Origins of mammals

Mammals evolved from reptiles called therapsids, and first appeared over 200 million years ago. The transition from reptile to mammal was a gradual one and involved a slow accumulation of mammalian features over a long period of time. One of these new mammalian features, found in all modern mammals, is a space behind the eye socket that allows extra room for muscles powering the jaws. Another is a hard bony palate that separates air moving through the nose from food moving through the mouth. Mammal-like reptiles also developed a new kind of jaw hinge, and increasingly complex teeth. The number of bones in the lower jaw progressively decreased from seven, until only one was left. Instead of being completely lost, the extra bones changed shape and took on a completely new role, helping to conduct sound waves from the eardrum to the inner ear. At some point in this long transition, fur, mammary glands, and a warm-blooded lifestyle also evolved.

One of the earliest mammals known, a species called Morganucodon, looked like a small weasel, and was only about 2.5 cm (about 1 in) long, while a related species, called Megazostrodon, was about the size of a shrew. These early mammals were almost certainly active at night and are thought to have lived on insects or small vertebrates.

By the Jurassic period, when dinosaurs dominated life on land, mammals had already been in existence for millions of years. During the long reign of the dinosaurs, several different groups of mammals evolved. Most of these groups have been named for the structure of their teeth. Morganucodon and Megazostrodon, for example, belonged to a group called the triconodonts—so called because their teeth had three cusps, or conical points, roughly in a line. Symmetrodonts had molars with three cusps arranged in a triangle, while multituberculates had rodent-like incisors, and large molars adapted for grinding up plant food. Teeth are made from hardy material that fossilizes well, and in many cases they provide scientists with the only clues that show how these early mammals lived.

When dinosaurs disappeared 65 million years ago, mammals found themselves in a world of unprecedented opportunities, without the ruling reptiles that had held them in check for so long. Some mammal groups, such as the multituberculates, eventually became extinct themselves, but others flourished and diversified as they filled the ecological roles that reptiles had left vacant. In particular, small placental animals developed a wide range of new diets, and an even bigger range of body forms. By about 30 million years ago, some of them had already taken to the seas, while on land their descendants included saber-toothed cats, the forerunners of today's pigs, and also giant browsers (leaf eaters) such as Indricotherium. Standing up to 5.5 m (18 ft) at the shoulder and weighing about 30 metric tons, this hornless relative of the rhinoceros was probably the largest mammal that has ever lived on land.

Despite their huge variation in size, these animals all shared the basic mammalian body plan that they had inherited from their distant ancestors. Most mammals still have four limbs, hair, and relatively large brains, yet their shared ancestry also explains similarities that are much less obvious. All mammals have four-chambered hearts, a muscular diaphragm separating their heart and lungs from their abdominal cavities, a lower jaw made of a single bone, and the same arrangement of tiny bones in the inner ears. Almost all mammals, including whales, mice, and giraffes, have seven vertebrae in their necks. The arms of humans, the flippers of seals, and the wings of bats all have the same number and arrangement of bones.

To receive our newsletters

Graphics from