HUMAN BIOLOGY FORM, FUNCTION, AND ADAPTATION

• Main Author: DeWitt, William (Author)
• Format: Book
• Language: English
• Published: Glenview, Ill. Scott, Foreman 1989
• Subjects: Human biology
• Online Access: Click Here to View Status and Holdings.

 Biology is the study of life, of living things. This simple and seemingly straightforward definition would be neither remarkable nor worthy of further consideration if it were not for one curious fact: no one can define life. Biologists are thus placed in the unenviable position of studying something they cannot define The impossibility of defining life has long fascinated philosophers and biologists alike. Up until the early nineteenth century, most scientists adhered to a concept known as vitalism, which contended that all living things possessed a "vital force" that characterized life and that served to distinguish the living from the inanimate. A basic premise of the vitalists was that the "vital force" was necessary for the production of organic compounds, those complex carbon-containing substances of which living organisms are composed. Conse quently, organic compounds were thought of as a unique characteristic of life, and life was defined as those entities capable of synthesizing organic com pounds. However, when the German chemist, Friedrich Wöhler, synthesized the organic compound urea in his laboratory in 1828, vitalism had to be abandoned, and biologists were left with no clear-cut way to define life. One of the reasons we cannot define life unequivocally is because there is not always a clear distinction between those entities we consider living and those we consider nonliving. For example, although the vast majority of scientists would not dispute the contention that a frog is living and a rock is nonliving, it is not always possible to obtain agreement when considering entities that have some characteristics we normally attribute to living systems, but also have other characteristics that we consider attributes of the nonliving world (Figure 1-3). Viruses, for instance, are borderline entities of this type. Biologists have long disputed whether viruses should be considered living or nonliving precisely because they have some characteristics of both the animate and the inanimate. They are composed of substances-primarily proteins and nucleic acids that are found as constituents of all living organisms, but they have no ability to function as independent entities. Instead, viruses are entirely dependent upon living organisms for their animate qualities. Their only life-like activity is reproduction, yet they are able to reproduce only within living organisms by using raw materials and synthetic machinery present in the cells they infect. Outside of a living organism, as when they are isolated in the virologist's test tube, viruses are essentially inanimate, and except for their chemical composition, they display none of the properties normally associated with life. Asking if such a borderline system is alive is much like wondering whether someone is happy-the ultimate judgment will depend on individual interpretation. In this context, it is clear that defining life becomes a philosophical argument. How, one may ask, can we study life if we can't define what we're studying? The answer to our dilemma is to recognize that although we cannot define life in any rigorous sense, we can list a number of characteristics that often distinguish living entities from nonliving ones. It is important to realize that any one entity that we would call living will not necessarily possess all of the characteristics on our list, but it will possess most of them.