The three major groups of mammals are monotremes, which lay external eggs; marsupials, which carry their babies in a pouch; and placental mammals, which carry and gestate their fetuses internally. Humans fall into the category of placental mammals. Examples of monotremes and marsupials are the platypus and the kangaroo, respectively.

Mammals increased in existence after the extinction of the dinosaurs, according to current fossil records. A fossil found in 1985 dates mammalian traits in animals of the Early Jurassic period about 195 million years ago. Within 10 million years after the dinosaurs became extinct, aquatic mammals, such as whales, and flying mammals, rodents and primates, evolved. As of 2014 there are over 5,400 total mammal species. Some characteristics of mammals are enlarged brains and diverse teeth, but all mammals have hair and nourish their young with milk.