The Cenozoic era, 65 million years ago to the present, is divided into two periods, the Tertiary and the Quaternary. The Tertiary period, 65 to 2 million years ago, encompasses the rebuilding of the animal kingdom at the end of the great Cretaceous extinction. From an unpromising beginning as small, nocturnal opportunists, mammals, along with the surviving birds, radiated into the most numerous and diverse life forms to inhabit Earth.

During the Quaternary, 2 million years ago to the present, dramatic climate changes reduced the overall diversity of animal life, yet saw the rise of another remarkable evolutionary opportunist—the bipedal hominid.

The earlier phase of the Quaternary, the Pleistocene epoch (2 million to 10,000 years ago), covers the alternating periods of glacial advance and retreat across the Northern Hemisphere and the corresponding effect on plant and animal life. The current chapter of Earth history, the Holocene epoch, deals with the interglacial period which...