The history of life on Earth, depicted on the scale of a 30-day calendar, is outlined in Figure 1.3. The profound changes that have occurred over the 4 billion years of this history are the result of natural processes that can be identified and studied using scientific methods. In this section, we will set the stage for the rest of this book by describing some of the most important of these changes. These six major evolutionary events will provide us with a framework for discussing both life's characteristics and how those characteristics evolved. By recognizing them, you will be able to better appreciate both the unity and diversity of life.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.