Selective Pressures Leading to Bipedalism in Hominids - Assignment Example

Selective Pressures Leading to Bipedalism in Hominids Various competing hypotheses have been put d to eluci bipedalism and its origin, particularly since bipedalism is the feature that defines the hominids. Bipedalism has been attributed to ecological pressures in the postural feeding hypothesis, while it has also been attributed to selective pressures of reproductive, sexual, and sexual conduct of hominids in the behavioral model (Sarmiento 62). In addition, the thermoregulatory model attributes the emergence of bipedalism to selective pressures related to reduced water requirements, reduced heat gain, increased cooling, and increased heat loss that led to a bipedal stance in tropical and hot climates. While habitual bipedalism does not seem like the most effective and fastest form of walking and running, it portends various advantages for hominids over specific specialized quadrupedal forms. Although the reason why early hominins took a bipedal stance is not entirely clear, majority of these hypotheses propose the importance of selection pressures based on the environment in driving bipedal evolution (Sarmiento 62). As climatic conditions resulted in the recession of forests, hominins started to move out of the forests into the grassland savannas where it was essential to assume bipedalism to aid in survival. The postural feeding hypothesis seeks to assert the fact that a shared postural specialization and adaptation of apes, as well as the arboreal postures of food gathering involving vertical climbing and arm-hanging, are adequate enough to have been a selective pressure encouraging bipedalism in hominids (Sarmiento 63). The anatomy of both the australopithecine and the behavior of chimpanzees are the major points that inform this model of selective pressures towards bipedalism. For instance, at least 80% of the time that chimpanzees spend in a bipedal stance is when they are feeding, of which they use arm hanging to stabilize their posture at least 90% of the time in terminal branches of the trees, while 52% of arm-hanging occurs in the trees’ central parts (Sarmiento 63). For australopithecines, their torso form has adaptations to arm-hanging, which may infer that australopithecines adapted to selective pressures requiring arboreal fruit gathering. This specialized and early mode of bipedalism evolved to become habitually bipedal much later. On the other hand, the behavioral model as an explanation for selection pressures resulting in hominid bipedalism concentrates on mechanisms of social behavior that influenced birthrate and survivorship. Human anatomy and sexual behavior have been hypothesized under this model to have resulted in a monogamous structure of mating, which can be viewed as a prerequisite to provision of resources by the man (Sarmiento 65). Using the upper body, as a result, of provisioning behavior to take food to their offspring and their mate is considered a strong factor of selection in hominid bipedalism. This is especially so because there was increasing pressure from the environment that required an adaptation to increase the rate of reproduction, as well as improving the rate of offspring survivorship. In addition, hominids were under pressure to protect their offspring from other carnivorous creatures, which made it essential for hominids to become bipedal to free up their forelimbs for protection purposes (Sarmiento 66). The thermoregulatory model, on its part, proposes that, as a selective pressure, thermoregulation necessitated the bipedal posture of hominids as a way to facilitate dissipation of heat, as well as reduce gains in heat. As hominids moved into the hot savanna region from the forests, a lean and upright posture exposed less area of the skin to the overhead heat of the sun (Filler 45). By being bipedal, the hominids could raise their average body surface some height above the ground. At this level above the ground, the hominid could take advantage of more favorable temperatures and wind speeds. An increase in the speed of wind directly translates into higher loss of heat due to convectional effects. In addition, by being bipedal, the hominids were able to conserve body water through a reduction in the need for sweating to cool the body by evaporation. Finally, through the vertical orientation conferred by bipedalism, there was a reduction in direct exposure to the sun, especially during the daytime when there was the most intense solar radiation (Johanson & Kate 26). Bipedalism and the timing of its appearance are critically essential in the assessment of these different hypotheses, as well as the selective pressures that they seek to incorporate into bipedal evolution (Filler 59). All of the hypotheses present selective pressures that all plausible and needed for evolution. These are energy and water conservation, avoiding injuries and predators, increasing the chance of offspring survival, ensuring their survival, providing for offspring and mates, and increasing access to food. These selective pressures will change in importance with regards to the variation of the environmental conditions (Johanson & Kate 28). What is most important is the condition that was prevalent during the time when the fossil records show the emergence of bipedalism. The convergence of toes and development of foot arches are evidenced in early footprints found on a paleosurface at Laetoli dated back to 3.56 mya. During this time, paleoecological reconstructions seem to indicate the presence of aquatic fauna and bush-land in Laetoli, as well as closed woodlands around the Hadar region (Filler 59). Bipedalism is directly evidenced by Australopithecus anamensis that can be dated back to between approximately 4 mya. A tibia from A. anamensis found in Kenya possesses Homo-like bipedal-derived characteristics. Ardipithecus ramidis, dated back to 4.4 mya, also evidences bipedalism through the anterior foramen magnum (Filler 59). In addition, the Australopithecus afarensis that is dated back to 3.4 mya shows bipedalism evidence that is more extensive. The thermoregulatory model, from a paleoecological perspective, however, does not fit with present evidence, particularly since bipedalism was established way before the dramatic alterations in the ecology of East Africa, although thermoregulatory selective pressures may have affected the direction of evolution for Homo species (Filler 61). The broad pelvis and short lower limbs of early Australopithecus with bipedal characteristics are not terrestrial adaptations but arboreal adaptations, just as with the longer forelimbs that enabled arm-hanging and greater food access. All these features are convergent in their support for ecological evidence. In addition, comparing A. anamensis to A. afarensis and A. africanus, the former has primitively large canines with the latter having moderately big ones. One would, therefore, expect earlier canine size reduction with regards to the behavioral model that proposes monogamous pair mating, in parallel to bipedal evolution. In this case, social behavioral changes suggested under this model were more likely to have occurred in the early evolution of hominids along with a larger cranial capacity (Filler 61). As a result, fossil chronology, paleoecology, and fossil morphology point to the postural feeding hypothesis as accounting for the selective pressures leading to bipedalism. Works Cited Filler, Aaron G. The Upright Ape: A New Origin of the Species. Franklin Lakes, NJ: New Page Books, 2009. Print. Johanson, Donald C, and Kate Wong. Lucys Legacy: The Quest for Human Origins. New York: Harmony Books, 2009. Print. Sarmiento, Esteban E. Generalized Quadrupeds, Committed Bipeds, and the Shift to Open Habitats: An Evolutionary Model of Hominid Divergence. New York: American Museum of Natural History, 2012. Print. Read More