Remnants

: VESTIGES

, EMBRYOS, AND BAD DESIGN

figure 18. The disappearing hindlimb structures in the spotted dolphin (Stenella attenuata)—evolutionary remnants of its four-legged ancestor. In the 24 day old embryo (left), the hindlimb bud (indicated by triangle) is well developed, only slightly smaller than the forelimb bud. By 48 days (right), the hindlimb buds have almost disappeared, while the forelimb buds continue to develop into what will be the flippers.

figure 18. The disappearing hindlimb structures in the spotted dolphin (Stenella attenuata)—evolutionary remnants of its four-legged ancestor. In the 24 day old embryo (left), the hindlimb bud (indicated by triangle) is well developed, only slightly smaller than the forelimb bud. By 48 days (right), the hindlimb buds have almost disappeared, while the forelimb buds continue to develop into what will be the flippers.

conception, we become completely covered with a fine, downy coat of hair called lanugo. Lanugo is usually shed about a month before birth, when it's replaced by the more sparsely distributed hair with which we're born. (Premature infants, however, are sometimes born with lanugo, which soon falls off.) Now, there's no need for a human embryo to have a transitory coat of hair. After all, it's a cozy 37 degrees C in the womb. Lanugo can be explained only as a remnant of our primate ancestry: fetal monkeys also develop a coat of hair at about the same stage of development. Their hair, however, doesn't fall out, but hangs on to become the adult coat. And, like humans, fetal whales also have lanugo, a remnant of when their ancestors lived on land.

The final example from humans takes us into the realm of speculation, but is too appealing to omit. This is the "grasping reflex" of newborn babies. If you have access to an infant, gently stroke the palms of its hands. The baby will show a reflex response by making a fist around your finger. In fact, the grasp is so tight that an infant can, using both hands, hang for several minutes from a broomstick. (Warning: don't try this experiment at home!) The grasping reflex, which disappears several months after birth, may well be an atavistic behavior. Newborn monkeys and apes have the same reflex, but it persists throughout the juvenile stage, allowing the young to hang on to their mother's fur as they're carried about.

It is sad that while embryology provides such a gold mine of evidence for evolution, textbooks of embryology often fail to point this out. I have met obstetricians, for instance, who know everything about the lanugo except why it appears in the first place.

As well as peculiarities of embryonic development, there are also peculiarities of animal structure that can be explained only by evolution. These are cases of "bad design."

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