Cenozoic era (sēnəzōˈĭk, sĕn–) [key], last major division of geologic time (see Geologic Timescale, table) lasting from 65 million years ago to the present. The Cenozoic is divided into the Tertiary (from 65 million years ago until 2 million years ago) and Quaternary (2 million years ago to the present) periods. Early in the Cenozoic, Greenland began to separate from Europe; Antarctica and Australia, and Africa and India also separated. The great Alpine-Himalayan mountain systems were formed; rifting with associated volcanic activity occurred in Africa, Europe, Asia, and Antarctica. In the late Cenozoic, the Cascade Range of volcanoes extended from southern British Columbia to N California, and represented a new volcanic arc superimposed on older structures. The volcanic arc is still periodically active today. Following the disturbances of the late Mesozoic era, the geography of North America had by the beginning of the Cenozoic attained substantially its present form. The only areas subjected to inundation by shallow marine waters were the Atlantic and Gulf coasts and a small area on the Pactific coast. Most of today's common plants and insects can be recognized in early Cenozoic fossils. Modern grasses appeared in the mid-Cenozoic. The animal life of the Cenozoic was dominated by mammals, which developed rapidly and were most numerous in the Tertiary period and then declined, with the exception of a few specialized types, in the Quaternary period. Theories explaining the decline or extinction of mammals during the Pliocene and Pleistocene epochs of the quaternary have ranged from a change in climate to the predation of humans. Cenozoic land mammals were never as large as the Mesozoic dinosaurs, but many were larger than today's mammals, and included beavers that grew to lengths of more than 7 ft (2 m), sloths as large as elephants, and birds up to 7 ft (2 m) in height. Early humans are thought to have evolved in Africa during the Pliocene and were widespread and accomplished toolmakers by the beginning of the Pleistocene. Around 1.6 million years ago, Homo erectus, a robust ancestor of modern humans, evolved (see human evolution). H. sapiens evolved more than a million years later in Africa and moved to Asia and Europe, supplanting earlier Homo species by 28,000 years ago, developed a form of agriculture in the Middle East c.10,000 years ago, and started writing and building the first cities c.4,000 years ago. See also geology.