No classification system is universally accepted; Mc Kenna & Bell (1997) and Wilson & Reader (2005) provide useful recent compendiums.George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v.
The line following the stem group Sphenacodontia split-off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals (Therapsida) in the early Mesozoic era.The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, and have been among the dominant terrestrial animal groups from 66 million years ago to the present.Findings from molecular genetics, for example, have prompted adopting new groups, such as the Afrotheria, and abandoning traditional groups, such as the Insectivora.The mammals represent the only living Synapsida, which together with the Sauropsida form the Amniota clade.The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon.
At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds.
The largest orders are the rodents, bats and Soricomorpha (shrews and allies).
The next three biggest orders, depending on the biological classification scheme used, are the Primates (apes and monkeys), the Cetartiodactyla (whales and even-toed ungulates), and the Carnivora (cats, dogs, seals, and allies).
While mammal classification at the family level has been relatively stable, several contending classifications regarding the higher levels—subclass, infraclass and order, especially of the marsupials—appear in contemporaneous literature.
Much of the changes reflect the advances of cladistic analysis and molecular genetics.
In human culture, domesticated mammals played a major role in the Neolithic revolution, causing farming to replace hunting and gathering, and leading to a major restructuring of human societies with the first civilizations.