The climate of the Cenozoic Era fluctuated over time: this geological period covered more than 65 million years, during which the Earth warmed and then drastically cooled. The Cenozoic Era began over 65 million years ago and continues today. It is sometimes referred to as the "Age of Mammals," as changing continental structure and more temperate conditions supported the growth of many new organisms, including aquatic species and land mammals.
The Cenozoic Era divides into several periods, including the Paleogene, Neogene and Quaternary Periods. The Paleogene Period spans the first part of the Cenozoic and is geologically distinct from the rest of the era. During this time, the Earth saw a warming trend. Sea temperatures increased, and the oceans supported more life forms. In addition to a warming climate, activity below the surface of the Earth's seas, primarily the release of methane hydrates from the sea floor, elevated water temperatures. In addition to new organisms, the continents on Earth assumed their modern shapes.
A cooling period followed the early warming trend of the Cenozoic Era, enabling the formation of glaciers. Forests dominated the landscape of Earth during the early part of the cooling phase but were replaced during the later half of the Cenozoic with plains and grasslands. Modern mammals emerged as well, including horses, whales, a more diverse population of fish and humans.