Tinkering

• The Panda’s Thumb: More Reflections in Natural History by Stephen Jay Gould
  Norton, 343 pp, £6.95, April 1981, ISBN 0 393 01380 4

Pandas are peculiar bears, which spend most of their days munching bamboo. To do this, they strip off the bamboo leaves by passing the stalks between their flexible thumb and the remaining fingers. But how can a panda have an opposable thumb, when in bears the thumb lies parallel to the fingers, and inseparable from them? In fact, the panda does not have a proper thumb at all: it has five parallel digits just like other bears. The apparent ‘thumb’ is a modification and extension of a small bone in the wrist. For Stephen Gould, this is a particular and fascinating fact, but it is also an illustration of a general principle. The principle is that evolution proceeds by tinkering with what is already there, and not by following the canons of optimal design. Had the panda been designed by the Great Artificer, He would not have been constrained to make its hand by modifying the hand of a bear, and would doubtless have come up with a more elegant, if less entertaining solution to the problem of stripping bamboo.