Mammals are a diverse group which has dominated terrestrial ecosystems for 65 million years, since the previous rulers, the dinosaurs, were obliterated by an incoming meteor, leaving only avians as descendants. When mammals evolved depends on what you consider a mammal. The earliest mammal-like animals, mammaliaformes, evolved during the Late Triassic, about 210 million years ago. The first true mammals evolved right at the start of the Jurassic, about 195 million years ago. Hadrocodium wui, a shrewlike animal the size of a paper clip, had the first truly mammalian jaw. It scurried around in the shadows of the dinosaurs, consuming insects.

For a long time, mammals were not very diverse, all resembling small shrews. About 125 million years ago, mammals diversified, leading to the three major groups of today: monotremes (Prototheria, including the platypus, echnidna, and extinct relatives), marsupials (Metatheria, including kangaroos, koalas, and many obscure living and extinct forms), and placental mammals (Eutheria, the dominant mammal taxon, including mice, elephants, whales, and humans, among many others). Although all had diverged from one another by 120 million years ago, placental mammals and marsupials are more closely related, both being members of the class Theria (meaning "beast"), while monotremes are more distantly related.

Monotremes (Prototheria) are a group that was once much more successful, but today have almost completely died out, leaving behind a mere five species: one platypus and four echnidnas. Monotremes means "single hole" in Greek, referring to their all-purpose cloaca, which they have in common with marsupials. Monotremes are the only mammals to lay eggs rather than nurture their young directly, although they are warm-blooded, provide milk for their young, have a single bone in their lower jaw, and are covered in a layer of fur. Monotremes keep their body temperature at about 32°C (90°F), compared to about 35°C (95°F) for marsupials, and 38°C (100°F) for placental mammals.

Marsupials (Metatheria) are a mildly successful group that includes 334 species. Marsupials once had a range that included much of the planet, but today are only really successful in Australia, where they are the dominant mammals. Australia and nearby islands include over 200 marsupial species, ranging from the familiar koala and kangaroo to the more unusual bandicoots and Tasmanian Devils. About 100 marsupial species are found in South America, mainly including possums, but also including unique specimens such as the diminutive Monito del Monte. 13 species wandered into Central America when it slammed into South America via the Isthmus of Panama, and a single species made it to North America -- the Virginia possum. Marsupials differ from mammals in their mode of reproduction, the presence of a cloaca, and their metabolic rate.

Placental mammals (Eutheria) are the most successful group of mammals on the planet today, and occupy a series of ecological niches occupied by the dinosaurs and the pelycosaurs before them. Placental mammals boast over 5,500 species. There are four primary groups including Rodentia (mice, rats, and other small mammals with fast-growing teeth), Chiroptera (bats), Carnivora (dogs, cats, bears, and other carnivores), and Cetartiodactyla (numerous herbivore species, such as deer, sheep, goats, buffalos, and whales). Placental mammals are found on every continent except Antarctica, and many have close mythological and symbolic relationships with human culture. They are distinguished by a high metabolic rate and their reproductive mode, where the embryo grows to maturity in a womb before it is born. Placental mammals are probably the most intelligent group of animals that has ever lived on the planet, and very recently (200,000 years ago) gave rise to the species that we belong to, Homo sapiens