Evolution of Pain

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.[1]

[1] 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.

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12 replies
      • Pain
        Pain says:

        I wasn’t sure about that since you suggested that the we could live without the pain. Long time ago, I read that the pain is bodies communication system. Many pain expressions don’t have a measurable indicators in the body – nothing shows up in the blood work, etc. Migraine stories are full of pain without tests that can identify it. I have noted that in the professional sports many athletes live and play with pain constantly. They are able to override, at least for some time, responses to the pain. Pain may indicate the need for the rest and recovery but it doesn’t prevent you from action if required.

        But then there are pains that you cannot ignore. Unless you are full of adrenaline and can do critical action before collapsing with the pain.

        Pain in phantom limbs is another strange aspect of the pain.

        Sorry for rambling. I’m doing sports that require long periods of tolerating pain. I have been interested in the pain evolution and characteristics for some while. I wanted to thank you for the article. I liked the statement that there are multiple pains that have evolved over the time to near flawless state. Pain is like a shark that has been around for very long time.

        Reply
        • Colin McGinn
          Colin McGinn says:

          It’s hard to think of a rationale for why pain should be necessary for survival–plants and simple organisms do without it. Presumably robots could manage without pain. We, as we are, need it to survive, because we have no fallback mechanism; but as a matter of principle it appears dispensable. I like the shark analogy.

          Reply
          • Pain
            Pain says:

            My hypothesis is that the pain helps organisms that need to explore dynamic environment for their survival. Basic organisms are usually constrained to the niche places. Robots are also created for niche purposes including the materials that help them stay relatively undamaged in the predicted environment. Humans are adopted to many environments. Pain constantly guides a person during the exploration.

            You could try to replace pain with sensors that inform you about the environment but there are too many touch points to the reality. Some are harmful, some useful, and many indifferent. Without pain, how would we learn to distinguish between these three states?

          • Colin McGinn
            Colin McGinn says:

            But a lot of pain reactions are completely reflexive. Robots could exist that respond intelligently to stimuli but have no pain. We can distinguish what is harmful from what is not perceptually; we don’t need pain.

  1. Pain
    Pain says:

    I feel you are overrating the ability to meaningfully adapt behavior based on the perceptual judgement. Most people know that eating junk food is bad, but they still do it. Most people know they need to exercise, but they don’t. Most people know that sleep is important, but they prioritize late night meaningless entertainment instead of early bed time.

    Are there people who can do it? Yes, there are. Are masses able to do it? No, they can’t. I don’t see how alternatives to pain could scale and be also self sufficient to sustain the adaptive behavior.

    I’m struggling to imagine a scenario where we could live without the pain. Constructing a sensing robot can be done, but I struggle to see this robot continue to adjust to the changing environment in a long run. I would bet on the robot’s quick extinction.

    The fact that there are no complex organisms (my unresearched assumption) that strive without the pain is a serious counterargument to your aspiration of world without the pain. There must be a significant fact that you are not considering.

    Reply
    • Colin McGinn
      Colin McGinn says:

      The question is rather like why sex exists: it’s a puzzle for evolutionary theory. We don’t know why, but there is presumably a good reason. Pain could be a spandrel or just an evolutionary accident–it might not be biologically optimal (many traits are not). A lot of pain seems completely without function, e.g. chronic pain from illness. The facile assumption that pain is somehow the best possible solution in the best of all worlds is what is questionable. It could be like having a large head.

      Reply
  2. David Khoo
    David Khoo says:

    Evolution doesn’t operate at an individual level. If an individual has suffered such severe bodily insult that it is in truly debilitating pain, it may be better at the gene level for that crippled individual to simply die than to take up more resources. Precisely by inducing death, the debilitation may be adaptive. Similarly, overt displays of pain may have social functions, but it may also be adaptive precisely because attracting predators results in the death of an individual that is unlikely to reproduce or whose offspring are less likely to be successful.

    Traits that result in death of specific individuals are not a puzzle for evolution. Otherwise we wouldn’t have species that die immediately after mating, or even allow themselves to be eaten by their mates after mating, and so forth. For that matter, death itself is a trait that is part of an evolutionary strategy (or the length of lifespan in general). Some species are biologically unaging, after all.

    Reply

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