Further Reading

Chaisson, Eric, and steve McMillan. Astronomy Today. 6th ed. upper saddle River, N.J.: Addison-Wesley, 2007. Comins, Neil F. Discovering the Universe. 8th ed. New

York: W. H. Freeman, 2008. melia, Fulvio. The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge: Cambridge university Press. 2003.

Quaternary The last 1.8 million years of Earth history are known as the Quaternary period, which is divided into the older Pleistocene and the younger holocene. Jules Desnoyers was the first to recognize that the rocks and unconsolidated deposits formed during this period were different from older deposits. Their characteristic boulder clays and other units deposited by glaciers in Europe reflected globally cool climates for the first part of the Quaternary, since glacier deposits were found in many parts of both the Northern and southern hemispheres. Global climate zones condensed near the equator, ice sheets covered about one-third of the continental surfaces, and desert regions converted to moist grasslands. Grasses, plants, and mammals experienced a rapid expansion.

Another major important discovery from this period was the recognition of the first human fossils, which became the basis for dividing the Quaternary into the older Pleistocene and the younger holocene, in which human fossils appear abundantly about 10,000 years ago. older primate fossils including early hominids are found in the older record going back several million years, and the record of human habitation in the Western hemisphere extends back to approximately 13,000 or 14,000 years ago. Genetic evidence suggests that humans are all descendants of a single female ancestor that lived somewhere in East Africa about 100,000 to 300,000 years ago, with the first hominids appearing about 4 million years ago.

See also Neogene; Pleistocene; Tertiary.

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