Читать книгу Your Body - The Fish That Evolved - Dr. Keith Harrison - Страница 15

Once a feature has been lost in evolution, it is almost impossible to resurrect it. Natural selection only acts on what is visible to it so it is more usual for evolution to provide solutions to new problems by modifying parts of the body currently in use. For example, birds evolved from early reptiles which evolved from early amphibians which, in turn, evolved from early fish. Early fish, amphibians and reptiles all had long tails and these were originally used for swimming. In their evolutionary journey into the air, birds lost their heavy, bony tails and replaced them with long, light tail-feathers. Only the stump of the ‘parson’s nose’ remains. When some birds returned to a watery existence and began to swim again (penguins for example), they did not re-evolve a long swimming tail, they used the limbs that propelled them when they flew, their wings, and now fly underwater. In response to this, the bones of the wing have become stronger and heavier than those of other birds to cope with the density of water, which is much harder to push through than air, and as a result penguins have lost the ability to fly in air. Penguins also float and swim on the surface of the sea but again a swimming tail did not re-evolve. Here the hind feet became webbed and push the bird forward as they do when it walks on land. The actions of paddling are very like those of walking.

The ideal engineering solution to the problem of how to propel a bird underwater might have been to give it a fish-like tail, but evolution does not plan and design bodies. Evolution is what happens because there is selection; it is not why there is selection. The penguin adapted to its new habits and survived because each generation was edited by natural selection. Small incremental changes modified the average appearance of penguins in such a way that they survived as underwater fish hunters. Nature does not demand the perfect solution, only a solution that works.

This gives us the answer to an age-old question that has plagued many mothers: ‘Why is giving birth so painful?’ The brutal explanation is, ‘Because it doesn’t have to be painless.’ Evolution doesn’t care whether birth is agony or ecstasy as long as it is successful. As long as healthy babies continue to be born, the accompanying pain has no relevance to natural selection, regardless of its relevance to the mother. The current degree of childbirth pain is probably a relatively recent phenomenon, increasing only over the last few million years as human brains have increased in size. With the brains of babies in the womb enlarging relative to their ancestors, a larger head has had to pass through the same size of birth canal. As this has continued to happen successfully there has been no drive for natural selection to increase the size of the canal. The result is pain.