Pleistocene Epoch, Earlier and longer of the two epochs that constitute the Quaternary Period. The Pleistocene began c. 2.6 million years ago and ended c. 11,700 years ago. It was preceded by the Pliocene Epoch of the Neogene Period and followed by the Holocene Epoch. At the height of the Pleistocene glacial ages, more than 30% of the land area of the Earth was covered by glacial ice; during the interglacial stages, probably only about 10% was covered. The animals of the Pleistocene began to resemble those of today, and new groups of land mammals, including humans, appeared. At the end of the epoch, mass extinctions occurred: in North America more than 30 genera of large mammals became extinct within a span of roughly 2,000 years. Of the many causes that have been proposed for these extinctions, the two most likely are changing environment with changing climate and disruption of the ecological pattern by early humans.