How would evolution have changed without the "reflex arc", if all reflexes had to go through the brain instead of the spinal cord? Would intelligent species be able to evolve?


3 Answers 3


There is no reason intelligence could not evolve under this situation. I am assuming only that for some reason communication of signals in that way was never attempted. What would be different? Well i could speculate the following:

  • Intelligence may actually evolve sooner as one needs to preempt attacks/threats rather than react to them. Intelligent tools such as traps would also be more effective so be more favored.
  • There may be more favorable evolution to having a head closer to the limbs. This would improve reaction times. This could be done by a smaller average size too.
  • Likely the largest dinosaurs would not have existed in their current state. They seem to have had a higher reliance of this mechanism in order to still react in reasonable time frames. The butt brains myth is an exaggerated version of this. This would hinder them a lot more than us. At very least a long appendage to reach leaves would be more safe than a long neck.
  • Butt Brains may actually exist. We can't go to the spine for fast reactions but we want the sensory brain high to shorten distance to eyes and keep eyes elevated. We may end up with an extra brain hemisphere (a southern one) that is situated in a different place in the body.
  • $\begingroup$ I like the "southern brain hemisphere" comment. $\endgroup$
    – octern
    Commented Oct 24, 2014 at 5:54
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    $\begingroup$ snicker"Butt brains"snickersnicker In all seriousness though, I didn't realize that that was a myth. Guess it's a good thing I didn't write the answer I was considering based upon that! This requires more research!! $\endgroup$
    – Kromey
    Commented Oct 24, 2014 at 15:33
  • $\begingroup$ I was actually just surprised that that was a common term for the theory. I tried googling a more eloquent explanation and that was what popped up. $\endgroup$
    – kaine
    Commented Oct 24, 2014 at 15:34

Why not? The primary purpose of the reflex arc is to speed up reaction time. It's there because it provided a helpful evolutionary survival trait. But I don't see how it's absence would affect the ability to have an intelligent species. It might even generate one faster, because of the extra wiring. The bigger question in my mind would be what benefit would a species get by 'not' having the reflex arc?

  • $\begingroup$ "The bigger question in my mind would be what benefit would a species get by 'not' having the reflex arc?" Evolution doesn't really work that way. What benefit do we have by not being able to see infrared? None, in fact, and our hunter/gatherer ancestors at least lost out on the significant advantage it would have given us -- but evolution just hasn't stumbled upon that random variation for us. Similarly, if evolution just hadn't stumbled upon the "reflex arc", we wouldn't have it. And in any case, "what advantage does not having it" is moot (unless the question is why something was lost). $\endgroup$
    – Kromey
    Commented Oct 22, 2014 at 17:33
  • $\begingroup$ @Kromey I'll give you that. I was thinking if some had the trait and some didn't what would the benefit be for those without? $\endgroup$
    – bowlturner
    Commented Oct 22, 2014 at 17:38
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    $\begingroup$ @Kromey You're right in general, though I suspect that the reflex arc may be representative of how our nervous systems worked before we had brains. In that case you'd be looking at a species that lost a trait, which I think is more likely to reflect some kind of adaptive pressure. $\endgroup$
    – octern
    Commented Oct 24, 2014 at 5:59
  • $\begingroup$ @bowlturner one scenario I can imagine is that a species has reflexes that are hard-wired for fitness in a particular environment. If the species moves into a new environment in which their reflexes are maladaptive, then they might just lose the trait. $\endgroup$
    – octern
    Commented Oct 24, 2014 at 6:03
  • $\begingroup$ @octern You may very well be right. On the other hand, perhaps that "pre-brain" simply developed into the brain, with the spinal cord and rest of the nervous system nothing more than simple nerves reaching outward from it. I think in terms of Earth biology you're right, but there are always other routes evolution could have taken. $\endgroup$
    – Kromey
    Commented Oct 24, 2014 at 15:30

At bottom, this gets into a discussion of the feasibility of non-distributed vs distributed neuronal plans, right? When we look around all we see, in higher animals at least, are degrees of distribution.

Cephalopods have neural centers in the head & in every tentacle, with the head containing only 1/3 the total neurons. That is a very high degree of distribution.

Most likely, intensive processing of coordinated perception/strategy/action will usually, perhaps inevitably, be favored in a highly integrated processing center (though the octopus may argue that point). Reflex arcs can be seen as hard-wired features adapted to optimize high-value survival stimulus-response action patterns & as such strictly limited in scope, but highly conserved. In the case of cephalopods, it cannot be that simple, by a long shot.

We don't see much or any of the completely non-distributed neuronal systems in nature, which strongly implies these reflex pathways were present early on in precambrian animal life, and were elaborated in tandem with the elaborating cerebral ganglion, etc. This also implies that the alternative is significantly inefficient/disadvantageous, by comparison, or was when body plans were being put thru their paces early on.

Some constraints which might inhibit the proposed non-distributed neuronal plan:

  • Reaction propagation delay,
  • Loss of opportunity for localized optimization in neuronal mapping, resulting in likely decreased brain sensory area efficiency profoundly impacting overall brain efficiency, increased diameter of the spinal cord/vertebrae, leading to loss of potential flexibility/agility/speed.
  • Greater susceptibility to spinal & brain infection.

Hard to see much upside, and therefore hard to see promising evolutionary opportunities for it to have proliferated within a viable body plan.

If anything, the lesson of the smartest of mollusks point in the opposite direction.

All that said, I'm not an evolutionary biologist, so if anyone who is cares to cut all this to shreds, have at it. ;)


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