The Class Mammalia consists of vertebrates that have hair and produce milk. Linnaeus classified mammals into one group, and even invented the name, based on the Latin mammae, referring to breasts. Mammals also have four-chambered hearts and teeth clearly differentiated into incisors, canines, and molars. There are or have been in the past about 5,000 genera of mammals. The surviving 1,135 genera contain about 4,700 species. This is far less than the three-quarter million species of beetles.

Mammals evolved from synapsids, one of the lineages of reptiles. One synapsid reptile of the late Paleozoic era was Dimetrodon, which had a huge sail of bony extensions along its back. Although Dimetrodon itself is unlikely to have been the ancestor of mammals, it may have shared a characteristic that may have been widespread among synapsids: warm blood. Modern mammals generate body heat internally (are endothermic) and usually maintain a constant body temperature (are homeothermic). Dimetrodon may have used its sail to warm its blood in the morning (by absorbing sunshine) and cool it off in midday (by radiating heat through the skin surface). This may have helped Dimetrodon to be, while not homeothermic, at least partly endothermic.

Synapsids included the pelycosaurs and the therapsids (from the Greek for "nurse"). Compared to earlier reptiles, therapsids had fewer skull bones, teeth that were more differentiated, and multiple sets of replacement teeth rather than just juvenile "milk teeth" replaced by adult teeth. One group of therapsids was the cynodonts (from the Greek for "dog teeth"), with teeth even more differentiated (doglike) than those of earlier therapsids. Small pits in the facial bones of cynodonts may have been openings for extensive blood vessels, and this has led some scientists to conclude that cynodonts were fully warm-blooded. Cynodonts such as Morganucodon and Sinocodon, which lived during the Jurassic period, represent nearly perfect "missing links" (actually not missing) between earlier synapsid reptiles and modern mammals. Morganucodon is considered one of the earliest mammals, even though it is intermediate between synapsid reptiles and modern mammals. The very fact that so many "missing links" have been discovered means that the line between reptiles and mammals is somewhat arbitrary. Like modern mammals, Morganucodon had juvenile milk teeth replaced by adult teeth; Sinocodon retained the ancestral reptilian pattern of multiple replacement sets of teeth.

By the Jurassic period, there were three major lineages of mammals: the monotremes, the multituberculates, and the therians. The modern descendants of the Jurassic monotremes are the duck-billed platypus and the spiny anteater (echidna) of Australia and nearby islands. They retain the primitive feature shared by most (but not all) reptiles of laying eggs. The mothers produce milk but do so from glands rather than from breasts. The multituberculate lineage became extinct during the Oligocene epoch of the Tertiary period. Other modern mammals evolved from the Jurassic therian lineage.

By the Cretaceous period, the therian lineage had already split into its two modern branches: the marsupials, which bear their young at a very early developmental stage, and the young nurse inside of a pouch; and the placentals, in which the fetus develops inside the uterus of the mother and is nourished through a placenta. The placenta, a structure vascularized by both the mother's and the fetus's blood vessels, allows the mother's blood to come close to but not in contact with the fetus's blood. This prevents the mother's body from launching an immune attack against the fetus. Therefore the fetus can stay inside the mother's body until highly developed. When born, the offspring has a relatively high chance of survival. In marsupials, on the other hand, the fetus must be born before the mother's body launches the immune attack. The fetus must crawl from the birth canal to a nipple, where it attaches and nurses until it is able to survive on its own. It is protected inside a pouch of skin. . .

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