Thursday, November 15, 2007

Five Bad Evolutionary Designs

Have you ever seen those nature documentaries where the narrator describes how perfectly designed, say, the cheetah, the polar bear, or the chameleon is for its environment? Of course, calling an animal perfectly designed is just shorthand for saying that the slow processes of evolution have led to that animal's well-adapted features. Yet evolution hasn't always generated the best designs, or at least not the best from an engineering perspective. In fact, some features seem downright poorly designed. This should come as no surprise when we understand a little about how evolution works.

When new features evolve in a species, they tend to build on already existing features. They aren't built from scratch. It's just too costly for evolution to go back to the drawing board and start over. This explains why lungs evolved from the swim bladders of ancient fish. This also explains why dolphins don't move their tails from side to side like fish do. The up and down movement of dolphin tails was built upon the galloping motion of their land-dwelling four-legged ancestors.

From an evolutionary standpoint new features don't need to have the best possible design either. They just need to be good enough to allow the organism to live long enough to reproduce. The evolution of the human body is no exception. We have body parts whose design is wanting, but they've been good enough to keep our species from going the way of the dinosaurs. Consider the following suboptimal designs in the human body.

Design One: The Pharynx

The human pharynx is the part of the throat that begins behind the nose and leads down to the voice box. It does double duty as a tube for breathing and for swallowing. But when you're swallowing you can't breathe, and when you're breathing you can't swallow. That's why humans run a serious risk of choking if the pharynx doesn't close at the right time when eating.

Curiously, human infants under six months and chimpanzees don't have this problem. But infants and chimps can't talk, and without our uniquely situated pharynx we wouldn't be able to talk either. The evolutionary advantage of talking must have outweighed the risks of choking in our early human ancestors. All in all, it's not a bad tradeoff, but engineers could certainly come up with a safer design especially if they didn't have to build on preexisting features.

Design Two: The Birth Canal

The Bible says that God punished Eve and her female descendants with painful childbirth because she partook of the forbidden fruit. Presumably, childbirth would be a snap if not for that damned curse. And not only has pain long accompanied childbirth, but so has death. The rate of mothers dying during childbirth in the United States in 1900 was about 65% higher than it is today.

You could blame God, or Eve, or the serpent. Or you could point the finger at bipedalism--walking upright on two legs. This evolutionary innovation forced a smaller pelvis on us. But bipedalism isn't the whole story. Humans have evolved big brains, and we needed a big container to hold those brains. This is why human infants are born more premature and helpless than most other mammals. Babies need to get through the birth canal before their heads get too big.

Design Three: The Jaw

The human jaw has too many teeth for its size. Many people have no room for wisdom teeth (third molars) if they get them, and a lot of people's teeth have to fight one another for limited territory, leading to crooked teeth and orthodontists. Impacted wisdom teeth can result in serious infections, and before modern dentistry these late eruptors could be deadly. If you couldn't eat, you died.

There's no consensus on why human jaws got smaller, but the fossilized jawbones of our Homo erectus ancestors show that, compared to us, they had massive jaws with huge molars. Some scientists suggest that our small jaw may have evolved in response to eating smashed and cooked food, so that over time there was little advantage to having a bigger jaw. Other scientists implicate the deactivation of a gene responsible for large jaw muscles, which may have allowed for our skulls to take on a new shape providing more room for those big brains of ours. Either way, we're stuck with small toothy mouths.

Design Four: The Appendix

This is a case of a vestigial organ if ever there was one. The human appendix has no known function, except perhaps to put money in surgeons' pockets. About the size of a finger, this organ is located at the beginning of the large intestine. Undigestible food that enters the appendix is normally forced out by muscular contractions. But when it isn't, the result is a potentially deadly infection.

The appendix is related to a digestive organ found in many other vertebrates. This is the cecum (pronounced SEE-cum), and it's largest in herbivores, where it helps to digest plant matter. Since evolution isn't keen on cleaning up after itself, we're left with a useless and potentially life-threatening organ. In fact, NASA is so concerned about appendixes bursting in outer space that it's considering requiring appendectomies prior to future long-term missions.

Design Five: The Spine

The spines of four-legged mammals work well horizontally. But when the spine stands up vertically, as in humans, it creates a lot of pressure on the vertebral discs. These discs can become compressed and slip, causing herniated discs and all manner of back pain and expensive therapy.

Standing on two legs must have benefited our early hominid ancestors who first adapted the upright position. A brand new spine designed for walking upright would have been an improvement, but instead evolution had to work with what it already had. There's no consensus on what advantages walking upright initially provided, but a widely-held view is that it freed up the hands for carrying food and manipulating objects. That's not a bad tradeoff, considering that it now allows us to rub our aching backs.

Building Upward

While evolution has created some wonderful adaptations that really do seem like optimal designs, it hasn't always done so. This is because novel features aren't designed from the ground up, but built up slowly on already existing features. Where an engineer would start from scratch, natural selection builds on whatever foundations it already has. As Richard Dawkins once put it: "Natural selection is like a robot that can only climb uphill, even if this leaves it stuck on top of a measly hillock. There is no mechanism for going downhill, for crossing the valley to the lower slopes of the high mountain on the other side."

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