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| Image modified from an original image by John G. Fleagle in Primate Adaptation and Evolution and published in Daniel Schmitt, “Review: Insights into the evolution of human bipedalism from experimental studies of humans and other primates,” The Journal of Experimental Biology |
Identifying transitional entities is a difficult enterprise in most disciplines; for paleoanthropologists it can be intensified by the unpredictable enlargement of the subject universe. The discovery of “Lucy” in the Afar Desert region of Northern Ethiopia in 1974 made a “momentous” addition to the known universe of hominid fossils. Not only an unusually complete find, but sufficiently older and different from other australopithecines to be classified as a new species, Australopithecus afarensis, “Lucy” generated worldwide interest. Since walking on two legs rather than four is the primary distinguishing characteristic of human locomotion, tracing the transition to fully erect two-footedness is central to understanding hominid, thus human, evolution. “Lucy’s” great age made its mode of locomotion a particularly compelling question, and the completeness of the find offered a rich fund of material for studying its locomotor anatomy.
Two members of the Department of Anatomical Sciences in the School of Medicine, Profs. Jack T. Stern, Jr., and Randall L. Susman, stepped forward, almost literally into the glare of the media, to apply their knowledge as comparative primate anatomists with experimental background in assessing relationships between form and function to perform a comprehensive functional analysis of this material, describing the stance and motion attributable to the size, shape and character of the hip, knee, ankle, and foot. Public attention focused on their conclusion, published in March, 1983 (“The locomotor anatomy of Australopithecus afarensis,” American Journal of Physical Anthropology, 60:3 (1983), 279–317) that “Lucy” was “very close to what can be called a missing link. It possesses a combination of traits entirely appropriate for an animal that had traveled well down the road toward full-time bipedality . . . . “
Scholarly debate focused on the question whether, terrestrial bipedality notwithstanding, “Lucy” consistently walked fully erect and as well as a modern human, or presented “a combination of arboreal and bipedal traits.” Prof. Stern reviews the current state of play as well as his and Prof. Susman’s original digression into fossil studies, which became a lifelong endeavor for both, in a recent reminiscence, marking the 25th anniversary of “Lucy’s” discovery (“Climbing to the Top: A Personal Memoir of Australopithicus afarensis,” Evolutionary Anthropology, 9:3 (2000), 113-33). The debate continues. Although a subsequently discovered, even older Australopithecine species, classified as A. anamensi has also been found to exhibit traits suggesting erectness and bipedality, and molecular genetics has begun to suggest new questions and contribute new information, understanding the locomotion of Australopithecus continues to be a central question, looking back ward predecessor species, and ahead toward the transition to the new species Homo; when did a “funny-walking biped” stop climbing trees except to write poetry about them?