When the first mammals appeared, dinosaurs were still the rulers of the Earth. Surrounded by huge, hungry, thunder-footed giants, many of the timid mammals—not much larger than rats and mice today—may have stayed in the shadows of the swamps or in the high branches of the soft-wood trees.
To have seen them at any time in that first million years of their existence, no one would have thought that mammals would one day rule the Earth, as they do now. How has it come about?
Mammals have several advantages over the earlier vertebrates. For one thing, they are warm-blooded; that is, they have built-in automatic temperature controls which keep their bodies at about the same temperature in either cold or warm weather. Most of them have furry coats to hold in their body heat in winter, and other ways to get rid of body heat in summer. The dinosaurs, lacking such body equipment, became so cold and sluggish in cold weather that they could not hunt food, defend themselves against enemies, or move to warmer areas.
Mammals have other advantages, too. Instead of laying eggs they bear their young alive. They do not run the risk—which the dinosaurs ran—of having their eggs eaten by others. Also, mammals care for their young. A mammal mother feeds her babies milk which she prepares in her body, and she teaches them.
Mammals can learn more than other animals. They have larger brains, greater curiosity. The more the young mammal learns from his parents, the more likely he is to grow up and have young of his own. Mammals of long ago thus tended to survive in larger numbers and pass their traits on to new generations.
Most important for mammals was the gradual improvement of their brains. More and more, they used intelligence, rather than size and strength, to deal with the environment. More and more, individuals survived life's dangers because, having better brains, they could change their behavior to meet new conditions.
