A major goal of this book is to show that fossil primates (including fossil humans) fit within evolutionary patterns seen among other mammals. What is a mammal? Mammals are a class of animals. That is, they are technically arranged in the zoological Class Mammalia, which is formally characterized by a number of distinctive traits. With the exception of birds, most of the animals with which we are most familiar are mammals. There are about 5,000 living mammal species, although they comprise only about 5 percent of all known animal species. The term “crown species” is often used to refer to living species, in contrast to fossil species, because the living animals appear at the top or crown of an evolutionary tree. Mammals have a very ancient and complicated evolutionary history. Figure 1.1 presents a simplified version of this evolutionary past. A complete and continuous fossil record documents the transition from first reptiles to first mammals. Although a number of “mammal-like reptile” or proto-mammal groups independently evolved mammalian traits, the first creatures identified as true mammals emerge about 200 mya (million years ago). Living mammals occur in three main groups: the ancient monotreme mammals of Australasia, and the closely related placental and marsupial mammals, whose origins are much more recent.