Thursday, May 13, 2010

13 Thursday

Aerobics & Stepdancing Term Paper
Muscles, Bones, and Evolution

As a bio major, I’m often surprised at how little I know about humans compared to fruit flies or nematodes. Today I’m surprised at how little I know about our muscles and skeleton. During exercise, it’s obvious that certain muscles are significantly weaker than others. Nothing in the body happens without some evolutionary purpose, like having an appendix (it was useful for ancestral animals that ate mostly leaves). I decided to look at why certain muscles are so weak and some injuries are so common.
The oldest known primate-like mammal is known as Plesiadapis; it lived 65 million years ago, weighed about 5 lbs. and looked a little bit like a weasel. Their skeletal structures revealed they were adept climbers with short, strong limbs, and gripping claws on all four limbs. The great apes, which are the closest living relatives to humans, are descendents of these tree-dwelling primates that also gave rise to hominids. Given ancestors that were most at home climbing and swinging in trees, it is no surprise that certain muscle groups in the upper body are strong and suitable for such movement. Similar to doing pull-ups, swinging through trees heavily involves the latissimus dorsi, biceps, and pectoralis major. These muscles are involved in pulling the body against gravity toward a tree limb. The deltoids and triceps work opposite these muscles, serving to lower the body slowly with gravity instead of simply letting it fall. Interestingly, these are two muscles of the upper body that tire particularly easily during exercise. The lowering muscles are much smaller and weaker than their lifting muscle-counterparts. One reason for this is gravity; whereas the first group of muscles works against gravity, the second group works with gravity. Another reason for this is that simply falling remains an option for tree dwellers, particularly when they can land safely on their feet. Leg muscles are typically larger than arm muscles, so a primate conserves energy by walking or jumping without needing to develop larger arm muscles.
Bipedalism (walking on two legs) allowed primates to use their forelimbs to get food or protect themselves, but skeletal adaptations were constrained by the build of their parents. The earliest terrestrial animals were fish that had adapted to breath air and move on land. These fish had four limbs, all of which it used for walking. The same is true of the reptiles, amphibians, and mammals that resulted from successive generations adapting to new environments. A great majority of terrestrial animals are quadrupeds (walk on four legs) and bipedalism is a relatively new concept in mammals. This change from walking on four legs to using only two was accompanied by drastic renovations to the skeleton.
Lower back pain is a brutally common and unpleasant condition resulting from disagreement between human anatomy and behavior. Our lower backs are also quite weak, and overuse can lead to injury. In a quadruped, the spine has a downward C-shaped curve, and the lower back and pelvis are directly over the hind legs. For example, while the lower back of a dog may bear considerable amount of weight while standing, it is carried much more efficiently. In the upright human spine, the lower back connects to the pelvis in the same way, curving slightly forward. In order to keep the body’s center of gravity over the feet, the spine has to curve backward again to offset the forward curve. As the spine connects to the skull, it curves forward again, giving it an overall S-shape. This change in posture created extra strain on the lower back. The rotation of the pelvis brought the lower back muscles down and away from the spine, decreasing their ability to support the back’s new range of motion. The decreased support at the lower back is the culprit of injuries from heavy lifting, problems with poor posture, and the back pain experienced by pregnant women and obese people.
The transition to bipedalism brought about other renovations to the lower body making certain areas prone to injury and wear. In particular, hips and knees suffer the consequences of altering the stance from gorilla to upright. Early bipedal primates had a wide stance and stood with knees slightly bent. In comparison, humans stand with feet and knees directly under the body, with the legs fully extended at the knees. Knee problems result from this adjustment for a number of reasons. First of all, the leg is now hyper extended at the knee, increasing the amount of wear at the kneecap. Secondly, in order to put the feet directly below the center of gravity given the width and rotation of the hips requires that the stance rotate at the knees. The consequence is greater pressure on the knee joint at an angle that accelerates wear.
As hinted at earlier, the pelvis plays a pivotal role in the evolution of structural weakness in humans. It constrains the shape of the upright spine to a stress-inducing S-curve and shortens the pelvis vertically. This reduces the size of the pelvic cavity, making childbirth more difficult for us than it was for early hominids. In gorilla stance, the hips are higher in the pelvis, connecting at the sides, and the legs extend forward and apart with respect to the pelvis. In the human stance, the hips connect lower on the pelvis at a different angle, and the legs extend downward and inward, putting the feet directly below the body. This increases the amount of wear at the hip joint from all directions. The joint is also smaller and therefore weaker due to the shortening of the pelvis.
With all of these problems and shortcomings that result from the evolution of humans’ upright posture, one might wonder what made it so advantageous during the process of natural selection. There are a lot of compelling reasons for why it’s good to be bipedal. 1) There are more predators in the forest than in the savannah, where you must walk. 2) You have a greater range of motion to do things like hunt with a spear. 3) Your upright stature gives you better control over your limbs, allowing you to use tools and build things like huts. 4) It’s easier to look behind you, because your body is not in the way. There are many more advantages to these changes, far more than the negative consequences that we focus on. Natural selection is constrained to modifying parts of the body structure that already exist. The body plan to which we owe our dominant status in the natural world comes at a price.

No comments:

Post a Comment