Cenozoic Era, last of the five major eras of geologic time, beginning about 65 million years ago and extending through the present. The Cenozoic, shortest of the geologic eras, is that period during which the modern world, with its characteristic geographical features and its animals and plants, came into being. Paleontologists often call the Cenozoic the age of Mammals because mammals became common during this time. During the Cenozoic, mammals evolved adaptations that allowed them to live in various environments; on land, in water, and even in the air. The Cenozoic is divided into two main sub-divisions: the Tertiary and the Quaternary. Most of the Cenozoic is the Tertiary, from 65 million years ago to 1.6 million years ago.
During the Tertiary period, earth's climates were generally warm and mild. In the oceans, marine mammals such as whales and dolphins evolved. Flowering plants and insects flourished. Grasses provided a food source that encouraged the evolution of grazing mammals. The last few million years of the Tertiary Period saw a gradual change in climate. Temperatures dropped steadily, leading into the severe freezing conditions of the Ice Age to come. .
The Quaternary period began at the end of the Tertiary Period, about 1.6 million years ago, and has continued to the present time. In the Quaternary period mammals that had evolved during the Tertiary peopled eventually faced a changing environment during the Quaternary Period. The Quaternary period saw the global temperature continue to fall, culminating in what is known as the Ice Age. So much of the earth's water was frozen in continental glaciers that the level of the oceans fell by more than 100 meters. This era is sometimes called the Age of Man because the earliest humans are believed to have evolved then. The fossil record suggests that the very earliest ancestors of out species appeared about 4.5 million years ago but that they did not look entirely human.