Evolution of Bipedality in Humans Essay

Bipedalism is a capacity, mostly associated to humans, to use two legs in locomotion. Its origin has been given importance and served as a topic of long debate among experts due to the possibility that this form of locomotion gave way for the development of modern human characteristics. Thus, a lot of theories were proposed and established in order to provide the logical explanation on its origin and development. The burden of dealing with this classical issue lies on the fact that locomotion is an activity that can not be fossilized. However, the Australopithecus afarensis fossils, the earliest hominid from Ethiopia and Tanzania, were found to live 3 million years after the appearance of hominid lineage around 6. 5 million years ago (â€Å"Bipedalism,† 2000). These fossils in a way may serve as evidence not only of bipedal locomotion but also hominid dental characteristics similar to apes. Bipedalism enabled humanoids to create tools used for the sustenance and defense of life and is also observed in other animal species (Dhingra and Jablonski, 2004). This trait is commonly attributed as a product of evolution for it coincided with the physiological development of the human brains. As such, bipedal locomotion has gone a very long and different history within animal kingdom. Bipedal Locomotion Human locomotion is described as a smooth flow of a series of actions including swing and stance phases (â€Å"Bipedalism,† 2000). In the swing phase, one leg shoves off through the toe then swings in slightly flexed position. As the foot creates contact on the ground through the heel, the leg becomes and remains extended to support the body. Then, as the leg moves in the swing phase, the body moves in the stance phase. On the other hand, chimpanzees are not capable of extending their knee-joints for a straight leg in the stance phase. They exert muscular power to support their bodies while their leg flexed gait denotes lack of toe off and heel strike in the swing phase. The human anatomical structure is fully developed for terrestrial locomotion while chimpanzee anatomy is adapted for climbing and knuckle walking (â€Å"Bipedalism,† 2000). Human anatomical adaptations include extensive and curved lower spine, a shorter and broader pelvis, a longer lower limbs and enlarged joint surface areas. In the swing phase, the weight of the body is shifted to the supporting leg while the balance is maintained at the unsupported side in the stance phase through contraction of gluteal abductor muscles in the hip. In chimpanzees, the gluteal abductors are not fully developed and their thigh bones do not slope inwards as in humans. Their feet are normally a little bit apart and in walking, they tend to shift their upper bodies from side to side so as to transfer the weight on each leg. It is a fact then that chimpanzees and gibbons can move through bipedal locomotion. Thus, as viewed by anthropologists, bipedalism is a hominid distinct adaptive capability (â€Å"Bipedalism,† 2000). Humanoids spend less energy through bipedalism than quadrupedalism because at a normal walking speed, the legs in a forward motion swing like pendulum then bringing back the forward momentum by slowing the swing foot before the fall (Hawks, 2005). Hence, this requires less muscular activity and energy than knuckle-walking. Moreover, bipedalism raises the head which gives a sharp range vision around the environment and making hands available for carrying tools, food items or other works. However, for early humanoids, bipedalism can offer disadvantages (Hawks, 2005). Without the ability to grasp through feet makes them unsecured from their predators. The loss of grasping foot made difficult for the early humanoids to climb through tress and escape predators, and for their young in clinging to their parents. Researches Findings Biologist and anthropologists have debated over bipedalism and proposed different hypotheses for possible explanation. However, these theories have their respective strengths and weaknesses on the plausible understanding with bipedal locomotion. Most of these theories correlated bipedalism to the animal species’ ventures on the savannah areas and was supported by the discovery of Lucy, a 3. 5 feet humanoid (Johanson, n. d. ). Standing up in savannah gives stunted humanoids a vigilant view on possible threat from their predators. Standing up in reaching out for foods is another possible explanation. In addition, an upright posture may also help our ancestors in cooling their bodies against the sultry environment. In the postural feeding hypothesis of Hunt (1994), bipedalism has evolved from the early habitual bipedal locomotion of humanoids due to arboreal food gathering. The behaviour of chimpanzees and the anatomical structures of australopithecine conformed to this ecological model. Eighty percent of bipedal locomotion in chimpanzee was observed during feeding with arm-hanging stabilizing the posture. In addition, the upper body anatomical structures of australopithecines were ascribed to arboreal bipedal fruit gathering adaptation. The behavioural model of Lovejoy (1981), on the other hand, attributed bipedalism to the social, sexual and reproductive behaviours of early humanoids. Their sexual characteristics and anatomical structures were believed to be an implication of monogamous mating. This sexual attitude led to provisioning behaviour of the humanoid male species wherein their upper limbs were used in giving food stuffs to their mates. Even Charles Darwin constructed a model in providing a logical explanation for humanoid origin and bipedalism. He postulated that bipedalism resulted from the terrestrial adaptation of quadrupedal species and the necessity for subsistence, required to giving freedom for hands to accommodate other activities such as making tools for food hunting (Hawks, 2005). Aside from this, he also assumed that the habitat change from woodland to savannah paved the way for the less importance of climbing. Then, other researchers augmented Darwin’s assumptions by elaborating that living in savannah made early humanoids to be vigilant over tall grasses and adapt in its sultry condition (Hawks, 2005). Thus, bipedalism provided them means for adjustments at these conditions by standing up leading to less sun exposure of the body. This model became the savannah model or killer-ape hypothesis. The aquatic theory and the theory on the use of tools are deemed to offer unreliable explanation (Johanson, n. d. ). Most of the time, primates avoid water due to ferocious predators. On the other hand, stone tools only appear around 2. 6 million years ago as contrasted with the 4. 2 million year-evidence of bipedal locomotion. According to Johanson (n. d. ), it is much better for the theorists to look not into the reason for the upright posture of our ancestors but rather on the advantages for the early humanoids that resulted to a behavioral change from quadrupedalism to bipedalism. Videan (2002) tested the different hypotheses for bipedalism (cited in Dhingra and Jablonski, 2004). The Carry Hypothesis described bipedalism as an adaptive means in the exploration of natural resources while carrying children, tools or foodstuffs. The Forage Hypothesis viewed it as a means towards the food gathering facilitation. Also, the notion that bipedalism gives animal species a sharp sense of sight in order to find good habitats; prey or escape from a predator is called as Vigilance Hypothesis. Display Hypothesis on the other hand, explained bipedalism as a gesture of threat for animal species. Based on the result of this study, Videan (2002) successfully correlated Carry, Vigilance, and Forage Hypotheses towards environmental adaptation of animal species. Nevertheless, the Display Hypothesis gave explanation for the sexual attraction among animal species. Dhingra and Jablonski (2004) scrutinized the bipedalism in several animal species like lizards, birds, kangaroos, and dinosaurs. They concluded that bipedalism resulted from animal’s natural attempt to adapt to their changing environment such as in escaping from predators or catching a prey, and in giving way to bodily parts for other functions like the forelimbs in humanoids for feeding and wings of the birds for flying. According to Stanford (2006), arboreal bipedal gait is different from arboreal quadrupedal stance for he observed that while feeding in tree crowns, Bwindi chimpanzees changed smoothly from four-legged to three-legged and even two-legged posture. This shift may have occurred in early humanoids. Modern gorillas are terrestrially adapted but based on observations; even adult male gorillas climb into tall trees for food. Thus, it is also possible for early humanoids to adapt a variety of bipedalism based on their ecological conditions. More than this, few evidences speak for terrestrial adaptation of modern chimpanzees considering the fact that they used to travel primarily on the ground and stay on trees only for sleeping or feeding purposes. These observations may denote the possibility of behavioral plasticity and arboreality in early humanoids. Based on the review conducted by Richmond, Begun, and Strait (2001), about the different studies on bipedalism, humans evolved from knuckle-walking and climbing ancestor as portrayed by evidences. These include terrestrial characteristics in the hands and feet, climbing skeletal structure, and knuckle-walking attributes in the wrist and hands. These features narrowed down the list of theories concerning bipedalism. Evidences from the paleobiology and paleoenvironments weakened the postulates based on monogamous social structure and open savanna-based bipedalism hypotheses. Rather, they suggested giving more importance on hypotheses based on food acquisition and carriage and a deep anatomical examination on the extant anthropoid behaviors. Conclusion No single factor may completely explain the origin of bipedalism. Perhaps the concoction of the factors considered in each theory that are deemed to be valid may provide much reliable reason for bipedal locomotion. The lack of complete knowledge on the anatomical structure of the early humanoids is a great burden in gaining a complete understanding on the origin of this locomotion (Hawks, 2005). Perhaps, a simple way of explaining bipedalism is that bipedal locomotion evolved as a result from the changes in the environment or social structure that is due to the availability of dietary resources to sustain life. It is hypothesized that in the late Miocene period, hominid food sources dispersed in some areas which forced hominids to travel, thus, led to the development of locomotion anatomical structures (â€Å"Bipedalism,† 2000). References Bipedalism. Human Evolution. Retrieved November 6, 2008, from http://www. stanford. edu/~harryg/protected/chp15. htm Dhingra, P. and Jabslonski. (2004). Comparative Bipedalism – how the rest of the animal kingdom walks on two legs. Retrieved November 6, 2008, from http://www. philosophistry. com/static/bipedalism. html Hawks, J. (2005). Why be bipedal? Retrieved November 6, 2008, from http://johnhawks. net/weblog/topics/bipedalism/why_be_bipedal. html Hunt, K. D. (1994). The Evolution of Human Bipedality. Journal of Human Evolution, 26, 183-202. Johanson, D. (n. d. ). What the Evidence Suggests. Retrieved November 6, 2008, from http://www. pbs. org/wgbh/nova/allfours/bipe-johanson. html Lovejoy, C. O. (1981). The Origins of Man. Science, 211, 341-348. Richmon, B. G. , Begun, D. R. , and Strait D. S. (2001). Origin of Human Bipedalism: The Knuckle-Walking Hypothesis Revisited. Yearbook of Physical Anthropology, 44, 70-105.