Dumb question, why do “sensitive” spots on the body need more nerves? Couldn’t you just have the normal touch-sensing nerves and map signals from specific spots on the body to stronger/pleasurable qualia in the brain?
Dumb question, why do “sensitive” spots on the body need more nerves? Couldn’t you just have the normal touch-sensing nerves and map signals from specific spots on the body to stronger/pleasurable qualia in the brain?
Not a dumb question. The shortest (and at a glance unsatisfactory) answer is because it works, and therefore it evolved that way.
Going in detail, first consider that for a feature to be evolutionarily selected for two things have to be true:
1. It must increase the fitness of the organism that carries it, i.e. the likelihood of its carrier having descendants as compared to non-carriers ( or be a side effect of another feature that improves fitness enough to be a net positive, etc etc )
2. It must be inheritable (and, in sexually reproduced organisms, mutually compatible during embryonic development).
One such a feature has reached dominance in a given population, as long as it continues to be important for fitness it cannot really be deprecated in favour of an alternative from scratch, even if that alternative is arguably better.
That's why, for instance, vertebrate ocular nerves connect to our retinas on the inside of our eyeball, resulting in us having a blind spot. Cephalopods, on the other hand, evolved their eyes independently the "reasonable" way, connecing their nerves from behind the eyeball. There's no way a vertebrate could mutate from scratch for its optical nerve to connect to the retina from behind without causing absolute mayhem in embryonic development. Our hacky solution for the blind spot? Let the brain hide it in software.
Going back to your question, some spots of the body being more sensitive than others became critical for evolutionary fitness long before nervous systems were complex enough to generate conscious qualia, let alone enough for them to be consistently involved in decision making. Furthermore, mapping of specific nerves to intensity of feeling on the CNS would imply complex hardcoding of something which is much easier to solve with "this place important, have more nerves", and maybe would even conflict with the fitness benefit of a CNS with enough neuroplasticity to learn anew during the development and lifetime of an organism.
So, in summary, the solution of having more nerves where it matters is simple, good enough, and has no reason to be rolled back in favour of a radically different alternative.
As a software dev, I think this is actually quite a satisfying and sensible answer. A simple reliable hardware solution in favour of a brittle “clever” software one
Nice post. Just this bit:
> Our hacky solution for the blind spot? Let the brain hide it in software.
I would say the solution is just having two eyes, since their respective blind spots don't overlap in the visual field.
I would also say that the brain doesn't hide the blind spots, but rather doesn't pay any attention to them in the first place. There's just a lack of information from them, and this deficit isn't normally noticeable because the other eye makes up for it. I think Dennett explains it that way somewhere, probably in Consciousness Explained
Having more independent samples helps filter out noise. If you had individual sensory neurons with outsized influence, then misfiring of such neurons would also have outsized influence.
This makes a lot of sense, thx!
Sounds plausible at least, but I think the question isn't necessarily making a valid assumption. Why do men have to have nipples? Why is our retina installed backwards? Why do sinuses drain upwards? It's just a path evolution took, it doesn't jump to some optimal design.
male nipples are developmental vestiges, the male condition is derived from response to embryonic testosterone, and is a developmental variation from default.
early stage embryos of both sexes are not easily distinguishable by genitalia, they look morphologically similar. later developmental events culminate in morphological rearrangement to male form.
lack of response to testosterone during development results n a curious state of affairs, where a person is genetically male, having x, and y chromosomes, develops according to a female plan. external appearances are female, with loss of secondary sex development in puberty.
Androgen insensitivity syndrome:
https://en.wikipedia.org/wiki/Androgen_insensitivity_syndrom...
Asking “why” questions about our body / evolution often (not always) gives informative answers. As in the example you gave: https://www.scientificamerican.com/article/the-purpose-of-ou...
Interesting finding, though it doesn't explain animals with properly-wired retinas: https://en.wikipedia.org/wiki/Cephalopod_eye
(Scientific American is throwing up a paywall even though they're only republishing https://theconversation.com/look-your-eyes-are-wired-backwar... At least they link to the original.)
Correct, though interestingly apropos to the discussion is that sex is one of the ways evolution is able to get around local maximums.
Fingers, for instance, not only have higher sensitivity, but also much higher spatial resolution due to the more dense nerve network.
I can't tell why other areas may have needed higher spatial resolution; maybe it was evolutionary important in the past, and remains today. Or maybe just adding more nerves due to a random mutation correlated with better reproductive outcomes due to a stronger signal, or higher sensitivity, so more nerves are present for no other reason.
Wait, so what you’re saying is, we can use our genitals in a pinch that could be as good as fingers for finesse?
> Dumb question, why do “sensitive” spots on the body need more nerves? Couldn’t you just have the normal touch-sensing nerves and map signals from specific spots on the body to stronger/pleasurable qualia in the brain?
Think of a television. What gives you a better picture, quadrupling the number of pixels or making the existing pixels 4x as intense?
Perhaps encoding "software" is more expensive in terms of codons? So it's cheaper/more likely to "implement" physically.
Sensing nerves aren't especially energy hungry when considering their volume in the human body, so evolution doesn't have much reason to minimize them.