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lived in earth burrows and manufactured their own heat when abroad were more likely to survive. The age of reptiles came to an end, and the age of mammals began.
    Even without an asteroid collision, the days of the cold-blooded dinosaurs were probably numbered. Sixty million years ago, the continents were drifting apart from their earlier union in a single super-continent. The continental masses moved away from the earth’s equator, and as they drifted, they rose and folded. Mountains uplifted and ocean currents changed. Cooler local climates came with the uplands and the sea currents. In place of the tropical forests, new kinds of plants were evolving. Given the cooler climate and the new plants, dinosaurs would have been hard-pressed.
    In this changing world, grasses evolved. Savanna and prairie spread over large sections of the continents. And with this new habitat came a burst of evolutionary activity among mammals. Both hunters and hunted evolved dramatically. Earlier plant-eaters were slow and clumsy creatures with broad heads, big bones, and elephantine feet. To take advantage of the new grasses and shrubs, some plant-eaters developed the ruminant digestive tract, which allowed cattle, deer, and antelopes to eat quickly, then go off to some safer place, regurgitate the unchewed food, and quietly chew and swallow it. That put a greater premium on the ability to escape, and deer and antelope developed longer legs, smaller feet, and relatively large bodies, to contain larger digestive systems and larger lungs. They developed a wrist bone that hinged top and bottom and so gave them greater flexibility and extension. The long bones of third and fourth digits fused to form the cannon bone. Gradually, the radius and the ulna fused into a single bone. Plant-eaters were increasingly designed to eat and run. Escape and wariness became the great advantages, and deer, camels, llamas, pigs, sheep, goats, bison, and cattle became the common herbivores.
    As the herbivores grew faster, the slow, broad-headed, short-legged creodonts and mianids that had preyed on their ancestors died out. The surviving predators developed longer limbs and biggerbrains. Out of these trends emerged the modern carnivores. Carnivores are defined by the presence of long sharp canine teeth and steeply ridged molars and premolars known as “carnassials,” which are used for shearing meat. Carnivores include bears, cats, hyenas, weasels, skunks, raccoons, and dogs. And the dogs, or canids, include foxes, coyotes, jackals, and wolves.
    The first wolflike canids emerged about three million years ago. In the Eastern Hemisphere, they gave rise to the various species of jackal; in the Western Hemisphere, to coyotes. The first wolves probably split from the coyote line about one million years ago in the New World and then migrated to the Old World. Perhaps seven hundred thousand years ago—perhaps more recently—the gray wolf emerged from the evolutionary mists in the Eastern Hemisphere. There is much debate about the exact line of descent. Ronald Nowak of the U.S. Fish and Wildlife Service believes the ancestors of today’s red wolf were in North America a million years ago, and that one descendant migrated to South America, where it evolved into the dire wolf, and another went north to Alaska and the Old World, where it evolved into the gray wolf. The gray wolf recrossed the land bridge to America sometime between three and six hundred thousand years ago. By the time of the Roman Empire, the gray wolf ranged over most of Eurasia and North America and boasted the largest range of any land mammal.
    Cats split off the line that would lead to dogs about forty million years ago. The differences between dogs and cats tell much about the kinds of choices that were being made. Dogs are relatively long-limbed and slender-bodied. The lower part of the leg is generally longer than the upper part, for greater leverage. Dogs evolved in open country, where hunting requires