Behavioral and phylogenetic implications of a narrow allometric study of Ardipithecus ramidus

Narrow allometry is used to compare Ardipithecus ramidus molar and body segment lengths and proportions to those of living primates, with the goal of reconstructing fossil behavior and exploring how lengths and proportions bear on phylogeny. Comparatively short hands and upper limbs suggest Ardipithecus was less adept at forelimb suspension and vertical climbing than are great apes. Its tibial and tarsal lengths, suggest bonobo-like leaping ability. Its short lower limbs, but long toes relative to humans, are not conducive to habitual bipedality. When terrestrial, Ardipithecus would have engaged in palmigrade quadrupedality. Compared to the semi-digitigrade baboon its long fingers and toes suggest a less marked terrestrial commitment and agree with carpal anatomy reflecting full palmigrady. Molar dimensions and surface areas are similar to those of baboons and drills, but greater than in chimpanzees, indicating a diet with less fruit and more roughage than that of chimpanzees. Ardipithecus dimensions reflect a generalized ape, able to move in trees and on the ground, and exploit food sources in woodlands, grasslands and/or flooded terrain. These abilities are well-suited to the mosaic habitats that characterize Africa at 11°N. Parsimonious reconstruction of the common human/African ape ancestor suggests the short upper limbs and metacarpals of Ardipithecus are too derived to belong to an exclusive human ancestor. Because parsimony is a theoretical construct and not an evolutionary reality, derived segment lengths alone do not prove conclusively Ardipithecus is not such an ancestor. Description in Ardipithecus of complex anatomy uniquely shared by humans and African apes, that leaves a record of reversals or parallelisms, would be a first step in showing whether this fossil qualifies as such an ancestor.