or How we traded half our legs for all of our arms.

We Homo sapiens are the only living members of the primate order who are bipedal, that is, we have a funny habit of walking around on two legs. Neither our early predecessors, whom we share with the apes, nor the apes themselves do this. How was the transition made? At first glance, it might even seem a disadvantage. We're so slow compared to four-legged animals that it boggles the mind and we have a relatively high center of gravity positioned over a very small surface area. Understanding how and why our ancestors became bipedal is an important step in understanding our evolution, as it is one of the few major traits that set us apart from the apes and our common ancestors.

In accordance with the currently favored model of evolution, for a trait to be carried on, it must occur as a mutation and then be favored by the environment in which the organism lives. So we must assume that the practice of ambulating on two limbs is no exception. In order for bipedalism to proliferate, it must have granted some advantages to the primates in which it appeared.

Most of the fossilized remains of our earliest predecessors have been discovered in Africa, east of the Rift Valley. This area is now mostly dry savannah, but it's believed that bipedal primates evolved either during or before the tectonic shifting that caused the area to dry up and allowed its forests to give way to grasslands. So we have a forested environment as the stage on which these strange, two-legged creatures make their debut. By the time they started walking bipedally, they must have already been spending some time on the ground, since the bipedal adaptation suggests walking on the surface and not swinging through the trees. Since they were most likely scavengers, they often had to walk long distances from one area to another in order to secure a fresh supply of food. Then, since they faced predators even in the forests, they would probably have to carry the food to a safe place before consumption. Bipedalism, though making them much slower at a running pace, is much more efficient for walking long distances. It also allowed them to have two limbs free to carry food, or, in the case of the mothers, their offspring.

Some other aspects of bipedalism are easier to understand if we examine the bonobo. The bonobo is thought to be the closest living relative to the common ancestor of apes and humans. The bonobo spends much more time walking on two legs than any other ape, and it displays sexual consciousness, using sex not only unconsciously, as a reproductive method, but also as a means of bonding and ensuring unity in a group. Our enjoyment of this particular behavior is thought to have appeared at the same time as bipedalism, with the disappearance of estrus and resulting constant sexual receptivity, which made reproduction much more convenient, and freed more time for food gathering; the male felt more secure leaving his companion during gathering because he didn't have to worry about her going into heat and fucking anything that moved.

Although they occurred after the initial development of bipedalism, certain things are thought to have reinforced it later, on the savannah, such as the ability to see farther due to a higher viewpoint. Also thought to be a later advantage was the cooling effect of exposing less area to solar radiation and keeping the torso and head in the area with cooler air currents, both of which could have fostered the development of our large, complex brains; the brains are liable to suffer a terminal meltdown when exposed to an excessive level of heat.

It may be slow and hard to get the hang of it, at first, but walking around on two legs does seem to have its advantages, not to mention the key role it played in our subsequent world domination, which will probably result in the destruction of most species on the planet, if not the very ability of the planet to sustain life. Well, maybe we'll evolve again, sooner rather than later.

This was originally an essay for my biological anthropology course. I followed the advice of sensei and noded my homework.

For more information about bipedalism on Everything2, check out Kaytay's excellent wu Major Features of Hominid Bipedalism.

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