This paper follows on from “The Cruel Gene” and “Pain and Unintelligent Design” on this blog.
Evolution of Pain
Pain has evolved over many millions of years. Presumably it had primitive forms that were subjected to natural selection. It was honed and whittled, modified and amplified. There are now several species of pain, each no doubt fine-tuned to achieve a certain end. It is very widespread among animals—if not universal, then close to it. If we think of it as a mental organ, we can model its evolution on the evolution of organs in general: there is a kind of structure-function matching, with the organ designed to have certain functional effects (ultimately survival and reproduction). It probably evolved very early given its survival utility, and it has persisted robustly over the millennia; it is unlikely to become obsolete or extinct. We can think of it as the eyes of the body: it “sees” danger and reacts to it. It is analogous to a perceptual faculty. Perhaps it evolved in prey animals (which includes nearly all animals, since nearly all are prey to some animal at some point), because they need to be sensitive to the teeth and claws of predators. Perhaps some animals perceive it more exquisitely than others, depending on their vulnerabilities. A lack of body armor would favor greater sensitivity to pain—there is no need for pain perception if your armor is impregnable to predators (and rivals). What we can be sure of is that pain was rigorously selected for according to its costs and benefits, like any other biological trait. It feels and acts the way it does for good reasons. There are two puzzles about this. First, the costs are considerable: yes, you avoid the painful stimulus, which is all to the good, but you also incur serious liabilities because of general debilitation. An animal in pain is often hobbled by the pain, unable to function, like an athlete in pain. True, this can be helpful if rest is needed, but nature is not always so obliging as to allow for such rest. Second, the pain gives rise to other effects than those of avoiding the painful stimulus, such as overt expressions of pain (grimaces, cries). What is the point of these? It is hard to avoid the conclusion that they are communicative: they let other animals know you are in pain. But this is puzzling, because it isn’t as if other animals will automatically come to your aid—instead of sensing weakness and attacking you. You might want to keep it a secret that your right leg is hurting, or that you have a headache and are not up for a session of head butting. So, pain might not be the simple adaptation we assume: too much downside and too many collateral effects. We can take it that this is not the result of insufficient fine-tuning, which will be rectified in due course; on the contrary, pain mechanisms must be highly adapted by now, not still replete with glitches. Pain responses have been well-nigh perfected over evolutionary time; natural selection has made them as wondrous and efficient as eyes. This will no doubt involve making pain as painful as is compatible with proper functioning, which is obviously pretty damn painful. Good pain is bad pain, so far as evolution is concerned (those selfish genes!). Pain has evolved to be bad, not mild and tolerable. And it has produced a biologically marvelous trait, truly spectacular: pain is remarkably bad, devilishly so. Natural selection has done its job and done it well: it has produced organisms that really hurt—as it has produced eyes that see really well. It has brought pain to the pitch of perfection, survival-wise; it’s hard to believe it could get any worse, subjectively speaking. It has labored long and hard to make us suffer, to ruin our days, even to make us wish for death. Quite an achievement! The genes must be proud of themselves, but the animal must carry the burden. That is the logic of evolution by natural selection playing itself out: animals have whatever traits enable the genes to survive, pleasant (orgasm) or unpleasant (pain). But there is a residual puzzle: why not use reflexes instead? The patellar reflex allows the organism to move rapidly and effectively, but no pain is involved; same for the blink reflex. So, why must a stubbed or squashed toe be accompanied by intense pain—why not arrange the nervous system so that the foot is quickly and reflexively withdrawn but without the intervening agony? The pain sensation doesn’t seem necessary to the function; it seems like a gratuitous (indeed sadistic) add-on. The only thing I can think of is that the pain is somehow necessary for ongoing flexible voluntary behavior in the presence of harmful stimuli, as in managing a broken bone or a burn. But though that seems true as a matter of empirical fact, it is difficult to see why it has to be true. Presumably it has just turned out over the course of evolutionary time that pain is a more efficient way of handling injury than a purely reflexive and pain-free method; but why this should be remains obscure. So, the existence of pain is something of an evolutionary puzzle, especially given its functional downside (its phenomenological downside counts for nothing in the evolutionary game). It clearly evolved over millions of years and is close to universal, but it’s puzzling why it exists at all as an adaptation. It must have evolved in multiple species many times (convergent evolution), but its rationale is far from obvious (unlike the eye). Pain is puzzling, biologically and philosophically, despite its undeniable reality. Some animals do quite well without it—jellyfish, insects, worms—but many live by it (literally), despite its manifest aversiveness. It is the only adaptation bequeathed to us that we would rather be without—that is intrinsically nasty. Suffering may be adaptive to life, but it is also the bane of existence. It is really the only thing that can make life not worth living, yet it exists to serve life. Pain is an enigma that we could live without.
 Imagine the amount of pain that has existed over the course of evolutionary time in all animal species. It doesn’t bear thinking about. Earth is the planet of pain.