Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young. Most mammals also possess sweat glands and specialised teeth, and the largest group of mammals, the placentals, have a placenta which feeds the offspring during gestation. The mammalian brain, with its characteristic neocortex, regulates endothermic and circulatory systems, the latter featuring red blood cells lacking nuclei and a large four-chambered heart maintaining the very high metabolism rate they have. Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) Bumble Bat to the 33-meter (108-foot) Blue Whale.
The word "mammal" comes from the Latin mamma ("breast"). All female mammals nurse their young with milk which comes out from special glands, the mammary glands. According to Mammal Species of the World, which is updated through periodic editions, 5,676 species were known in 2005. These were distributed in 1,229 genera, 153 families and 29 orders. In 2008 the IUCN (International Union for Conservation of Nature) completed a five-year, 17,000-scientist Global Mammal Assessment for its IUCN Red List, which counted 5488 accepted species at the end of that period. The class is divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter containing the infraclasses Metatheria (including marsupials) and Eutheria (the placentals). The classification of mammals between the relatively stable class and family levels having changed often, different treatments of subclass, infraclass and order appear in contemporaneous literature, especially for Marsupialia.
Except for the five species of monotremes (which lay eggs), all living mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders, in descending order, are Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders, depending on classification scheme, include the Primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales) and the Carnivora (dogs, cats, weasels, bears, seals, and their relatives).
The early synapsid mammalian ancestors, a group which included pelycosaurs diverged from the amniote line that would lead to reptiles at the end of the Carboniferous period. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first true mammals appeared 220 million years ago in the Triassic period. Modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period. Phylogenetically, the clade Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals). This means that some extinct groups of "mammals" are not members of the crown grou Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes.