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consistency index = 0.69) because the later "robusts" resemble early Homo in so many features. These features include many aspects of basicranial flexion, loss of prognathism (muzzle), changes in the anterior dentition, and encephalization. The postcrania, although not included in this analysis, support the view that at least A. robustus and early Homo are monophyletic relative to other species of early hominid.
Whatever the true phylogeny is, and there can be only one, the fact remains that homoplasy is commonplace. Some resemblances appeared independently and not because of evolution from a common ancestor that possessed the same feature. Either adaptations for heavy chewing evolved twice or basicranial flexion, orthognathism, reduced anterior dentition, and encephalization each evolved more than once.
Bipedalism and the Postcranium
However the specific phylogeny of Hominidae is reconstructed, the important point is that these species are closely related to H. sapiens , and, in general, the more recent in time the species is, the more derived features it shares with our species. The earliest species, A. afarensis, is the most primitive in the sense that it shares the fewest of these derived traits and retains a remarkable resemblance to the common ancestor of African apes and people in many craniodental features. Its postcranium, however, is highly derived (McHenry, 1994a). It is fundamentally reorganized from the form typical of apes to that specific to Hominidae (McHenry, 1982, 1986, 1991, 1994a; Berge, 1993; Johanson et al., 1982; Latimer, 1991; Latimer and Lovejoy, 1989, 1990a, 1990b; Lovejoy, 1978, 1988).
Figure 2A presents features in which the postcranium of A. afarensis differs from African apes and approaches the condition characteristic of humans. The most significant features for bipedalism include shortened iliac blades, lumbar curve, knees approaching midline, distal articular surface of tibia nearly perpendicular to the shaft, robust metatarsal I with expanded head, convergent hallux (big toe), and proximal foot phalanges with dorsally oriented proximal articular surfaces. A commitment to bipedalism in A. afarensis is also shown by the 3.5 million year (Myr) Laetoli footprints, which show very human-like proportions, arches, heel strike, and convergent big toes (McHenry, 1991; Leakey and Hay, 1979; Tuttle, 1987; White, 1980).
The nature of A. afarensis implies that bipedalism evolved well before the appearance of most other hominid characteristics. The appearance of bipedalism is sudden in the sense that it involved a complex alteration of structure in a relatively short period of time. Unfortunately, the fossil record does not yet include hominid postcrania predating