Paranthropus: Where Do Things Stand?

Diversity-dependent speciation and extinction in hominins

The search for drivers of hominin speciation and extinction has tended to focus on the impact of climate change. Far less attention has been paid to the role of interspecific competition. However, research across vertebrates more broadly has shown that both processes are often correlated with species diversity, suggesting an important role for interspecific competition. Here we ask whether hominin speciation and extinction conform to the expected patterns of negative and positive diversity dependence, respectively. We estimate speciation and extinction rates from fossil occurrence data with preservation variability priors in a validated Bayesian framework and test whether these rates are correlated with species diversity. We supplement these analyses with calculations of speciation rate across a phylogeny, again testing whether these are correlated with diversity. Our results are consistent with clade-wide diversity limits that governed speciation in hominins overall but that were not quite reached by the Australopithecus and Paranthropus subclade before its extinction. Extinction was not correlated with species diversity within the Australopithecus and Paranthropus subclade or within hominins overall; this is concordant with climate playing a greater part in hominin extinction than speciation. By contrast, Homo is characterized by positively diversity-dependent speciation and negatively diversity-dependent extinction-both exceedingly rare patterns across all forms of life. The genus Homo expands the set of reported associations between diversity and macroevolution in vertebrates, underscoring that the relationship between diversity and macroevolution is complex. These results indicate an important, previously underappreciated and comparatively unusual role of biotic interactions in Homo macroevolution, and speciation in particular. The unusual and unexpected patterns of diversity dependence in Homo speciation and extinction may be a consequence of repeated Homo range expansions driven by interspecific competition and made possible by recurrent innovations in ecological strategies. Exploring how hominin macroevolution fits into the general vertebrate macroevolutionary landscape has the potential to offer new perspectives on longstanding questions in vertebrate evolution and shed new light on evolutionary processes within our own lineage.

Hominin fossils from Kromdraai and Drimolen inform Paranthropus robustus craniofacial ontogeny

Ontogeny provides critical information about the evolutionary history of early hominin adult morphology. We describe fossils from the southern African sites of Kromdraai and Drimolen that provide insights into early craniofacial development in the Pleistocene robust australopith Paranthropus robustus. We show that while most distinctive robust craniofacial features appear relatively late in ontogeny, a few do not. We also find unexpected evidence of independence in the growth of the premaxillary and maxillary regions. Differential growth results in a proportionately larger and more postero-inferiorly rotated cerebral fossa in P. robustus infants than in the developmentally older Australopithecus africanus juvenile from Taung. The accumulated evidence from these fossils suggests that the iconic SK 54 juvenile calvaria is more likely early Homo than Paranthropus. It is also consistent with the hypothesis that P. robustus is more closely related to Homo than to A. africanus.

Calcium isotopic ecology of Turkana Basin hominins

Diet is a major driver of hominin evolution, but most of the geochemical evidence relies on carbon isotopes (δ¹³C). Here, we report enamel stable calcium isotope (δ^44/42Ca) values against δ¹³C values for several hominins and co-existing primates in the Turkana Basin area, circa 4 to 2 Ma. Australopithecus anamensis clusters with mammal browsers, Kenyanthropus platyops is distinct from A. anamensis in foraging into more open environments and the coexisting Theropithecus brumpti encompasses both the grazer and omnivore/carnivore domains. Early Homo is remarkable for its wide distribution in δ^44/42Ca values, possibly reflecting omnivorous and opportunistic preferences. Paranthropus boisei is uniquely distributed in the δ¹³C versus δ^44/42Ca iso-space being distinct from all other hominins from the Turkana Basin area as well as from the co-existing Theropithecus oswaldi. Several hypotheses are explored to discuss the unique δ^44/42Ca values of Paranthropus boisei including significant differences observed with δ^44/42Ca values recently reported for P. robustus from South Africa, questioning the monophyly of this genus.

Non-traditional stable isotopes, such as of calcium, have potential to expand our understanding of ancient diets. Here, Martin et al. use stable calcium isotopes recovered from fossil tooth enamel to compare the dietary ecology of hominins and other primates in the Turkana Basin 2-4 million years ago.

Variation among the Dmanisi hominins: Multiple taxa or one species?

OBJECTIVES

There is continuing controversy over the number of taxa documented by the Dmanisi hominins. Variation may reflect age and sex differences within a single population. Alternatively, two (or more) distinct species may be present. Our null hypothesis states that just one population is represented at the site.

MATERIALS AND METHODS

We assess the likely sources of variation in endocranial capacity, craniofacial and mandibular morphology, and the expression of characters related to aging and sex dimorphism. We use the coefficient of variation and a modified version of Levene's test for equal variances to compare trait variation at Dmanisi with that in fossil hominins and modern Homo sapiens from Africa.

RESULTS

Skull 5 presents a low, massive vault, and a muzzle-like lower face. Other individuals have larger brains and more globular vaults. Despite such variation, the five crania share numerous features. All of the mandibles possess marginal tubercles, mandibular tori, and a distinctive patterning of mental foramina. Relative variation at Dmanisi is comparable to that in selected reference groups. Further growth anticipated in Skull 3, age-related remodeling affecting the D2600 mandible, pathology, and sex dimorphism can account for much of the interindividual variation observed. The preponderance of evidence supports our null hypothesis.

DISCUSSION

Sources of the variation within ancient Homo assemblages remain poorly understood. Skull 5 is a very robust male, with a brain smaller than that of both a juvenile (Skull 3) and a probable female (Skull 2). Skull 1 has the largest brain, but cranial superstructures do not clearly mark this individual as male or female. It is likely that the Dmanisi hominins represent a single paleospecies of Homo displaying a pattern of sex dimorphism not seen in living hominids.

A new method to quantify mandibular corpus shape in extant great apes and its potential application to the hominoid fossil record

OBJECTIVES

Mandibular corpus robusticity (corpus breadth/corpus height) is the most commonly utilized descriptor of the mandibular corpus in the great ape and hominin fossil records. As a consequence of its contoured shape, linear metrics used to characterize mandibular robusticity are inadequate to quantify the shape of the mandibular corpus. Here, we present an alternative to the traditional assessment of mandibular shape by analyzing the outline of the mandibular corpus in cross-section using landmarks and semilandmarks.

MATERIALS AND METHODS

Outlines of the mandibular corpus in cross-section between M₁ and M₂ were quantified in a sample of hominoids and analyzed using generalized Procrustes analysis, Procrustes ANOVA, CVA, and cluster analysis. Corpus breadth and width were also collected from the same sample and analyzed using regression, ANOVA, and cluster analysis.

RESULTS

Analysis of corpus outline shape revealed significant differences in mandibular corpus shape that are independent of size and sex at the genus level across hominoids. Cluster analysis based on the analysis of corpus outline shape results in almost all specimens grouping based on taxonomic affinity (99.28% correct classification). Comparison of these results to results using traditional measures of mandibular robusticity shows that analysis of the outline of the corpus in cross-section discriminate extant great apes more reliably.

CONCLUSION

The strong taxonomic signal revealed by this analysis indicates that quantification of the outline of the mandibular corpus more fully captures mandibular corpus shape and offers the potential for greater power in discriminating among taxa in the hominoid fossil record.