The well-known anthropologist W.M. Krogman often talked about the problems modern humans have in some of their biological features as being the result of the way we evolved, what Krogman called the "scars of human evolution." Discuss a biological complex that you believe Krogman might have been referring to, and include in your essay some reasons why he might have called these a scar of human evolution.
As hominids began walking upright, they underwent extensive morphological adaptations. The human backbone was forced to accommodate to new vertical weight-bearing stresses. As a result of this comparatively recent evolution in the spine, modern humans regularly experience lower back pains, which can be labeled as "a scar of human evolution."
In modern humans, the vertebral column surrounds and protects the spinal cord as it descends from the brainstem at the foramen magnum to the lower trunk. The vertebral column provides the body's main axial support, but it still remains flexible; the spine is comprised of 24 individual bones, called vertebrae, and two fused bones, the coccyx and the sacrum. The first 7 vertebrae located in the neck are cervical vertebrae, followed by 12 thoracic vertebrae that connect to the rib cage, and 5 lumbar vertebrae that make up the lower back. The sacrum is composed of 5 vertebrae (forming the back of the pelvis) that fuse together during adulthood. The coccyx (the tailbone) is made up of irregularly shaped vertebrae that fuse together between the ages of four and six years. The thoracic and sacral curvatures form during fetal development. The cervical curve forms when a human infant begins to hold up its head. The concave lumbar curve forms when a young human child begins to walk. These features maintain the balance and support necessary for bipedalism.
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In quadrupeds, there is a gently C-shaped curve that makes the thoracic region of the spine slightly convex. The human biped, however, has an S-shaped spine resulting from opposing curvatures (in the cervical and lumbar spinal regions) grafted onto the C-shape curvature of a quadruped. The lower back (lumbar) vertebrae form a very acute curve in humans by placing the last two inter-vertebral discs at a marked angle to the passage of body weight. The spinal curves in the biped allow the weight of the body to be carried directly over the hip joint sockets in the midline, by bringing the center of gravity closer to the hips.
The weight of a biped is borne down the spine to the sacrum, where it passes to the hips and then through the two legs. Because the amount of weight increases progressively down the spine, the vertebrae of a biped are increasingly larger as they approach the lumbar region. In contrast, weight bearing does not increase along the spine of a quadruped, and so the vertebrae remain of nearly equal size in the different regions of the spine.
Intervertebral discs of connective tissue separate each vertebra. The outside is made up of several layers of fibrocartilage. The inside is the nucleus, which is filled with a pulp that has the consistency of jello. The nuclei of the disc act to cushion each vertebra and absorb shock.
Once a human reaches adulthood, the functioning of the spine goes downhill. With age, the spongy disks between the vertebrae lose moisture and elasticity; the pulp inside loses flexibility and becomes less supple. The harder, drier disks lose height, bringing the vertebrae closer together. Many individuals develop serious complications of the vertebral column.
Intervertebral disc breakdowns affect millions of people worldwide; many suffer serious chronic pain, and life long disability. A ruptured disc, "a slipped disc" in common parlance, occurs when an intervertebral disc becomes thinner and compressed, causing a herniation of the disc's contents and pressure on the spinal nerves, most importantly, the main sciatic nerve. A tear in the annulus fibrosis on an intervertebral disc allows the soft nucleus pulpous to ooze out. This herniation results in loss of muscle. An example of this muscular degeneration is foot drag. Almost 90 percent of herniations happen in vertebrae 4 and 5 in the lumbar vertebrae. These vertebrae have the most acute curve, and produce our upright posture. Because these two vertebrae are so angled, they bear the most differential weight and, accordingly, suffer the greatest wear and tear.
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Most people can recover from intervertebral disc breakdown without surgery; the pulp that is oozing out will eventually retract from the nerve because it loses moisture and it shrinks. Nonetheless, there are more than 4 million operations a year in the United States alone to correct disc herniations. If an intervertebral disc breaks down and results in pressure on the spinal cord, it can be life threatening. The most immediate symptom is the loss of bladder and bowl control.
Another serious complication of the vertebral column is osteoarthritis - the breakdown and eventual loss of the cartilage between the vertebrae. This results in thickening of the joints and supporting ligaments and the growth of bony spurs that can close in around the exits for the spinal nerves. Osteoarthritis can lead to spinal stenosis, which is "the narrowing of the central spinal canal or its lateral recesses" (Weinstein).
Lower back pain was not as big as a problem for our ancestors. There are three theories to explain this fact. The first explanation is that the small body of our ancestors led to less force through lumbar system. The second explanation is that our ancestors were not couch potatoes. Our ancestors had incredibly powerful muscles (robust bones and large articulations), as a result of constant physical activity. This helped to preserve the lower back. The third explanation is that our ancestors did not live as long as modern humans, as so they did not reach the age where back problems emerge. In support of this theory is the fact that the mean age for disk surgery is 42 years old.
Today, more than 70 percent of adults suffer from back pain at one point in their lives, and 30 percent have had it in the last 30 days. There are many ways to treat and prevent back pain in modern humans. The most important is exercise, and increasing our muscle tone of the back and legs. Minimizing sitting is also good, because chairs offer uneven distribution of weight. Surgery is a treatment option, but it should be the last option; surgery can be very painful, and sometimes it does not alleviate the pain. Treating back pain currently costs $26 billion a year; which is 2.5 percent of total health care costs in the United States.
Lower back pain is a biological complex that Krogman would have clearly labeled as a scar of human evolution. It is the inevitable product of bipedalism and longevity. Hopefully medical science will advance to a point where it can more effectively alleviate this painful consequence of human evolution.
- Deyo, Richard A. "Low Back Pain." Blackboard. Web.
- Mann, Alan. "Bipedalism, Power Point Presentations 1 and 2." Lecture.