Evolution Of Bipedal Locomotio Essay Research Paper

Subject: Evolution of Bipedal Locomotion

What are we? To the biologist we are members of a sub-species called Homo sapiens sapiens, which represents a division of the species known as Homo sapiens. The most interesting aspect about our species is that we are able to and can walk upright on our hind legs at all times. This is defiantly not the usual way of getting around for a mammal. The view of evolution is to see it as the product of steady environmental pressure exerted on each species, improving its adaptation to its habitat. This pressure, which has been termed natural selection is responsible for this. Natural selection works on the premise that in any population, no two individuals are exactly alike. Most of the differences between them are inherited from their parents, and are thus capable of being passed along to the next generation. Each generation will be slightly different from the one before it, and a little better adapted to the prevailing condition (Hand, 1993).

Habitually walking around on the hindlimbs, leaving the forelimbs free for other jobs, is an unusual mode of locomotion. Once our ancestors had adopted an upright stance, many things associated with being human became possible, such as fine manipulation with the hands, and the carrying of food back to a base camp (Leakey, 1981). This does not suggest that some four million years or so ago primitive hominids evolved upright walking in order to use their hands in refined ways or to develop a food-sharing economy. Indeed, this cannot be the case, because these behaviors did not arise until several million years after the development of upright walking. Nevertheless, the origin of bipedalism must be seen as one of the major steps, if not the major step, in human evolution.

A principle characteristic of the earliest hominids was probably the adoption of an upright posture and a certain way of walking with the posture. When looking at why these changes occoured, it is important to realize that these changes did not occour as in inevitable trend towards modern man. Why did our ancestors start to walk upright and when did they start to, are questions which need to be examined when figuring out the Evolution of Bipedal locomotion. Upon examination of all the evidence, there seems to be many different explanations.

The most significant claim is that Autralopithecus afarensis is probably the first ancestor to all homonids. When studying the bones of A. afarensis they look very modern below the neck. For example, the knee looks very much like a modern human joint; the pelvis is fully adapted for upright walking; and the foot with both modern and primitive features, is adequately structured for bipedalism. Some of the bones in the feet are slightly curved, and look like the bones you would expect to see in a human ancestral who climbed trees (Hunt, 1993). The adoption of upright walking, combined with an analysis of the environments in which hominid bones are found, implies that our ancestors of two million years ago lived in much more open country than did their forebears (Hand,1993). It is Probably at this point in human evolution with respect to bipedalism, A. afarensis was starting to leave behind its arboreal habits. Even still A. afarensis was probably capable of climbing trees in times of danger, and it is likely that they slept in trees, as baboons do, in areas where there are no caves for shelter. However, the body skeleton of these early humans was very different from ours, most notably in their limb proportions. For example Lucy a almost complete skeleton of an A. afarensis, shows clearly that while the arms of A. Afaresis were proportioned pretty much like ours, the legs were rather short (Hand,1993).

With more time being spent on the ground, and the animals moving at a faster speed than before, entirely new demands were made upon the foot. In the foot anatomy, when comparing early hominids and humans, there is a difference between the ways in which weight is transmitted along early hominids and human feet. In an early hominid moving bipedally, the weight is carried along the side of the foot, and the push is passed through the middle of the row of toes. In a human, body weight is transmitted along the outside of the foot, then internally across the ball of the foot, and finally push-off is made by the big toe, a much more efficient arrangement for striding (Hunt, 1993). The choice of aligning all of the toes might have been influence by the basically ape-like hominid s tendency to stand and monitor his surroundings. This would have helped develop strong hind feet capable of taking the entire body s weight. The keeping of a permanently upright stance probably came before rather than after the foot s reorganization for fast movement along the ground. Given some reorganization of muscles and bones it can be as economical of energy to stride on two legs as to run on four (Kingdon,1993). Some curvature in the foot bones as shown before, probably indicated that early humans had retained some climbing abilities. This was probably the middle step between primitive and modern features in an evolving foot which would be perfect for bipedalism. In Lucy s feet it has been found that there are subtle but significant differences. For example, the contours are human like but the ankle joints are still chimplike in the flexibility (Kingdon,1993).

It is importat to look at the subtle differences between the knee joints of a modern human, a chimpanzee and Australopithecus afarensis. The A. afarensis and the modern knee joints resembles each other as they both have a “carrying angle,” typical of todays human bipeds. In the chimpanzee the thigh and shin bones form a straight line, which is an ideal arrangement for a form that carries its weight on all four limbs. However, when walking on two legs this means that at each pace the body s center of gravity must be swung inefficiently in a circle around the supporting leg. In comparison, in the human and A. afarensis the thigh and shin bones form an angle, so that the thighs come together at the knees, and the feet trace a single straight line (Berge, 1990).

In comparing the pelvis in australopithecine and modern humans fossils of australopithecine, show that these hominid s pelvis are relatively shallow and rounded, and the femurs tilt toward the midline. This indicates that the trunk is being supported by the pelvis and body weight is being transmitted directly downward through the knees when the individual stands erect (Berge, 1990). In the pelvis of a chimpanzee, australopithecine and a modern human the distance between left and right acetabuli in australopithecine is less than in humans. This increase in the distance, in humans was apparently the result of selection for a relatively large birth canal to help facilitate the passage of newborn infants with larger crania than possessed by australopithecine newborns. In bipedal hominids, the pelvis assumes a broad, shallow, bowl-like shape with a widened but shortened ilium, thus bringing the sacrum closer to the acetabulum, the temoral socket. These changes, together with forward curvature of the lower spine and further flattening of the thoracic cage, help transmit weight of the trunk directly to the legs, producing a balanced center of gravity along a vertical axis (Strickberger, 1990)

As well as the anatomical aspect of bipedalism in human evolution, selective pressure or natural selection plays the important role in bipedalism in human evolution. For example, some of the consequences of having a bipedal posture in a unshaded extremely hot environment would be that a bipedal human reduces the area of the body receiving both the direct rays of the sun and the heat radiating from the earth. The bulk of the body is also raised high off the ground, thus benefiting from the 22cooling effect of the wind.

A leading selective pressure would be food, in the way it s collected and processed, is vital in determining how an animal will behave. By taking this approach towards bipedalism we introduce a modification in a species. An animal living in woodland or bush savanna spends most of its time on the ground while traveling from one food source to the next. Living primates do stand or walk upright for the most part when they are feeding and on the ground (Leakey, 1981). So it is reasonable to say that a small homonide would have walked upright readily when it was on the ground. This habit would have been reinforced if it spend a good deal of time feeding while on the ground, taking fruits, berries and nuts from low bushes, for instance.

Bipedal locomotion is still extremely beneficial to the hominids. This is because it has enabled the freeing of hands, which could be used for the making and using of tools. When using the bipedal stance, which enhances height, animals have a wider view of the surrounding area and terrain thus improving their field of vision. It helps them in the early spotting of predators over tall grass and enables them to wade in water or pursue game or seek protection from predators, which is critical to their survival. It is also thought that in Bipeds, less of the body is exposed directly to the sun compared to quadrapeds. They also have the advantage that proportionatly more of there body is further away from the ground, therefore less heat from the ground will be effecting them. (Turnbaugh, 239).

Improvement in reproduction is considered one of the benefits of the Bipedals compared to the quadrapeds Bipedalism enables adult to carry food manually to their females and offspring. This mode of provision reduced the need for females to be continuously in foraging both for themselves and their attached offspring as in other competing hominids, thereby offering three important advantages (Strickberger, 1990): (1) a home position which can be considered stable, which helps in providing a more constant social relationships and perhaps closer mother-infant relationships that improve the infants chance of survival, (2) The reduction in injuries to infant because infants no longer were attached to a mother that is continuously mobile and (3) a reduction in the time between each birth by allowing more offspring to be cared for successfully. Thus, the life style introduced by bipedalism, long distance foraging, continuous sexually activity, and their many behaviors probably improved survivorship (Strickberger, 1990).

The Evolution of bipedal locomotion was caused by various selection processes and adaptations which occoured over a long period of time. The reason why these changes occoured are not simple, but on examination of the evidence do seem quite clear. The feet of our ancestors were put under different strains and they adapted in such a way that it benefited the species. This enabled them to stand for longer period of times, run faster and basically ensure that they had a better chance of survival. The knee joints also had to adapt, as the animal could no longer rely on using all four limbs to stand. The joints developed in such a way that it was possible to stand up straight, bipedally. The Pelvis was also forced to changed, facilitating a different center of gravity. It also allowed for the different shape of cranium/ foramen magnum, to be able to pass through the birth canal. Natural selection is also important to take into consideration when talking about the evolution of bipeds. It was because of this selection, that certain characteristics were passed on from one generation to the next. Efficient bipedal movement is a trait that appeared very early, and it is probably the single most important development in the emergence of man.