Theories abound on why our hominid ancestors made the evolutionary leap from brachiating and knuckle walking to bipedalism and some of them are quite creative indeed.
A recent piece of joint research facilitated by three American universities involved the observation of energy consumption and muscle use for both chimpanzees and humans. Experiments allowed the humans to walk normally, while the chimpanzees were coaxed into bipedal walking and knuckle walking.
A device measuring oxygen usage and computer-assisted graphing were employed to record the reactions of specific muscles, and the the results were surprising.
Originally, it was believed that bipedal walking consumed less energy. Sure enough, this particular study did show that humans consumed 75% less energy and calories compared to knuckle walking.
However, of the five chimps that were studied, three were found to have consumed more energy walking on their hind legs than on all fours. Also, one used the same energy in both walking styles. And interestingly, a lone chimp used less energy in the upright walking position.
The findings indicate that, just as some humans are natural climbers, some chimpanzees are natural walkers. Much like humans, apes have differences in anatomy. Those with larger hips seem more able to take longer strides, especially when paired with the usage of the right muscles for walking.
This could indicate that some anatomically adapted ancient hominids might have chosen to walk upright to conserve energy when covering long distances. According to this theory, others mimicked them, and the rest is history.
A very different reason for bipedalism may have to do with brachiating through trees or wading through bodies of water. These activities require the use of the hind legs and a relatively upright position.
Still another theory states that in order to lessen the chances of heat stroke, our ancestors stood up to prevent the surface of their backs from being exposed to the sun.
All of these factors may have contributed directly or indirectly to the development of bipedalism, but there is one other theory that trumps them all in terms of novelty—sex appeal.
Recent findings on ancient Ardipithecus ramidus describe how “Ardi” males “stood up” to increase their chances of getting sexual favors. In their society, females would watch over and raise the young while males foraged for food.
Ardi males soon discovered that the females were willing to offer sex in return for a steady supply of provisions. Having two arms free to carry fruits and tubers thus had a practical advantage.
Of course, all this would have happened as long as six million years ago. But considering all the incredible things humans have done to satisfy their sexual desires over the ages since, perhaps standing up and walking for sex doesn’t seem so strange at all.
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