Ten Strange Consequences Of Evolution

Ten Strange Consequences Of Evolution: "

The human body in itself is a powerful piece of evidence in favor of evolution — many of our physical traits make little sense other than as leftovers from back when we were fishy or four-legged. Smithsonian Magazine examines some of the oddest and most troublesome quirks:

Hiccups

The first air-breathing fish and amphibians extracted oxygen using gills when in the water and primitive lungs when on land—and to do so, they had to be able to close the glottis, or entryway to the lungs, when underwater. We descendants of these animals were left with vestiges of their history, including the hiccup. In hiccupping, we use ancient muscles to quickly close the glottis while sucking in (albeit air, not water). One of the reasons it is so difficult to stop hiccupping is that the entire process is controlled by a part of our brain that evolved long before consciousness, and so try as you might, you cannot think hiccups away.

Backaches

The backs of vertebrates evolved as a kind of horizontal pole under which guts were slung. It was arched in the way a bridge might be arched, to support weight. Then, for reasons anthropologists debate long into the night, our hominid ancestors stood upright, which was the bodily equivalent of tipping a bridge on end. Standing on hind legs offered advantages—seeing long distances, for one, or freeing the hands to do other things—but it also turned our backs from an arched bridge to an S shape. The letter S, for all its beauty, is not meant to support weight and so our backs fail, consistently and painfully.

Unsupported intestines

Once we stood upright, our intestines hung down instead of being cradled by our stomach muscles. In this new position, our innards were not as well supported as they had been in our quadrupedal ancestors. The guts sat atop a hodgepodge of internal parts, including, in men, the cavities in the body wall through which the scrotum and its nerves descend during the first year of life. Every so often, our intestines find their way through these holes—in the way that noodles sneak out of a sieve—forming an inguinal hernia.

Choking

In most animals, the trachea (the passage for air) and the esophagus (the passage for food) are oriented such that the esophagus is below the trachea. In a cat’s throat, for example, the two tubes run roughly horizontal and parallel to each other before heading on to the stomach and lung, respectively. In this configuration, gravity tends to push food down toward the lower esophagus. Not so in humans. Modifications of the trachea to allow speech pushed the trachea and esophagus further down the throat to make way. Simultaneously, our upright posture put the trachea and esophagus in a near-vertical orientation. Together these changes leave falling food or water about a 50-50 chance of falling in the “wrong tube.” As a consequence, in those moments in which the epiglottis does not have time to cover the trachea, we choke. We might be said to choke on our success. Monkeys suffer the same fate only rarely, but then again they can’t sing or dance. Then again, neither can I.

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