Читать книгу Quantum Evolution: Life in the Multiverse - Johnjoe McFadden - Страница 28

The standard textbook illustration of evolution is the development of the modern horse. Darwin’s friend and colleague Thomas Huxley worked out its sequence from more than two hundred species of fossil horses from Europe and America, using it to champion Darwin’s theory. Horses are members of the family perissodactyles (animals with odd-numbered toes) that also includes rhinos and tapirs. The first perissodactyles were dog-like browsing animals weighing only about twenty kilos that first appeared in the North American forests roughly sixty million years ago. The subsequent evolution of the horse is thought to have been in response to a changing environment as woodlands gave way to open savannah. Gradually, over millions of years, many new species appeared with fewer toes, longer legs adapted for fast running and stronger jaws with big teeth adapted for grazing. Each new species was only slightly modified from its likely progenitor, but over millions of years there was a gradual increase in size and parallel changes in bone structure. Most of the new species became extinct, particularly during the last great Ice Age, but seven species of modern horse survived, which include the domesticated horse, asses and zebras.

The standard interpretation of the fossil record of horses, and indeed other animals, is one of gradual evolution. At any point in time there would have been many individual horses, all slightly different. Natural selection would have favoured the more successful variants so that, over the course of many thousands and millions of years, there would have been a gradual shift in horse shape, size and toe bones, to suit their new environment.