The History ofLife on Earth

When you want to know what time it is, you probably look at your watch, or at the clock on the wall or on your computer. You could also listen for an announcement of the time on the radio or television. But suppose the electric power system failed and you lost your watch. How would you tell time then? You would use the cue that people have used during most of human history: the cycle of day and night. We are so accustomed to having time-measuring devices all around us that we forget that these devices are recent inventions. When Galileo studied the motion of a ball rolling down an inclined plane about 400 years ago, he used his pulse to mark off equal intervals of time.

The development of the science of biology is intimately linked to changing concepts of time, especially of the age of Earth. Biology as we know it could not and did not develop very far until about 150 years ago, when geologists provided evidence that Earth was ancient. Before 1850, most people believed that Earth was only a few thousand years old. Charles Darwin could not have developed his theory of evolution by natural selection if he had not read the works of Charles Lyell, who was England's leading geologist during Darwin's lifetime. Lyell suggested that existing land-forms could be explained by the action, over very long time periods, of the same forces that are still acting on them today. That is, Lyell argued that it is not necessary to postulate sudden catastrophes as the reason for dramatic geological changes. As we pointed out in Chapter 1, Darwin's theory was based on the assumption that Earth was very old and that millions of years were available for life's evolution.

The goals of Part Four of this book are to document the history of life on Earth, to describe patterns of evolutionary change, and to investigate the agents that cause them. We begin this chapter by asking, How do we know that Earth is ancient? What is the evidence that life evolved early during Earth's history and has continued to evolve since then? We will first examine how events in the distant past can be dated. We will review the major changes in physical conditions on Earth during the past 4 billion years, look at how those changes have affected life, and describe some patterns in the evolution of life. In Chapter 23 we will discuss

Sunset at Stonehenge Even the earliest humans felt the need to keep track of time and seasons.The arches of Stonehenge,an astronomical "timepiece"on Salisbury Plain in England, date back to about 2000 B.C., and radioisotope dating has revealed some wooden structures unearthed here to be more than 8,000 years old.

Structure Radioisotope

the processes by which life evolves. In subsequent chapters, we will see how biologists determine the evolutionary histories of organisms and how the millions of species that live today (as well as many more that became extinct) were derived from a single common ancestor.

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Essentials of Human Physiology

Essentials of Human Physiology

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.

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