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Regarding the articular surfaces of the fibula of A. afarensis, Stern and Susman (1983, p. 305) wrote that they provide “evidence for a significant component of arboreality in the behavior of A. afarensis.” Johanson’s supporters such as Latimer disagreed with this analysis (Latimer et al. 1987). In overall appearance, however, the lower part of Lucy’s fibula, the part that connects with the ankle, is different from that of a human being and almost identical to that of the pygmy chimpanzee (Susman et al. 1984, p. 130).

Stern and Susman (1983, p. 302) argued that Lucy’s foot could be bent back further than in humans. “This trait would seem to be useful in reaching for branches with the feet and in hindlimb suspension,” they noted (Stern and Susman 1983, p. 299). According to Stern and Susman (1983, p. 300), Lucy’s ankle was structured so that she would have “had difficulty in maintaining a vertical orientation of the trunk and might have progressed bipedally in a manner unlike that of humans and more like that of an African ape.”

Johanson’s supporters took a completely opposite position, namely, that the ankle of A. afarensis was almost totally adapted for a humanlike, terrestrial bipedal gait, making impossible any substantial arboreal behavior. Latimer and Lovejoy in particular have published several articles micro-analyzing every curve of the A. afarensis foot and ankle bones as proof of exclusive terrestrial bipedalism (Gomberg and Latimer 1984, Latimer et al. 1987, Latimer and Lovejoy 1990a, Latimer and Lovejoy 1990b).

We note, however, that an author of a recent survey (Groves 1989) takes the side of Stern, Susman, Tardieu, Oxnard, and others who have argued for a substantial component of arboreality in Australopithecus afarensis and the australopithecines generally. Groves (1989, p. 310) said that in the australopithecines “bipedal locomotion was only part of a pattern which also incorporated sophisticated climbing ability.”

J. H. Prost (1980) of the University of Chicago concluded that the australopithecines, including Lucy, were primarily quadrupedal vertical climbers. “Quadrupedal vertical climbing produced a large number of . . . traits which have incorrectly assumed to have been bipedal adaptations,” stated Prost (1980, p. 186).

According to Prost (1980, p. 175), the australopithecines, including A. afarensis, would have possessed, in addition to their aboreal capabilities, the capacity for “facultative terrestrial bipedalism.” The word facultative means “optional” or “taking place under some conditions but not under others.” In other words, the predominantly arboreal australopithecines, if the situation demanded, would have been able to move bipedally on the ground, perhaps in running from one tree to another some distance away. This type of behavior is observed in many primates, including chimpanzees, orangutans, and gibbons. So the fossil evidence in no way obligates one to attribute to A. afarensis any specifically human locomotor behavior. According Prost (1980, p. 188), the first true terrestrial bipedal hominid was Homo habilis (as understood before the discovery of the apelike OH 62 individual) or early Homo erectus.

R. H. Tuttle posited the existence of preaustralopithecine hominids displaying a kind of arboreal bipedalism. He called them hylobatians, after the genus Hylobates, which includes the modern gibbon. Tuttle (1981, p. 90) stated: “Vertical climbing on tree trunks and vines and bipedalism on horizontal boughs were conspicuous components of their locomotor repertoire. They commonly stood bipedally while foraging in trees . . . Short bursts of bipedal running and hindlimbpropelled leaps may have been important for the manual capture of insects and small vertebrates with which they supplemented their vegetable fare.”

According to Tuttle (1981, p. 89), the Hadar hominids “had curved fingers and toes, strong great toes and thumbs, and other features that suggest they were rather recently derived from arboreal hominids [his