An Evolutionary Approach to Pain (1994)

AN EVOLUTIONARY APPROACH TO PAIN GROVER C. PITTS* This study explores the evolutionary significance of pain and examines criteria possibly useful in deciding which nonhuman species can experience pain. First, problems of definition will be addressed before approaching pain's evolutionary significance in humans. Then, criteria for assessing pain capability will be considered in preparation for the question of pain perception in other species. We can expect to arrive at some theories regarding how much pain other species experience (ranging from probable to possibly highly probable) but few, if any, firm convictions. Considering the barriers to studying perception in other species, firm convictions may be beyond reach. Two general types of pain are recognized: acute or "quick" pain, which usually initiates evasive behavior, and chronic or "slow" pain, which may induce protective inactivity favoring recovery. Many definitions of pain in people are necessarily anthropomorphic and possibly misleading when applied to nonhuman species. Some investigators have avoided this and other problems by adopting the behavioral definition that pain is the phenomenon characterized by withdrawal or flight in response to stimulation in any animal, vertebrate or invertebrate. The shortcomings of this approach (including its inadequate treatment of neural and evolutionary factors) have been put into perspective by Bunge [I]. The physiological, neural, and behavioral factors characterizing pain in a person and, say, an insect are so very different as not to justify inclusion under a common rubric. Since enough is now known about these factors to justify this article, I have chosen not to employ the behavioral definition. Pain, understood by layperson and clinician as conscious suffering, and its absence observed by the surgical patient during anesthesia, has long-standing priority over the behavioral definition . The difficulties resulting from coupling the two ill-understood concepts of "pain" and "consciousness" are probably no greater than would *Professor emeritus of physiology, School of Medicine, University of Virginia, Charlottesville . Address: 110 Woodstock Drive, Charlottesville, Virginia 22901.© 1994 by The University of Chicago. AU rights reserved. 0031-5982/94/3702-0849$01.00 Perspectives in Biology and Medicine, 37, 2 ¦ Winter 1994 | 275 have resulted from applying the simplistic behavioral definition in this study. In what follows I shall focus on pain as a conscious unpleasant sensation of variable intensity produced by near, or actual, tissue damage. Consciousness has been the subject of much philosophical discussion [2, 3] having limited usefulness here. However, Homer Smith [4] helpfully characterized consciousness in humans as follows: (a) It keeps the brain's sensory screen aglow during the intervals between consecutive stimuli, thereby introducing the time dimension; (b) It enables the individual to integrate present and past experience (stored in memory) and to project an anticipated future; (c) It introduces the possibility of individual choice between alternative actions. Since pain is a conscious experience , the mental processing that Smith associates with consciousness is available for processing pain stimuli. The massive evolutionary significance of this fact will be addressed next. Initially pain attaches a variable degree of aversion to an experience, thereby evoking behavioral responses with survival value. This aversion is recorded in memory. The insect, possessing a hard-wired set of negative tropic reflexes, does not need to think about what to do when it encounters a noxious stimulus because its avoidance behavior is dictated by those reflexes. But a conscious man in a similar situation is faced with a decision-making crisis. He will consult his memory for relevant past experience and choose a course of action probably best suited to the circumstances. For a simple example, suppose a primitive man encounters a large predator and is conscious of its capability for imposing great pain. He recalls from past observations that this species is too heavy-bodied to climb, so he ascends the nearest tree; whereas the individual depending upon negative tropic reflexes simply runs away and perhaps is overtaken. Thus there are major evolutionary advantages of the human pain system, for example: (a)Humans can memorize a virtual encyclopedia of threatening individuals , species, and scenarios which enable them to avoid pain and/or bodily harm before it becomes inevitable; (b)Humans can transmit from generation to generation by word of mouth (as some other species do by parental...