The Pliocene, the last epoch of the Tertiary Period, stretched from 10 to 2 million years ago. As it began, the Rio Grande Rift was forming in Colorado-New Mexico and the Tetons rose in Wyoming. Other geologic highlights of this Epoch included the formation of the Galapagos Islands (8 million years ago), the development of the San Francisco volcanic field of northern Arizona (6 MYA), the sculpting of the Grand Canyon (which continues today), the arrival of the Salinia Terrain (now Southern California), the formation of Kauai (5 MYA), the opening of the Gulf of California, the development of Oahu (4 MYA), the rise of the Sierra Batholith (which continues today), the arrival of Panama to connect North and South America, the formation of the Long Valley volcanic field in California, the appearance of Easter Island (3 MYA) and the opening of the Lake Baikal Rift (2.5 MYA).
New life forms of the Pliocene included sheep, goats, cattle, antelope and bison in Eurasia, mammoths in Africa and megatherium, a giant ground sloth, in South America; sea lions, seals, walruses and saber-toothed cats also appeared during this Epoch. Gorillas split from the human ancestral line 9 million years ago and chimpanzees followed two million years later.
Australopithecus, thought to represent the earliest hominid, appeared in the Rift Valley of Africa 4.5 MYA, followed by Homo habilis 2.5 MYA; the latter species had opposable thumbs and was the first hominid to use stone tools.
Toward the end of the Pliocene, Earth's climate was cooling. An ice cap had formed over the North Pole and glaciers appeared on Greenland. The stage was set for the Pleistocene Ice Age, which began about 2 million years ago; many climatologists believe that the Holocene, in which we live, is just a warm interglacial period of the Pleistocene and that another phase of glaciation will develop in 5-10 thousand years.
