Mammals are a recent addition by geological time standards, first appearing in the Triassic period.
The geological past is defined as a series of Eons, which are subdivided in to Eras, which are subdivided in to Periods, and the Periods are further subdivided in to Epochs. The various periods of history stretch across a time span of thousands or millions of years and are used by scientists to help sort out the geological and evolutionary history of Earth.
The Proterozoic eon of the Precambrian supereon is the most recent of three eons within the Precambrian, and is the only one to contain any periods. The eon begins 2.5 billion years ago and starts with the Siderian period, followed by the Rhyacian period, the Orosirian period, the Statherian period, the Calymmian period, the Ectasian period, the Stenian period, the Tonian period, the Cryogenian period and finally the Ediacaran period, which ended 542 million years ago. Notable developments within these periods were the buildup of oxygen in the atmosphere and the increased complexity of multicelled organisms.
The Paleozoic era began about 542 million years ago and lasted until roughly 251 million years ago. It began with the Cambrian period, which was followed by the Ordovician period, the Silurian period, the Devonian period, the Carboniferous period and finally the Permian period. The Paleozoic era started with the Cambrian explosion, which saw a huge expansion of life forms as well as a dramatic increase in the diversity and size of life on Earth.
The Mesozoic era began about 251 million years ago and lasted until 186 million years ago. It started with the Triassic period, which was followed by the Jurassic
period and then the Cretaceous period. These periods were characterized by the flourishing of the dinosaurs and the first mammals. Later, larger dinosaurs began to appear, as well as the first birds and the original seed-bearing plant life.
The Cenozoic era is the most recent and is divided in to three different periods. It starts with the Paleogene period, followed by the Neogene period and then the Quaternary period. The Paleogene and Neogene periods are also sometimes combined in to the Tertiary period. This era was characterized by a further diversification of life forms, in particular the continued development of mammals, including primates, an animal order to which human beings belong.