Phylogenetics and dating with confidence
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, monkeys, and allies), the Cetartiodactyla (whales and even-toed ungulates), and the Carnivora (cats, dogs, seals, and allies).
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.
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.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.In some classifications, extant mammals are divided into two subclasses: the Prototheria, that is, the order Monotremata; and the Theria, or the infraclasses Metatheria and Eutheria.The marsupials constitute the crown group of the Metatheria, and include all living metatherians as well as many extinct ones; the placentals are the crown group of the Eutheria.