| The central idea of chemical evolution is that life evolved from nonlife. But aside from insight based on biochemical knowledge and laboratory simulations of some key events on primordial Earth, do we have any direct evidence that life could have developed from nonliving molecules? The answer is yes. The smallest and simplest entity that sometimes appears to be alive is a virus. We say "sometimes" because viruses seem to have the attributes of both nonliving molecules and living cells. Virus is the Latin word for "poison," an appropriate name since viruses are often a cause of disease. Although they come in many sizes and shapes—a typical example is the polio virus, shown here magnified 300,000 times—all viruses are smaller than the size of a typical modern cell. Some are made of only a few thousand atoms. In terms of size, then, viruses seem to bridge the gap between cells that are living and molecules that are not.
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Viruses contain some proteins and genetic information (in the form of DNA or the closely related molecule RNA) but not much else—none of the material by which living organisms normally grow and reproduce. How, then, can a virus be considered alive— When alone, it cannot; a virus is absolutely lifeless when isolated from living organisms. But when inside a living system, a virus has all the properties of life. Viruses come alive by transferring their genetic material into living cells. The genes of a virus seize control of a cell and establish themselves as the new master of chemical activity. Viruses grow and reproduce copies of themselves by using the genetic machinery of the invaded cell, often robbing the cell of its usual function. Some viruses multiply rapidly and wildly, spreading the disease and—if unchecked—eventually killing the invaded organism. In a sense, then, viruses exist within the gray area between the living and the nonliving. |
INTERLUDE 28-1 The Virus 