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NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

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International Standard Book Number-13: 978-0-309-14838-2 (Book)

International Standard Book Number-10: 0-309-14838-3 (Book)

International Standard Book Number-13: 978-0-309-14839-9 (PDF)

International Standard Book Number-10: 0-309-14839-1 (PDF)

Library of Congress Control Number: 2010921862

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Cover: Cover design by Francesca Moghari. Front Cover: Five fossil human skulls show how the shape of the face and braincase of early humans changed over the past 2.5 million years (from left to right: Australopithecus africanus, 2.5 million years old; Homo rudolfensis, 1.9 million years old; Homo erectus, ~1 million years old; Homo heidelbergensis, ~350,000 years old; Homo sapiens, ~4,800 years old). The images are courtesy of the Human Origins Program of the Smithsonian Institution and photo credits include Chip Clark, Jim DiLoreto, and Don Hurlbert, all of the Smithsonian Institution. The bottom image is a composite image with an oxygen isotope record from and courtesy of James Zachos. The cracked mud image is courtesy of Free Nature Pictures. The woodland image is courtesy of Kaye Reed, National Science Foundation. Back Cover: The drill rig is courtesy of Jason Agnich, University of Minnesota-Duluth. The wetland, savannah, and ground examination images are also courtesy of Kaye Reed.

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