania limits, to some extent, inferences about the formation of the Sterkfontein assemblage. However, carnivore tooth marks on some fossil specimens and an overall broad similarity in patterns of skeletal part representation between Sterkfontein and primate bone assemblages created by extant carnivores suggest that carnivores did have some involvement in the accumulation of the fossil hominid assemblage. Thus, this study provides support for the “carnivore‐collecting hypothesis” of Brain (Brain [ 1981 ] The Hunters or the Hunted? Chicago: University of Chicago Press), which implicates large carnivores as prominent collecting agents of hominid body parts in Sterkfontein Member 4. Evidence of bone surface damage is, however, too scant to make confident inferences about specific carnivore taxon/taxa involved in hominid bone collection at the site. Am J Phys Anthropol, 2004. © 2004 Wiley‐Liss, Inc.     

An odontometric assessment of variability inAustralopithecus afarensis

Odontometric data are utilized to investigate both the extent of variation in the Pliocene hominid remains from Hadar and Laetoli and whether this variation is best explained as resulting from sexual dimorphism or from the presence of more than one species in the sample. Coefficients of variation for the Hadar/Laetoli dental elements are compared with those from other established Plio-Pleistocene hominid taxa and extant pongids. Results indicate that while CVs for the central cheek teeth (M1/1 and M2/2) tend to be rather high, the variability does not consistently exceed ranges of variability for extant anthropoids and other primate species. Thus odontometric data do not disprove the null hypothesis that the Hadar/Laetoli sample can be accommodated within a single species. Therefore, although the Hadar/Laetoli sample tends to exhibit less canine variability than is found among sexually dimorphic apes, odontometric variation in this sample is more likely due to sexual dimorphism than the presence of multiple taxa in the sample.     

Beyond leopards: tooth marks and the contribution of multiple carnivore taxa to the accumulation of the Swartkrans Member 3 fossil assemblage

The ca. 1.0 myr old fauna from Swartkrans Member 3 (South Africa) preserves abundant indication of carnivore activity in the form of tooth marks (including pits) on many bone surfaces. This direct paleontological evidence is used to test a recent suggestion that leopards, regardless of prey body size, may have been almost solely responsible for the accumulation of the majority of bones in multiple deposits (including Swartkrans Member 3) from various Sterkfontein Valley cave sites. Our results falsify that hypothesis and corroborate an earlier hypothesis that, while the carcasses of smaller animals may have been deposited in Swartkrans by leopards, other kinds of carnivores (and hominids) were mostly responsible for the deposition of large animal remains. These results demonstrate the importance of choosing appropriate classes of actualistic data for constructing taphonomic inferences of assemblage formation. In addition, they stress that an all-encompassing model of assemblage formation for the hominid-bearing deposits of the Sterkfontein Valley is inadequate and that each must be evaluated individually using not just analogical reasoning but also incorporating empirical data generated in the preserved fossil samples.     

The Australopithecus assemblage from Sterkfontein Member 4 (South Africa) and the concept of variation in palaeontology

Interpreting morphological variation within the early hominin fossil record is particularly challenging. Apart from the fact that there is no absolute threshold for defining species boundaries in palaeontology, the degree of variation related to sexual dimorphism, temporal depth, geographic variation or ontogeny is difficult to appreciate in a fossil taxon mainly represented by fragmentary specimens, and such variation could easily be conflated with taxonomic diversity. One of the most emblematic examples in paleoanthropology is the Australopithecus assemblage from the Sterkfontein Caves in South Africa. Whereas some studies support the presence of multiple Australopithecus species at Sterkfontein, others explore alternative hypotheses to explain the morphological variation within the hominin assemblage. In this review, I briefly summarize the ongoing debates surrounding the interpretation of morphological variation at Sterkfontein Member 4 before exploring two promising avenues that would deserve specific attention in the future, that is, temporal depth and nonhuman primate diversity.     

Human taxonomic diversity in the pleistocene: Does Homo erectus represent multiple hominid species?

Recently, nomina such as “Homo heidelbergensis” and “H. ergaster” have been resurrected to refer to fossil hominids that are perceived to be specifically distinct from Homo sapiens and Homo erectus. This results in a later human fossil record that is nearly as speciose as that documenting the earlier history of the family Hominidae. However, it is agreed that there remains only one extant hominid species: H. sapiens. Has human taxonomic diversity been significantly pruned over the last few hundred millennia, or have the number of taxa been seriously overestimated? To answer this question, the following null hypothesis is tested: polytypism was established relatively early and the species H. erectus can accommodate all spatio-temporal variation from ca. 1.7 to 0.5 Ma. A disproof of this hypothesis would suggest that modern human polytypism is a very recent phenomenon and that speciation throughout the course of human evolution was the norm and not the exception. Cranial variation in a taxonomically mixed sample of fossil hominids, and in a modern human sample, is analyzed with regard to the variation present in the fossils attributed to H. erectus. The data are examined using both univariate (coefficient of variation) and multivariate (determinant) analyses. Employing randomization methodology to offset the small size and non-normal distribution of the fossil samples, the CV and determinant results reveal a pattern and degree of variation in H. erectus that most closely approximates that of the single species H. sapiens. It is therefore concluded that the null hypothesis cannot be rejected. © 1993 Wiley-Liss, Inc.     

Echoes from the past: New insights into the early hominin cochlea from a phylo-morphometric approach

We investigate cochlear variation, an indirect evidence of auditory capacities among early hominins and extant catarrhine species, in order to assess (i) the phylogenetic signal of relative external cochlear length (RECL) and oval window area (OWA), (ii) the evolutionary model with the highest probability of explaining our observed data, (iii) some hominin ancestral nodes for RECL and OWA. RECL has a high phylogenetic signal under a Brownian motion model, and is closely correlated with body mass. Our model-based method has the advantage over parsimony-based methods of incorporating branch lengths in a phylo-morphospace, and this shows RECL shifted towards significantly higher values at the Homo erectus-Homo sapiens node. We also observe that the StW 53 and KB 6067 fossil specimens from Sterkfontein and Kromdraai likely represent one or two distinct, smaller-bodied and less derived hominin form(s) compared to Paranthropus specimens represented at Swartkrans.     

Patterns of tooth crown size and shape variation in great apes and humans and species recognition in the hominid fossil record

It has been suggested that patterns of craniodental variation in living hominids (Gorilla, Homo, Pan, and Pongo) may be useful for evaluating variation in fossil hominid assemblages. Using this approach, a fossil sample exhibiting a pattern of variation that deviates from one shared among living taxa would be regarded as taxonomically heterogeneous. Here we examine patterns of tooth crown size and shape variation in great apes and humans to determine 1) if these taxa share a pattern of dental variation, and 2) if such a pattern can reliably discriminate between samples that contain single species and those that contain multiple species. We use parametric and nonparametric correlation methods to establish the degree of pattern similarity among taxa, and randomization tests to assess their statistical significance. The results of this study show that extant hominids do not share a pattern of dental size variation, and thus these taxa cannot be used to generate expectations for patterns of size variation in fossil hominid species. The hominines (Gorilla, Homo, and Pan) do share a pattern of shape variation in the mandibular dentition; however, Pongo is distinct, and thus it is unclear which, if either, pattern should be expected in fossil hominids. Moreover, in this case, most combined-species samples exhibit patterns of shape variation that are similar to those for single hominine species samples. Thus, although a common pattern of shape variation is present in the mandibular dentition, it is not useful for recognizing taxonomically mixed paleontological samples.     

Cladistic analysis of early Homo crania from Swartkrans and Sterkfontein, South Africa

The phylogenetic relationships of early Pleistocene Homo crania from the South African sites of Swartkrans and Sterkfontein were investigated through cladistic analyses of 99 morphological characters. The Swartkrans Member 1 specimen SK 847 and the Stw 53 cranium from Sterkfontein Member 5A were treated as separate operational taxonomic units (OTUs), distinct from the three species of early Homo-H. erectus, H. habilis, and H. rudolfensis-that are recognized from the Plio-Pleistocene deposits of East Africa. The cladistic analyses differed in the treatment of the South African OTUs (separate Swartkrans and Sterkfontein OTUs vs. a single Swartkrans+Sterkfontein OTU). PAUP 4.0 was used to construct cladograms and address hypotheses about relationships. In the analysis that treated the South African specimens as a single OTU, the position of that OTU was stable as a separate branch on the Homo clade between H. rudolfensis and [H. habilis+(H. erectus+H. sapiens)]. When SK 847 and Stw 53 were treated as separate OTUs, the majority of most parsimonious trees indicated that they were positioned in similar positions as the combined South African Homo OTU; that is, as separate branches between H. rudolfensis and [H. habilis+(H. erectus+H. sapiens)], with the Swartkrans OTU generally occupying a more derived position. The position of the Sterkfontein OTU was more stable than that of the Swartkrans OTU, which was found in several other positions among the minimum length trees. Running the analyses with only those characters preserved by SK 847 and Stw 53 resulted in similar topologies for minimum length trees, although the positions of Stw 53, SK 847, and H. habilis exchanged places in some trees. In no case was an exclusive sister relationship between either South African OTU and a particular species of Homo supported statistically. Both South African OTUs differ from H. habilis in the fewest number of cladistic characters.     

Falsification of a single species hypothesis using the coefficient of variation: a simulation approach.

Dental variation remains an important criterion for assessing whether a morphologically homogeneous fossil primate sample includes more than one species. The Coefficient of Variation (CV) has commonly been used to compare variation in a fossil sample of unknown taxonomic composition with that of extant single-species samples, in order to determine whether more than one species might be present. However, statistical tests for differences between fossil and single species reference sample CVs often lack power, because fossil samples are usually small and confidence limits of the CV are consequently large. The present study presents a new methodology for using the CV to test the hypothesis that a sample represents only one species. Simulated sampling distributions of single-species and pooled-species CVs are generated based on variation observed in dental samples of extant Cercopithecus species. These simulated distributions are used to test a single-species hypothesis for 13 different combinations of two or three sympatric Cercopithecus species across four dental characteristics at different sample sizes. Two different ways to generate the reference value of the CV are used. Results show the proposed methodology has substantially greater power than previous methods for detecting multiple-species composition, while maintaining an acceptable Type I error rate. Results are also presented concerning the dependence of power on sample size and on the average difference between means in a pooled-species combination.     

Sagittal cresting in the South African australopithecines

The evidence for sagittal cresting, and more generally the position of the temporal lines is reviewed in the South African australopithecine sample. The position of the lines is dependent on both the allometric relation of the masticatory apparatus to cranial size and on individual variation. In the Swartkrans specimens, with generally bigger body size, the influence of allometry predominates, actually overshadowing the influence of individual variation. At Sterkfontein and Makapansgat with generally smaller body size and a resulting smaller allometric ratio, individual variation has a greater influence. Of the eleven adult South African specimens, the four largest are crested. The one smaller crested specimen comes from Sterkfontein. The crested Makapan specimen is intermediate in size. The pattern of australopithecine cresting is somewhat different from other hominoids, and is part of a total morphological pattern suggesting adaptation to a diet requiring powerful crushing during mastication.     

Dental metric comparisons of Morotopithecus and Afropithecus: implications for the validity of the genus Morotopithecus

Morotopithecus bishopi and Afropithecus turkanensis are two large-bodied hominoid primates from early Miocene deposits of eastern Africa. Researchers have used both cranial and postcranial characters to distinguish these two species. Unfortunately, of the fossil material attributed to each, only the face, palate, and upper dentition are preserved well enough in both species for direct comparisons. There are currently no known directly comparable postcranial elements. In this study, we reevaluated dental characters argued to distinguish the type specimens of Morotopithecus from Afropithecus: relative size of the upper premolars and M3. Exact randomization methods were used to address two questions. First, is it possible to find the degree of dental-size difference observed between Morotopithecus (UMP 62-11) and Afropithecus (KNM-WK 16999) within extant African hominoids? Second, what is the probability of observing the levels of difference found between the fossils among pairs of extant individuals? Metric differences in relative premolar and M3 size were calculated between all possible pairs within the extant sample and the observed difference of the fossil pair was then compared to the resulting distribution of extant pairs. The observed size differences for all comparisons in the fossil teeth were well within the variation observed in the extant African hominoid samples (p>0.05). In light of these results and other currently available cranial evidence, we suggest that the type specimens of Morotopithecus and Afropithecus are not different enough to support taxonomic distinction.