Since the beginnings of life on Earth, all animal have gradually changed. Some have developed (or “lost”) legs, while others have grown horns, beaks or wings. Some kinds have died out altogether, giving way to others that are better suited to their environment. This process of change and adaptation is called evolution.
Mammals are descended from mammal-like reptiles called pelycosaurs that lived on Earth about 300 million years ago. The first warm-blooded animals were probably the cynodonts. Small, fast-moving carnivores, the cynodonts may have had fur rather than scales. Fur is an essential feature of warm-blooded animals, as it helps them to keep their bodies at a stable temperature.
The first true mammals evolved about 210 million years ago, when the dinosaurs dominated the Earth. In order to avoid the dinosaurs, the mammals came out only at night. Gradually, the three main groups of mammals evolved: marsupials, monotremes and placental mammals. The extinction of the dinosaurs meant that there was suddenly an absence of any large animals on land. This gave the mammals the opportunity they needed to spread rapidly and evolve into many different species.
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