Biology  5/e   Raven/Johnson
	Student   Online Learning Center

The life that we see around us on this planet is the result of 3.5 billion years of natural selection and evolution. The earth formed approximately 4.5 billion years ago and bacterial life appeared within 1 billion years of the earth's formation. Ever since then, natural selection has been occurring, evolutionary changes have accumulated in populations, and new species have arisen. As humans, we are most interested in the evolutionary events that led to the appearance of our own species, Homo sapiens, some 500,000 years ago. This chapter outlines our evolutionary roots and reminds us that the evolution of species did not and does not occur in a vacuum, but under particular environmental and geographical conditions. Our evolutionary heritage includes the ancestors of insectivorous tree shrews, nocturnal prosimian primates with grasping fingers and toes and binocular vision, and apes with opposable thumbs and larger brains. Africa was the cradle of human life. Over 4 million years ago the first hominid, the first of the human line, appeared there. This was a bipedal creature called Ardipithecus ramidus. Africa also gave rise to all the australopithecine species and the genus Homo. Different researchers have recognized anywhere from three to seven species of Homo. Homo habilis appeared approximately 2 million years ago, H. erectus approximately 1.5 million years ago, and H. sapiens less than 0.2 million years ago.

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