The first life must have come from nonlife. All matter, living and nonliving, is made up of chemicals. The smallest chemical units are atoms, which bond together into molecules (the properties of these units are the subject of Chapter 2). The processes of chemical evolution that led to the appearance of life began nearly 4 billion years ago, when random inorganic chemical interactions produced molecules that had the remarkable property of acting as templates to form similar molecules. Some of the chemicals involved may have come to Earth from space, but chemical evolution continued on Earth.
The information stored in these simple molecules enabled the synthesis of larger molecules with complex but relatively stable shapes. Because they were both complex and stable, these molecules could participate in increasing numbers and kinds of chemical reactions. Certain types of large molecules are found in all living systems; the properties and functions of these complex molecules are the subject of Chapter 3.
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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.