The Cenozoic era is the time span in which we are living now. It began 66 million years ago right after the Mesozoic. This present time era is divided into two periods, the Tertiary and the Quaternary (Anthropogenic). The Tertiary is characterized by the formation of the newest and largest mountain ranges on the planet, which have the highest peaks, such as Himalaya, Andes, and the Rocky Mountain Range. The Quaternary, on the other hand, is marked by the four glaciation ages and the emergence of big carnivores and human beings. This era also contains the top strata (layers) of the Earth's crust.
If the Cenozoic era is divided into two well-defined periods, each one of these periods consists of epochs. Thus, the Tertiary period is further subdivided into the Paleocene, Eocene, Oligocene, Miocene, and Pliocene. It is very important to point out that during the Miocene the Earth's temperature began to drop as large tracts of forests and jungles started to disappear, giving way to the appearance of the plains, savannas, and steppes, with massive proliferation of graminae (grass) and ruminant herbivores, such as bovines (cattle), ovines (sheep), caprines, and deer. Inordinate numbers of grass grazers triggered the proliferation of big cats and other carnivores, and also human beings; the hunters. The appearance of Homo sapiens would take place during the Quaternary, which geologists divided into two epochs: the Pleistocene and the Holocene (present time); during the Pleistocene, the first humans emerged.
General Characteristics
During the Cenozoic era the present distribution of continents and oceans occurred. The very beginning of the era saw the completion of the breakup of the formerly unified southern continental mass, Gondwana, into the separate continental blocks of South America, Africa, the Indian subcontinent, Australia, and Antarctica, divided by the newly formed basins of the Indian Ocean and the southern part of the Atlantic Ocean —a process that had been under way since the Mesozoic. By the middle of the Cenozoic, Eurasia and Africa formed the continental mass of the Old World, joined by the mountain structures of the Mediterranean geosynclinal belt. The collision of tectonic plates brought about the orogenesis (formation of mountains) of most of the highest mountain ranges on Earth today: the Alps, Himalaya, Andes, Rocky, Cascade Range, etc. During the Quaternary, the large plains rich top soil continue to build up, with grass being the most abundant plant on Earth.
At the beginning of the Cenozoic, the large reptiles, which had predominated during the Mesozoic, became extinct and were replaced by mammals that, with birds, constituted the nucleus of the terrestrial vertebrates of the Cenozoic era. On most continents the higher placental mammals became dominant, and only in Australia (which became isolated before these mammals appeared on a large scale) did unique marsupials and, to some extent, monotremes develop. During the early Paleogene, mammals were represented almost exclusively by small primitive forms. By the middle Paleogene almost all the orders existing today had appeared, as well as several groups that subsequently became extinct. A great variety of mammals evolved and thrived.
