Nesse1998 - The principles of evolution by natural...

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T houghtful contemplation of the human body elicits awe in equal measure with perplexity. The eye, for instance, has long been an object of wonder, with the clear, living tissue of the cornea curving just the right amount, the iris adjusting to brightness and the lens to distance, so that the optimal quantity of light focuses exactly on the surface of the retina. Admiration of such apparent perfection soon gives way, however, to consternation. Contrary to any sensible design, blood vessels and nerves traverse the inside of the retina, creating a blind spot at their point of exit. The body is a bundle of such jarring contradictions. For each exquisite heart valve, we have a wisdom tooth. Strands of DNA direct the development of the 10 trillion cells that make up a human adult but then permit his or her steady deterioration and eventual death. Our immune system can identify and destroy a million kinds of foreign matter, yet many bacteria can still kill us. These contradictions make it appear as if the body was de- signed by a team of superb engineers with occasional interventions by Rube Goldberg. In fact, such seeming incongruities make sense but only when we investigate the origins of the body’s vulnerabilities while keeping in mind the wise words of distinguished geneti- cist Theodosius Dobzhansky: “Nothing in biology makes sense except in the light of evo- lution.” Evolutionary biology is, of course, the scientific foundation for all biology, and bi- ology is the foundation for all medicine. To a surprising degree, however, evolutionary bi- 86 Scientific American November 1998 The principles of evolution by natural selection are finally beginning to inform medicine Evolution and the Origins of Disease by Randolph M. Nesse and George C. Williams Copyright 1998 Scientific American, Inc.
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Scientific American November 1998 87 Constraints Example: The design of the human eye leads to a blind spot and allows for detached retinas. The squid eye is free of such problems. Defenses Example: Symptoms such as cough or fever are not defects but in fact are the body’s de- fenses in action. Novel environments Example: The human body has only recently adopted its current environment, filled with former rarities such as high-fat foods. Trade-offs Example: Overdesign of any one system, such as a pair of unbreak- able arms, would upset the entire organism’s functioning. Conflicts Example: Human beings are in con- stant battle with other organisms that have been fine-tuned by evolution. Cholera bacterium ILLUSTRATIONS BY CRAIG KIEFER CNRI/SPL/PHOTO RESEARCHERS, INC. LAURIE GRACE SQUID RETINA HUMAN RETINA Copyright 1998 Scientific American, Inc.
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ology is just now being recognized as a basic medical science. The enterprise of studying medical problems in an evolu- tionary context has been termed Dar- winian medicine. Most medical research tries to explain the causes of an individ- ual’s disease and seeks therapies to cure or relieve deleterious conditions. These efforts are traditionally based on con-
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