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Bipedalism Evolution Theories Facts And Resources

What is Bipedalism

Bipedalism means walking upright on two feet rather than on four feet or the hands and feet. Fossil evidence shows hominids, the evolutionary forerunner to human beings, began walking bipedally even before their brains became significantly larger, around 6 million or 7 million years ago. For this reason, scientists believe bipedalism was somehow important to the progression of the hominid beyond the level of the other primates. Thus, paleontologists consider bipedalism to be one of four major stages in the evolution of hominids into humans. The other three stages are terrestriality, encephalization, and civilization.

Theories as to Why There Was a Shift To Bipedalism

There are many different theories to explain what caused this shift to walk on two feet. One theory is that it reduced the amount of body surface getting direct exposure to the strong African sun. Another is that it helped hominids walk along lower branches while supporting their upper bodies in the higher branches. In the water, the ability to stay upright may have prevented drowning. Still, another theory is that bipedalism was a key factor in helping hominids spot predators, while others think walking upright aided hominids in reaching fruit growing on trees. These theories may be correct, but it isn’t easy to pinpoint which may have been the most decisive factor or factors in the shift.

Two professors, Adam Sylvester of the University of Tennessee and Patricia Kramer of the University of Washington, used chimpanzees to make a comparative study of how much energy primates must expend when they keep having to sit up or stand up to reach food, confront a predator or accomplish other tasks that require being upright, as opposed to how much energy they expend to remain on two feet between such tasks. They concluded that the higher a primate had to reach, the more energy-efficient it was to remain bipedal. This energy efficiency also holds for walking on two feet for certain distances between tasks, depending on the size of the primate. In addition, the professors pointed out that human babies learning to walk likewise use a distance-related, instinctive decision-making process to determine if they will attempt to walk to the next place they want to go to or if they will get down and crawl there.

Like human toddlers today, it is likely that at first, hominids did not stay bipedal for long distances. A typical scenario might have been a hominid standing up on its rear two feet to reach for a piece of fruit, spotting another tempting fruit hanging from a nearby tree, and instinctively realizing it would take less effort to shuffle over to that second fruit on two feet rather than drop down on all fours, amble over to the tree and stand up again. It is also likely that these evolving hominids did not walk or run smoothly as they started staying on two feet but rather shuffled or toddled as some primates still do.

What is the Significance of Bipedalism in Human Evolution?

No matter which factors contributed more or less to the shift, it is clear that this change in stance had great significance in human evolution. Bipedalism dramatically changed the hominids’ relationship to the world around them and opened up many new opportunities. However, it is likely that the ability to walk developed gradually. Modern man’s gait is the end product of a long evolutionary process during which people underwent physical changes to adapt to living on solid ground.

What Other Changes did Bipedalism Cause?

The shift to bipedalism caused, during the time, many physiological and anatomical changes. The foot formed an arch as it became adapted for supporting the body’s full weight. Because they were no longer needed for grasping, the toes became shorter. In hominids, weight was borne by the heel and then transmitted to the outer foot and then the middle toes, while in humans, it is the ball of the foot and the big toe that receive the final weight load. This apparently helps conserve energy.

Human hip joints adapted for upright walking by enlarging and broadening, which caused the spinal column to sit closer to the hips for more stable support. The spine also assumed a slight S-curve shape to center weight over the legs to help with balance on two feet.

Knee joints also became larger to support more weight. When a human walks, the knees are straight, and the thighs turn slightly inward, rather than outward as with hominids, to help with balance. Legs got longer, and the muscles adapted so that the legs could support locomotion without the need for the arms to help swing them forward. This freed up arms to carry objects while walking. The bone structure of the upper arms and legs also changed accordingly.

Even the human skull adapted to the new style of walking. As a result of bipedalism, the base of the skull where the spinal column attaches to the head shifted to put the head’s weight behind the spine. Once that happened, the head could be held erect without the help of muscles in the face. That is why the human face eventually became flatter than that of other primates. Bipedalism also caused major changes in the shape of the female pelvis, so it is easy for scientists to identify a skeleton as male or female.

What Was the First Bipedal Fossil Discovered?

The first bipedal hominid fossil discovered intact was “Lucy,” found in 1974 in Ethiopia. Lucy’s skeleton has been digitally archived, allowing students and scientists to examine her bones and compare them with the skeletons of apes and humans. Much of the information available today on bipedalism is a result of studying Lucy.