The Origin of Bipedalism
The origin of bipedalism, a defining feature of hominids, has been attributed to several competing hypothesis. The postural feeding hypothesis (Hunt 1996) is an ecological model. The behavioral model (Lovejoy 1981) attributes bipedality to the social, sexual and reproductive conduct of early hominids. The thermoregulatory model (Wheeler 1991) views the increased heat loss, increased cooling, reduced heat gain and reduced water requirements conferred by a bipedal stance in a hot, tropical climate as the selective pressure leading to bipedalism.
Hunt's postural feeding hypothesis asserts that the arboreal food gathering postures of arm-hanging and vertical climbing, a shared adaptation and postural specialization of apes, are sufficiently common to influence anatomy. Both chimpanzee behavior and australopithecine anatomy inform the model. Eighty percent of chimpanzee bipedalism is during feeding with arm-hanging stabilizing the posture 93% of the time in terminal branches and 52% in the central parts of trees. Torso form in australopithecines features adaptations to arm-hanging, inferring australopithecine adaptation to arboreal bipedal fruit gathering. According to Hunt, this early and specialized origin of bipedalism only later evolved into habitual bipedal locomotion.
The behavioral model, as presented by Lovejoy, focuses on social behavioral mechanisms that influence survivorship and birthrate. Human sexual behavior and anatomy are hypothesized as implying a monogamous mating structure, a social form seen as prerequisite to male provisioning. Provisioning behavior with the upper limbs used to transport food to a mate and offspring is seen as a strong selection factor for bipedality by directly improving offspring survivorship and increasing reproductive rate.
Wheeler's thermoregulatory model proposes, as the selective pressure, bipedalism conferring reduction in heat gain and facilitation of heat dissipation. Bipedalism raises the mean body surface higher above the ground, where more favorable wind speeds and temperatures prevail. Greater wind flow translates to higher convective heat loss. Bipedalism reduces evaporative cooling requirements and conserves body water. Vertical orientation also minimizes direct solar exposure during the time of day when the solar radiation is most intense.
The timing of the appearance of bipedalism is of critical importance in assessing these competing hypotheses. The models all present plausible selective pressures needed for evolutionary change: food access, provisioning, survivorship assurance, increase in offspring, predator and injury avoidance and energy and water conservation. Under different conditions the individual importance of these pressures will change. The important question is what conditions prevailed at the time that bipedalism appears in the fossil record.
Early footprints evidencing a convergent toe and well-developed arches were found at Laetoli, on a paleosurface tuff dated to 3.56 ±0.2 mya (Klein 1999:170). Paleoecological reconstructions for that time include bushland and aquatic fauna at Laetoli and closed woodland at Hadar. Direct evidence of bipedality in Australopithecus anamensis dates from between about 3.9 and 4.2 mya (Leakey, et. al. 1995). An A. anamensis tibia from Kenya has bipedal derived Homo-like characteristics. Bipedality is also inferred for the 4.4 mya Ardipithecus ramidis by the anterior foramen magnum (White, et. al. 1994). The pelvis and lower body of A. afarensis, dating after 3.4 mya., provides even more extensive evidence of bipedalism.
Vrba's "turn-over pulse" hypothesis supports a major climate change, with onset of drier conditions and diminution of wooded habitats, beginning in the Pliocene around 2.5 mya. Faunal ratios evidence a change from frugivores to grazers spanning from 2.3 to 1.8 mya, inferring a mosaic of ecological conditions during that span. Habitat reconstruction based on faunal associations with hominid fossils demonstrate that Australopithecus species lived in wooded and well watered environments (Reed 1997). Homo is the first hominid known to have adapted to open, savannah-like habitats, well after the evolution of bipedality. Arm-hanging anatomy persists long after the onset of bipedal characteristics, indicating occupation of wooded niches for the early bipedal hominids.
Thus, from an paleoecological perspective, the thermoregulatory model does not fit the evidence. Perhaps thermoregulatory selective pressure influenced the direction of Homo evolution, favoring greater height, but bipedalism is well established millions of years before the dramatic change in African ecology and before the change in leg length seen in early Homo. The short lower limbs and broad pelvis of early bipedal Australopithecines are arboreal adaptations, not terrestrial, as are long forelimbs enabling greater access to food and facilitating arm-hanging. These features converge in support of the ecological evidence.
A. anamensis had primitive large canines. Both A. afarensis and A. africanus have moderately large canines. With the behavioral model, which hypothesizes monogamous pair bonding and reduction in mate competition as changing social factors antecedent to bipedalism, one would expect an earlier reduction in canine size, in parallel with the evolution of bipedalism. The social behavioral changes suggested by this model are far likelier to have occurred, if at all in early hominid evolution, in tandem with a greater social capability conferred by the larger cranial capacity seen in early Homo. As with the paleoecology evidence insinuating habitat change, the hominid encephalization is far later in the sequence than bipedalism. Fossil morphology, paleoecology and the fossil chronology converge in support of the postural feeding hypothesis.
Hunt, Kevin D., 1996. The postural feeding hypothesis: an ecological model for the evolution of bipedalism. South African Journal of Science 92:77-90.
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Leakey, Meave G., Craig S. Feibel, Ian McDougall and Alan Walker, 1995. New four-million-year-old hominid species from Kanapoi and Allia Bay, Kenya. Nature 376:565-571.
Lovejoy, C. Owen, 1981. The Origins of Man. Science 211:341-348.
Reed, Kaye E., 1997. Early hominid evolution and ecological change through the African Plio-Pleistocene. Journal of Human Evolution 32:289-322.
Wheeler, P. E., 1991. The thermoregulatory advantages of hominid bipedalism in open equatorial environments: the contribution of increased convective heat loss and cutaneous evaporative cooling. Journal of Human Evolution 21:107-115.
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