Following the Precambrian Time, Paleozoic Era, and Mesozoic Era on the geologic time scale is the Cenozoic Era, which began 65 million years ago and continues to the present. After the Cretaceous-Tertiary, or, K-T, Extinction at the end of the Cretaceous Period of the Mesozoic Era, which eliminated 80 percent of all species of animals, the Earth found itself needing to rebuild.
Now that all dinosaurs besides birds were extinct, mammals had the opportunity to flourish. Without competition for resources from dinosaurs, mammals had the opportunity to grow. The Cenozoic was the first era that saw humans evolve. Much of what is commonly thought of as evolution has happened in the Cenozoic Era.
The Cenozoic Era Begins
The first period of the Cenozoic Era called the Tertiary Period has been divided into the Paleogene and Neogene periods. Most of the Paleogene Period saw birds and small mammals become more diverse and grow greatly in numbers. Primates started to live in trees, and some mammals adapted to live part-time in the water. Marine animals didn't have such luck during this period when massive global changes resulted in many deep-sea animals going extinct.
The climate had cooled significantly from tropical and humid during the Mesozoic Era, which changed the types of plants that did well on land. Lush, tropical plants were replaced by deciduous plants, including the first grass. The Neogene Period saw continuing cooling trends. The climate resembled what it is today and would be considered seasonal. Toward the end of the period, however, the Earth was plunged into an ice age. Sea levels fell, and the continents came to roughly the positions they hold today.
Many ancient forests were replaced with expansive grasslands as the climate continued to dry out, leading to the rise of grazing animals such as horses, antelope, and bison. Mammals and birds continued to diversify and dominate. The Neogene Period is also considered the start of human evolution. During this time the first human-like ancestors, the hominids, appeared in Africa and moved into Europe and Asia.
Humans Start to Dominate
The final period in the Cenozoic Era, the current period, is the Quaternary Period. It began in an ice age where glaciers advanced and retreated over parts of the Earth that are now considered temperate climates, such as North America, Europe, Australia, and the southern part of South America. The Quaternary Period is marked by the rise of human dominance. Neanderthals came into existence and then went extinct. The modern human evolved and became the dominant species on Earth.
Other mammals continued to diversify and branch off into various species. The same happened with marine species. There were a few extinctions over this period due to the changing climate, but plants adapted to the various climates that emerged after the glaciers retreated. Tropical areas never had glaciers, so lush, warm-weather plants thrived all during the Quaternary Period. Areas that became temperate had many grasses and deciduous plants, while slightly colder climates saw the re-emergence of conifers and small shrubs.
No End in Sight for the Cenozoic Era
The Quaternary Period and Cenozoic Era continue today and likely will remain until the next mass extinction event. Humans remain dominant, and new species are discovered daily. While in the early 21st-century climate is changing once again and some species are going extinct, no one knows when the Cenozoic Era will end.