The Pliocene Epoch, a relatively short period in geologic history (10-2 million years ago), covers a very important phase in early human evolution. Early in the Pliocene, as the Rio Grande Rift was developing, the Tetons were rising and the Galapagos Islands were forming, gorillas diverged from the common ancestor of chimps and humans.
By the middle of the Epoch, coincident with the development of the San Francisco Volcanic Field in northern Arizona and the docking of the Salinia Terrain to form Southern California, chimps diverged from the human ancestral line. Then, about 4.5 million years ago, as Kauai's formation gave way to Oahu's development and the Baja of California began to rift from the Mexican mainland, Australopithecus appeared in East Africa; the males of this earliest known hominid weighed 100 lbs., twice the size of females.
Near the end of the Pliocene, as the Sierra Batholith was rising, Panama was drifting in to connect the Americas and Easter Island was developing from marine volcanoes, Homo habilis arose in Africa. This human ancestor, equipped with opposable thumbs, was the first hominid to use stone tools; he immediately preceded Homo erectus, the first hominid to use fire and the first of our ancestors to leave Africa.
