By Rob Dunn
Smithsonian.com, November 19, 2010
From hiccups to wisdom teeth, the evolution of homo sapiens has left behind some glaring, yet innately human, imperfections
Natural selection acts by winnowing the individuals of each generation, sometimes clumsily, as old parts and genes are co-opted for new roles. As a result, all species inhabit bodies imperfect for the lives they live. Our own bodies are worse off than most simply because of the many differences between the wilderness in which we evolved and the modern world in which we live. We feel the consequences every day. Here are ten.
1. Our cells are weird chimeras
Perhaps a billion years ago, a single-celled organism arose that would ultimately give rise to all of the plants and animals on Earth, including us. This ancestor was the result of a merging: one cell swallowed, imperfectly, another cell. The predator provided the outsides, the nucleus and most of the rest of the chimera. The prey became the mitochondrion, the cellular organ that produces energy. Most of the time, this ancient symbiosis proceeds amicably. But every so often, our mitochondria and their surrounding cells fight. The result is diseases, such as mitochondrial myopathies (a range of muscle diseases) or Leigh’s disease (which affects the central nervous system).
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. Importantly, the entryway (or glottis) to the lungs could be closed. When underwater, the animals pushed water past their gills while simultaneously pushing the glottis down. 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). Hiccups no longer serve a function, but they persist without causing us harm—aside from frustration and occasional embarrassment. 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.
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.
4. 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.
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.
6. We're awfully cold in winter
Fur is a warm hug on a cold day, useful and nearly ubiquitous among mammals. But we and a few other species, such as naked mole rats, lost it when we lived in tropical environments. Debate remains as to why this happened, but the most plausible explanation is that when modern humans began to live in larger groups, our hair filled with more and more ticks and lice. Individuals with less hair were perhaps less likely to get parasite-borne diseases. Being hairless in Africa was not so bad, but once we moved into Arctic lands, it had real drawbacks. Evolution has no foresight, no sense of where its work will go.
7. Goosebumps don't really help
When our ancestors were covered in fur, muscles in their skin called “arrector pili” contracted when they were upset or cold, making their fur stand on end. When an angry or frightened dog barks at you, these are the muscles that raise its bristling hair. The same muscles puff up the feathers of birds and the fur of mammals on cold days to help keep them warm. Although we no longer have fur, we still have fur muscles just beneath our skin. They flex each time we are scared by a bristling dog or chilled by a wind, and in doing so give us goose bumps that make our thin hair stand uselessly on end.
8. Our brains squeeze our teeth
A genetic mutation in our recent ancestors caused their descendants to have roomy skulls that accommodated larger brains. This may seem like pure success—brilliance, or its antecedent anyway. But the gene that made way for a larger brain did so by diverting bone away from our jaws, which caused them to become thinner and smaller. With smaller jaws, we could not eat tough food as easily as our thicker-jawed ancestors, but we could think our way out of that problem with the use of fire and stone tools. Yet because our teeth are roughly the same size as they have long been, our shrinking jaws don’t leave enough room for them in our mouths. Our wisdom teeth need to be pulled because our brains are too big.
Many of the ways in which our bodies fail have to do with very recent changes, changes in how we use our bodies and structure our societies. Hunger evolved as a trigger to drive us to search out food. Our taste buds evolved to encourage us to choose foods that benefited our bodies (such as sugar, salt and fat) and avoid those that might be poisonous. In much of the modern world, we have more food than we require, but our hunger and cravings continue. They are a bodily GPS unit that insists on taking us where we no longer need to go. Our taste buds ask for more sugar, salt and fat, and we obey.
10 to 100. The list goes on.
I have not even mentioned male nipples. I have said nothing of the blind spot in our eyes. Nor of the muscles some of use to wiggle our ears. We are full of the accumulated baggage of our idiosyncratic histories. The body is built on an old form, out of parts that once did very different things. So take a moment to pause and sit on your coccyx, the bone that was once a tail. Roll your ankles, each of which once connected a front leg to a paw. Revel not in who you are but who you were. It is, after all, amazing what evolution has made out of bits and pieces. Nor are we in any way alone or unique. Each plant, animal and fungus carries its own consequences of life's improvisational genius. So, long live the chimeras. In the meantime, if you will excuse me, I am going to rest my back.