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Paleobiological Implications of the Ardipithecus ramidus Dentition

Gen Suwa et al.

Teeth are highly resilient to degradation and therefore are the most abundant specimens in the primate fossil record. The size, shape, enamel thickness, and isotopic composition of teeth provide a wealth of information about phylogeny, diet, and social behavior. Ardipithecus ramidus was originally defined in 1994 primarily on the basis of recovered teeth, but the sample size was small, limiting comparison to other primate fossils. We now have over 145 teeth, including canines from up to 21 individuals. The expanded sample now provides new information regarding Ar. ramidus and, using comparisons with teeth of other hominids, extant apes, and monkeys, new perspectives on early hominid evolution as well.

In apes and monkeys, the male's upper canine tooth usually bears a projecting, daggerlike crown that is continuously sharpened (honed) by wear against a specialized lower premolar tooth (together these form the C/P3 complex). The canine tooth is used as a slicing weapon in intra- and intergroup social conflicts. Modern humans have small, stublike canines which function more like incisors.

All known modern and fossil apes have (or had) a honing C/P3 complex. In most species, this is more developed in males than females (in a few species, females have malelike large canines, either for territorial defense or for specialized feeding). The relatively large number of Ar. ramidus teeth, in combination with Ethiopian Ar. kadabba, Kenyan Orrorin, and Chadian Sahelanthropus [currently the earliest known hominids at about 6 million years ago (Ma)], provide insight into the ancestral ape C/P3 complex and its evolution in early hominids.

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Dentitions from human (left), Ar. ramidus (middle), and chimpanzee (right), all males. Below are corresponding samples of the maxillary first molar in each. Red, thicker enamel (~2 mm); blue, thinner enamel (~0.5 mm). Contour lines map the topography of the crown and chewing surfaces.

In basal dimensions, the canines of Ar. ramidus are roughly as large as those of female chimpanzees and male bonobos, but their crown heights are shorter (see figure). The Ar. ramidus sample is now large enough to assure us that males are represented. This means that male and female canines were not only similar in size, but that the male canine had been dramatically “feminized” in shape. The crown of the upper canine in Ar. ramidus was altered from the pointed shape seen in apes to a less-threatening diamond shape in both males and females. There is no evidence of honing. The lower canines of Ar. ramidus are less modified from the inferred female ape condition than the uppers. The hominid canines from about 6 Ma are similar in size to those of Ar. ramidus, but (especially) the older upper canines appear slightly more primitive. This suggests that male canine size and prominence were dramatically reduced by ~6 to 4.4 Ma from an ancestral ape with a honing C/P3 complex and a moderate degree of male and female canine size difference.

In modern monkeys and apes, the upper canine is important in male agonistic behavior, so its subdued shape in early hominids and Ar. ramidus suggests that sexual selection played a primary role in canine reduction. Thus, fundamental reproductive and social behavioral changes probably occurred in hominids long before they had enlarged brains and began to use stone tools.

Thick enamel suggests that an animal’s food intake was abrasive; for example, from terrestrial feeding. Thin enamel is consistent with a diet of softer and less abrasive foods, such as arboreal ripe fruits. We measured the enamel properties of more than 30 Ar. ramidus teeth. Its molar enamel is intermediate in thickness between that of chimpanzees and Australopithecus or Homo. Chimpanzees have thin enamel at the chewing surface of their molars, whereas a broad concave basin flanked by spiky cusps facilitates crushing fruits and shredding leaves. Ar. ramidus does not share this pattern, implying a diet different from that of chimpanzees. Lack of thick enamel indicates that Ar. ramidus was not as adapted to heavy chewing and/or eating abrasive foods as were later Australopithecus or even Homo. The combined evidence from the isotopic content of the enamel, dental wear, and molar structure indicates that the earliest hominid diet was one of generalized omnivory and frugivory and therefore differed from that of Australopithecus and living African apes.

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