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THE BLIND MOLE RAT-A FAT, FURRY SAUSAGE OF A RODENT WITH TINY LEGS AND NO EYES VISIBLE-IS HELPING EVOLUTIONARY BIOLOGISTS FIGURE OUT HOW THE HUMAN LOST HIS TAIL.
Most discussions of biological evolution concern the acquisition of useful traits. As Darwin showed, the process involves natural selection: an individual animal that possesses some usefull characteristic is more likely than another animal to survive and leave behind offspring with similar advantages. But evolution is also about losing traits as well as acquiring them, about discarding old features that have become less useful. The long list of ancestral traits that were lost or reduced in the course of human evolution includes tails, body hair, wisdom teeth, the ability to synthesize vitamin C, the size of our teeth and appendix, the thickness of our skulls, and the bony browridges over our eyes. Likewise, snakes lost their legs, whales lost most of their sense of smell, and the dodo and many other birds on remote, predator-free islands lost their power of flight. The process is common enough to be unsurprising. Yet it pays to remember that such traits took remarkable luck as well as many thousands or millions of years to arise in the first place. Why would any animal cavalierly dispense with such hard-won victories? What evolutionary force drove their loss?
It is not at all obvious how natural selection could provide the answer. What harm would it really do us to sport a tail? Why do we survive better without one? A tail wouldn't get seriously in our way: we could occasionally use it for balance or gesturing, otherwise curl it up or hold it aside, and let it protrude through a rear-facing fly in the back of our trousers or skirt. That would hardly constitute a disadvantage. So why did every single one of us end up without a tail? Why would it be such a tragedy if we had somewhat larger teeth, a larger appendix, or a thicker skull?
Those turn out to be difficult questions. Darwin proposed two alternative solutions but was unable to decide between them, and biologists continue to debate them today. They are most easily understood by considering the corresponding two explanations for why a derelict building, once abandoned,...