How to Define Life: A Hierarchical Approach

HOW TO DEFINE LIFE: A HIERARCHICAL APPROACH AYDIN ORSTAN* More than 3,000 years ago, the Mesopotamians wrote about the adventures of Gilgamesh, the king of the city of Uruk, and his struggle to become immortal. Mesopotamians believed that the gods who created them had retained immortality for themselves and allotted death to the humans. Even Gilgamesh, who was two-thirds god and one-third man, was destined to die like ordinary humans. But he rebelled against his faith and went on a long and dangerous journey to find his ancestor Utnapishtim. According to legend, Utnapishtim and his wife were the only humans who had ever attained immortality. At the end, however, Utnapishtim's secret of eternal life was to disappoint Gilgamesh. Utnapishtim and his wife were granted immortality for having survived the flood that had once destroyed the rest of humanity. There was nothing Gilgamesh could do to duplicate this feat [I]. Gilgamesh failed to achieve immortality. But his preoccupation with life and death was contagious. Both in literature and real life, countless people throughout the ages have tried to postpone aging, overcome death, and understand what life and death are all about. At about the turn of the seventeenth century, a new class of people, the scientists who were infected with the Gilgamesh virus, began to ponder on the puzzle of life. They observed, experimented, and hypothesized. And they also tried to define life and living organisms in more or less scientific terms. In his 1878 book, Lectures on the Phenomena ofLife Common to Animals and Plants, Claude Bernard gives a list of some of these definitions [2]. My favorite one is that of the eighteenth-century anatomist Bichat, who defined life as "the ensemble of the functions that resist death." I wonThis essay received honorable mention in the 1988 Dwight J. Ingle Memorial Writing Award Competition for young authors. ?Institute of Gerontology, University of Michigan, 300 North Ingalls, Ann Arbor, Michigan 48109-2007.© 1990 by The University of Chicago. All rights reserved. 0031-5982/90/3303-0680$01.00 Perspectives in Biology and Medicine, 33, 3 ¦ Spring 1990 | 391 der how he would have defined death. Bernard himself believed that only those objects that were the products of human minds could be defined . And life, being a natural phenomenon, was therefore undefinable . Furthermore, he also thought that there was no need to define life: "When one speaks of life, one is understood on the subject without difficulty, and this is enough tojustify the use of the term in an unequivocal way" [2]. The scientists and philosophers have nevertheless continued to come up with more definitions, which, expectedly, have slowly become more practical and less poetic than those listed by Bernard. Most of the contemporary definitions are based on the reproductive abilities of living organisms. For example, according to Horowitz, three of the properties of living organisms, mutability, self-duplication, and heterocatalysis, constitute a definition of living matter [3]. A few definitions that are not directly based on reproduction have also been proposed . For example, Dillon defined life as the "capability of synthesizing proteins in at least sufficient quantity to replace those that are catabolized by normal processes" [4]. And some scientists have based their definitions on the collective properties of living organisms. Allee's definition of life as the total of five common characteristics of living organisms (metabolism, growth, reproduction, the ability to adjust to the environment, and cooperation) is in this group [5]. It does not take much effort to realize that these definitions are little more than arbitrary lists of things the living organisms do. And for this reason, when it comes to figuring out how and why the living organisms do what they do, they are not of much use. But, as Bernard realized more than a century ago, biologists in general seem to be able to carry out their inquiries perfectly well without a rigorous definition of life. This is indicated by the tremendous advancement of biological knowledge during the past few centuries. We have learned a lot about how the living organisms carry out their vital functions, and, given enough time, we will undoubtedly learn more. So why bother with a definition...