AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 73:41-63 (1987) Evolution of P3 Morphology in Australopithecus afarensis

The Australopithecus afarensis dental sample exhibits a wide range of variation, which is most notable in the morphology of the lower third premolar (P3). P3 morphology in the A. afarensis sample ranges from the primitive sectorial extreme in AL 128-23 to the derived, bicuspid (molarized) extreme in AL 333w-1. In this paper, the degree and patterning of variation of the 20 known A. afarensis P3s are examined and the evolutionary implications are discussed. Initially, a series of dental and mandibular metric criteria are evaluated to determine whether this sample may be analyzed as a single species. From the metrics, it is clear that the single species hypothesis cannot be rejected. Next, a series of morphological criteria is devised to measure P 3 molarization. Taken as a whole, the A. afarensis P3 sample displays more variation than a sample of modern hominoids (Pan troglodytes) and shows a slight trend toward increased molarization through time. When separated by sex, the A. afarensis sample still displays greater variation than the chimpanzee sample; however, only the male A. afarensis specimens show a trend toward increased molarization. Additionally, the male A. afarensis P 3 s are more molarized than the female, a pattern that is seen as well (though less markedly) in the chimpanzee sample. The trend toward increased molarization over time indicates selection for grinding in A. afarensis. The sexual differences parallel those seen in the postcrania (cf. Stern and Susman: Am. J. Phys. Anthropol. 60.-279-318, 1983), as the females tend to retain the primitive condition, while the males display the derived morphology. Consequently, a model of sexual differences in niche exploitation, with the females exploiting a more arboreal environment, would seem to be supported by both the dental and postcranial evidence. The caninelpremolar complex has changed significantly over the course of hominoid evolution. Many theories of hominid origins have stressed changes in the function of this complex from shearing to grinding. Reduction of projecting canines and development of bicuspid lower third premolars (P3s) have been associated with early hominids’ development of tools (Dart, 1957; Darwin, 1871; Washburn and Lancaster, 1968). Other theories of hominid origins have emphasized a shift in dietary regime that resulted in the evolution of a grinding masticatory complex characterized by molarized P3s with thick enamel and by reduced canines that would not impede movement of the jaw (Jolly, 1970, 1973; Simons and Pilbeam, 1971; Wolpoff, 1979,1980; Wolpoff and Russell, 1981; contra Kay, 1981). However, it is now known from the dental remains of A ustralopithecus afarensis that at least some of the earliest hominids possessed a pongidlike canineffs complex, as indicated by the presence of single-cusped P ~ s , large, projecting maxillary and mandibular canines, and characteristic shearing wear on at least one of the mandibular canines (BMNH18773) (White, 1981a; Wolpoff, 1980) and possibly some of the P 3 s (i.e., AL 288-1 and AL 128-23). Consequently, it now appears that fully reduced canines and molarized P 3 s were not part of the initial hominid adaptation. Received March 21, 1985 revision accepted December 19,1986.