The functional role of the Carabelli trait in early and late hominins.

The Carabelli trait is a dental feature that forms along the lingual margin of the protocone of deciduous and permanent maxillary molars. It is variably expressed, ranging from a small pit or furrow to a large cusp, and its development seems to be associated with crown size and molar cusp spatial configuration. The degree of expression of the Carabelli trait differs systematically between hominin taxa, and for this reason, it has been used extensively in the reconstruction of their phylogeny. However, the functional implications of having a large Carabelli trait remain unclear. In this study, we analyze the macrowear pattern of maxillary molars of early and late hominins using the occlusal fingerprint analysis method, an approach based on digital models of teeth that helps in reconstructing occlusal dynamics occurring during mastication. Tooth crowns with a small Carabelli cusp generally exhibit larger wear contact areas that extended cervically, while two additional new occlusal contact areas are common in teeth characterized by a large Carabelli cusp. These wear areas are created at the beginning of the chewing cycle, when occluding with the slopes of the lingual groove of the lower molars, between the metaconid and entoconid cusps. Advancing tooth wear leads to a slight enlargement of Carabelli occlusal contacts increasing their functional area. A steep inclination could be mechanically important in food reduction and in balancing the functional load distribution during mastication contacts. Steep wear areas are particularly developed in primates that process foods characterized by tough and fibrous textural properties. Future biomechanical and microwear texture analyses could provide additional information on the mechanical adaptation of this dental trait.

Ages-at-death distribution of the early Pleistocene hominin fossil assemblage from Drimolen (South Africa).

OBJECTIVES: A prevailing hypothesis in paleoanthropology is that early Pleistocene hominin bones were accumulated in South African caves by carnivores, which used those shelters, and the trees surrounding them, as refuge and feeding sites. We tested this hypothesis at the site of Drimolen, by comparing its hominin age-at-death distribution to that of the nearby and roughly contemporaneous site of Swartkrans. MATERIALS AND METHODS: We employed standard dental aging systems in order to categorize the Drimolen hominin teeth into age classes of 5 years each. We then compared the age-at-death distribution for Drimolen with the published data available for the Swartkrans hominins. RESULTS: Age-at-death distributions indicate that the age category "young adults" is the best represented age category at Swartkrans and the most poorly represented one at Drimolen. Moreover, Drimolen has a preponderance of infant specimens. Both sites have a low frequency of old adult specimens. CONCLUSIONS: Differences observed in frequencies of the age-at-death categories suggest different mechanisms of hominin skeletal accumulation at Drimolen and Swartkrans. Swartkrans' frequency curve reflects mortality in a population subjected to predation and is thus consistent with the carnivore-accumulating hypothesis. In contrast, the Drimolen curve is similar to that of wild populations of living apes. Living primates have been observed exploiting caves as sleeping shelters, for nutritional, security, drinking, and thermoregulatory purposes. We suggest that similar cave use by Pleistocene hominins can explain, in large part, the accumulation of hominin bones at Drimolen. Such a conclusion is another illustration of the growing awareness that a "one-size-fits-all" taphonomic model for South African early Pleistocene hominin sites is probably insufficient.

Dental ontogeny in pliocene and early pleistocene hominins.

Until recently, our understanding of the evolution of human growth and development derived from studies of fossil juveniles that employed extant populations for both age determination and comparison. This circular approach has led to considerable debate about the human-like and ape-like affinities of fossil hominins. Teeth are invaluable for understanding maturation as age at death can be directly assessed from dental microstructure, and dental development has been shown to correlate with life history across primates broadly. We employ non-destructive synchrotron imaging to characterize incremental development, molar emergence, and age at death in more than 20 Australopithecus anamensis, Australopithecus africanus, Paranthropus robustus and South African early Homo juveniles. Long-period line periodicities range from at least 6-12 days (possibly 5-13 days), and do not support the hypothesis that australopiths have lower mean values than extant or fossil Homo. Crown formation times of australopith and early Homo postcanine teeth fall below or at the low end of extant human values; Paranthropus robustus dentitions have the shortest formation times. Pliocene and early Pleistocene hominins show remarkable variation, and previous reports of age at death that employ a narrow range of estimated long-period line periodicities, cuspal enamel thicknesses, or initiation ages are likely to be in error. New chronological ages for SK 62 and StW 151 are several months younger than previous histological estimates, while Sts 24 is more than one year older. Extant human standards overestimate age at death in hominins predating Homo sapiens, and should not be applied to other fossil taxa. We urge caution when inferring life history as aspects of dental development in Pliocene and early Pleistocene fossils are distinct from modern humans and African apes, and recent work has challenged the predictive power of primate-wide associations between hominoid first molar emergence and certain life history variables.

Extended male growth in a fossil hominin species.

In primates that are highly sexually dimorphic, males often reach maturity later than females, and young adult males do not show the size, morphology, and coloration of mature males. Here we describe extended male development in a hominin species, Paranthropus robustus. Ranking a large sample of facial remains on the basis of dental wear stages reveals a difference in size and robusticity between young adult and old adult males. Combined with estimates of sexual dimorphism, this pattern suggests that male reproductive strategy focused on monopolizing groups of females, in a manner similar to that of silverback gorillas. However, males appear to have borne a substantial cost in the form of high rates of predation.