Children often ask the most intelligent questions. “Why,” asks an inquisitive child, “can’t Uncle Bob grow a new leg like a starfish does?” Why not indeed? If lizards regrow lost tails, starfish lost arms, and fish lost fins, why can we

not even regenerate a lost finger? It is remarkable that this question seldom bothers adults, even biologists. The answer, in general evolu- tionary terms, is that natural selection will not maintain capacities that are unlikely to be useful or that have costs that would exceed the expected benefits.Serious damage to the brain or heart was uniformly fatal before the era of modern medicine, and the ability to regenerate these tissues could not be selected for. An individual who lost an arm in a Stone Age accident could bleed to death in a few minutes. If the bleeding were somehow controlled, the victim would likely soon die of tetanus, gangrene, or other infection. Any process that might have allowed our remote ancestors’ arms to regenerate has gradually been lost by the accumulation of mutations that have not been selected against.

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But what about the loss of a finger? This would not be as likely to cause death as the loss of a whole arm, and such injuries often do heal under Stone Age conditions. Why not regenerate the finger instead of merely healing the wound? The explanation given in the previous paragraph will not suffice here. We suggest instead two other factors. The first is merely that this regenerative ability would not be used very often and would not produce a major benefit. Most people do not lose fingers, and if they do, the long-term impairment need not be serious. A nine-fingered Neanderthal might live to the ripe old age of fifty. Another reason, which we have already repeatedly emphasized, is that every adaptation has costs. The capacity to regenerate dam- aged tissue demands not only the cost of maintaining the machinery to make this possible but also the cost of a decreased ability to con- trol harmful growths. A mechanism that allows cell replication increases the risk of cancer. It is dangerous to let mature, specialized tissues have more than the minimum needed capability to repair likely injuries.