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In the Light of Evolution Volume IV: The Human Condition
skeleton found at Aramis (Middle Awash, Ethiopia) and dated to 4.4 mya. According to the researchers who described the specimen, some features characteristic of the modern human pelvis that are strongly related to bipedal posture—because they permit abduction during walking—can already be appreciated and are also found in the australopithecines and later hominids: a short iliac isthmus, a slightly broadened and sagittally oriented ilium with a weak greater sciatic notch, and a strong, anterior inferior iliac spine formed by a separate ossification center. In addition, the pubic symphysis would have been superoinferiorly short, differing from the tall symphysis in chimpanzees.
The authors of the study of the skeleton of A. ramidus maintain that the common ancestor of humans and chimpanzees was not a brachiator like living chimpanzees, but “was probably a palmigrade quadrupedal arboreal climber/clamberer that lacked specializations for suspension, vertical climbing or knucle-walking” (White et al., 2009). It was also claimed that A. ramidus occupied a different ecological niche than extant chimpanzees because the study of stable isotopes has shown that they consumed some C4 plants (a type mostly represented in East Africa by grasses and sedges) as part of their diet (~10–25%), whereas extant chimpanzees are almost pure C3 (forest green plants) feeders.
Although bipedal posture had been established, it would have undoubtedly been a more primitive form than that of Australopithecus. The postcranial skeleton of A. ramidus is, in general, very different from that of Australopithecus afarensis. If Australopithecus anamensis resembles A.afarensis postcranially, and A. ramidus is the direct ancestor, the passage from one adaptive plateau to another would have occurred in a relatively short period, ≤200 kya (from 4.4 to 4.2 mya). Thus, we could speak of a rapid evolution, at least in comparison with the subsequent stability in the body plan, which would not change during at least the subsequent 2 million years of evolution. But it is also possible that the skeleton of A.ramidus from Aramis corresponds to a later population than the population (of the same species) that gave rise to Australopithecus. In this case, the mother and daughter species would have coexisted, implying that this transition is not an example of the phyletic mode of evolution but rather of speciation or ramification (branching evolution) and further, of a special type [“like a parental Hydra buds off young individuals” in the words of Eldredge and Cracraft (1980)], because only a part of the ancestral species would have given rise to the descendant.
HEADS AND BODIES
The neo-Darwinians, in general, gave more weight to the phyletic mode of evolution and maintained a very lineal notion of human evo-