Were Australopithecines Ape-Human Intermediates or Just Apes? a Test of Both Hypotheses Using the "Lucy" Skeleton
Senter, Phil, The American Biology Teacher
To effectively introduce the evidence for evolution, it is important that a biology teacher have a basic grasp of the anatomy of australopithecines. This is because the presence of evolutionary sequences in the fossil record is one of the main lines of evidence for evolution, and no fossil evolutionary lineage generates more interest than our own. As evolutionary intermediates between apes and humans, the australopithecines form a prominent part of that lineage, and their anatomy is a beautiful illustration of the anatomical transition from ape to human.
A good way to become familiar with australopithecine anatomy would be to use a point-by-point comparison of their skeletons with those of apes and humans. Unfortunately, such is lacking in most biology textbooks (e.g., Starr & Taggart, 2004; Campbell et al., 2009), including those on evolution (e.g., Volpe & Rosenbaum, 2000; Barton et al., 2007). This makes it difficult for the biology teacher to illustrate exactly how australopithecines are intermediate between apes and humans. Here, I compare an australopithecine skeleton with those of the chimpanzee (Pan troglodytes) and the modern human species (Homo sapiens).
A point-by-point comparison of ape, australopithecine, and human anatomy presents another opportunity that should be seized. Such a comparison can be used to test two competing claims that can be treated as testable hypotheses: (1) the consensus among mainstream scientists that australopithecine anatomy is intermediate between those of apes and humans and (2) the young-earth creationist claim that australopithecines are "just apes," unrelated to humans (Mehlert, 2000; Line, 2005; Murdock, 2006). In a time of rampant creationism (Mazur, 2005; Miller et al., 2006), it is important not to dismiss the latter claim out of hand but to explicitly put it to the test.
Hundreds of australopithecine specimens are known, but to keep this study simple and to avoid overwhelming the nonspecialist reader, I chose to use a single australopithecine specimen: AL 288-1, nicknamed "Lucy." Discovered in Ethiopia in 1974 (Johanson et al., 1982), AL 288-1 is particularly appropriate to use for this test of hypotheses, for several reasons. First, at 40% complete (Johanson et al., 1982), it is one of the most complete australopithecine skeletons known to date; most of the skull is missing, but the preserved portions of the jaw, dentition, vertebral column, pelvis, and limbs are sufficient to test the two hypotheses with traits from a variety of skeletal regions. Second, cast replicas of the entire known skeleton of Lucy are commercially available--as are those of chimpanzees and humans--so this test of hypotheses can be repeated by any academic or other entity with an appropriate budget without having to travel to see the fossil itself. Third, AL 288-1 represents a species, Australopithecus afarensis, with important phylogenetic significance. It predates both our own genus (Homo) and the later australopithecines with which early Homo coexisted, and it is possibly ancestral to both (Stein & Rowe, 2006). Other specimens of A. afarensis show that AL 288-1 is similar enough to its conspecifics to serve as an exemplar of the species (Drapeau et al., 2005). Fourth, it is a particularly famous fossil, and the public's familiarity with it continues to increase as a result of publicity generated by its current tour through the United States.
The hypothesis that Lucy is anatomically intermediate between apes and humans predicts that a point-by-point comparison will reveal a mixture of apelike features and humanlike features in the specimen. The hypothesis that Lucy is just an ape predicts that such a comparison will only reveal apelike features in the specimen. Some proponents of the latter hypothesis recognize that australopithecines had upright bipedal locomotion but do not consider this a feature linking australopithecines with humans (Mehlert, 2000; Murdock, 2006). …