Understanding the extinction of Australopithecus and origins of Paranthropus and Homo in South Africa has been hampered by the perceived complex geological context of hominin fossils, poor chronological resolution, and a lack of well-preserved early Homo specimens. We describe, date, and contextualize the discovery of two hominin crania from Drimolen Main Quarry in South Africa. At ~2.04 million to 1.95 million years old, DNH 152 represents the earliest definitive occurrence of Paranthropus robustus, and DNH 134 represents the earliest occurrence of a cranium with clear affinities to Homo erectus. These crania also show that Homo, Paranthropus, and Australopithecus were contemporaneous at ~2 million years ago. This high taxonomic diversity is also reflected in non-hominin species and provides evidence of endemic evolution and dispersal during a period of climatic variability.

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The frontal sinuses are cavities inside the frontal bone located at the junction between the face and the cranial vault and close to the brain. Despite a long history of study, understanding of their origin and variation through evolution is limited. This work compares most hominin species’ holotypes and other key individuals with extant hominids. It provides a unique and valuable perspective of the variation in sinuses position, shape, and dimensions based on a simple and reproducible methodology. We also observed a covariation between the size and shape of the sinuses and the underlying frontal lobes in hominin species from at least the appearance of Homo erectus . Our results additionally undermine hypotheses stating that hominin frontal sinuses were directly affected by biomechanical constraints resulting from either chewing or adaptation to climate. Last, we demonstrate their substantial potential for discussions of the evolutionary relationships between hominin species.

Objectives: Twenty-four dental specimens from the Drimolen Main Quarry (DMQ) are described. This increases the number of DMQ Paranthropus robustus specimens from 48 to 63 and DMQ Homo specimens from 8 to 12. This allows reassessment of the proposed differences between the DMQ P. robustus assemblage and that of Swartkrans. Analysis conducted assesses intraspecific and inter-locality variation.Materials and Methods: We examined the P. robustus and early Homo assemblages from South Africa. Morphology was observed using a hand lens and a binocular microscope. Mesiodistal and buccolingual measurements were taken using plastictipped calipers. Summary statistics were generated and patterns of variability in P. robustus were assessed through box plots and Mann-Whitney U tests.Results: Comparison between the expanded DMQ and Swartkrans P. robustus assemblages demonstrates overlap in size. Ten dental variables show statistically significant differences.Discussion: The expanded P. robustus sample allowed us to re-examine previous analyses of differences in tooth size between the samples. While analyses presented here show a high degree of overlap in the MD and BL dimensions of the two assemblages, significant differences were found in the mean values of these variables in the postcanine maxillary teeth-consistent with previous analyses. Two current hypotheses may explain this pattern: 1) dental size increase through the P. robustus lineage or 2) different sample composition between the two sites. Small sample sizes for all permanent dental classes in the DMQ assemblage represents a limitation on this analysis and interpretations thereof. Any addition to the DMQ or the Swartkrans samples may alter these results.

Classic depictions of human evolutionary ecology cast Homo as predator and other hominins, including Paranthropus robustus, as prey. Such hypotheses rest on a small number of fossils that exhibit evidence of carnivore predation, including the iconic SK 54 cranium from Swartkrans in South Africa. Here we demonstrate that the SK 54 cranium shares its closest affinities with H. erectus rather than P. robustus. Demonstrating that Homo was prey for leopards at Swartkrans weakens the historically significant hypothesis that Homo was better able to avoid predation because of being behaviourally and technologically advanced compared to Paranthropus. Subsequent ideas about hominin paleobiology derived from this hypothesis warrant reconsideration.

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