Biology 2010 Student Edition

Published by Prentice Hall
ISBN 10: 9780133669510
ISBN 13: 978-0-13366-951-0

Chapter 19, History of Life - Assessment - 19.3 Earth's Early History - Understand Key Concepts - Page 567: 21

Answer

d. Oxygen

Work Step by Step

Initially very little oxygen was present on the earth. It is important to remember that the earth has changed a lot over time, allowing for different species to exist.
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