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I was wondering: why most, well, pretty much all organism with a brain have it right in front of their bodies or at the top.

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    $\begingroup$ I believe the general argument goes like this: it's expected that the sense organs appear toward the anterior end of the organism (you want to see, smell, hear what's in front of you). For the ease and efficiency of processing inputs, it makes sense that nerve bundles appear close the sense organs. These nerve bundles eventually became brains. $\endgroup$ – p.s.w.g Feb 15 '15 at 20:41
  • $\begingroup$ I agree with @p.s.w.g. $\endgroup$ – AliceD Feb 15 '15 at 21:06
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    $\begingroup$ @p.s.w.g - while your explanation makes sense, it doesn't hold true, especially in insects (butterflies 'taste' with their feet, etc.). It does hold true for eyes, though. Optic lobes are a large part of insect brains. Interesting to contemplate. $\endgroup$ – anongoodnurse Feb 15 '15 at 22:16
  • $\begingroup$ @anongoodnurse - humans feel touch, heat and pain in their limbs. Fact stands that neuronal cell bodies congregate in the anterior parts in humans and insects alike. I think that's the PO's question - why is the brain (i.e., ganglions in insects) situated in the head region? $\endgroup$ – AliceD Feb 16 '15 at 2:47
  • $\begingroup$ The wiki article on cephalization may help. $\endgroup$ – Remi.b Apr 19 '15 at 17:33
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TL;DR Visual and auditory systems are crucial for survival, especially in higher animals. Also, visual/auditory systems have great number of cells. For processing to be effective, it should be performed closer to data acquisition (eyes).

First of all, for example, in mouse or human, central nervous system includes brain and spinal cord. Since spinal cord is extremely important at regulating different physiological processes as well as in performing complicated motor tasks, it would be unjust to not think about CNS as a whole.

If you talk only about brain part of CNS, it makes sense to locate important processing pathways near sensitive organs for two reasons. First, it saves material on running long axons from many sensory cells. Secondly, longer axons deliver information slower. If there is a sense that is important for survival, than information in that modality must be delivered and processes as quick as possible.

In comments to the question @anongoodnurse mentioned that "butterflies 'taste' with their feet". I would argue that a) taste in butterflies is not vital for immediate survival and b) number of taste buds is very limited, whereas retinal cells are count up to more than thousand per eye.

But what about humans? We also able to "taste with our feet", e.g. feel temperature. For survival it is crucial to be able to escape hot surface as fast as we can. And we do, thanks to a direct reflex arc going from sensory cells to spinal cord and motor neurons. Which brings back point about CNS as a whole system. Crucial information and immediate decisions are made closer to sensitive organs.

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I was wondering: why most, well, pretty much all organism with a brain have it right in front of their bodies or at the top.

I believe you're confounding origins. Complex brains -- like those in us and other mammals -- are situated in the position that has been the best-protected over millions of years of evolution.

Evolution did not decide where the front of the animal should be and then put the brain there. Rather, evolutionary pressures resulted in the brain being an excellent survival tool, and the species that could protect it best kept its benefits. It just so happened that, since complex brains require an enormous amount of resources and are quite fragile, wherever the organism could best protect the brain became its "front".

In mammals that became a skull. In some insects it's in the thorax or abdomen. In sea slugs... well, there really isn't a complex brain in sea-slugs... so the difference between their front-end and back-end is pretty minimal. Same with worms and other very simple creatures.

So think of it like this:

Brain = Good for Survival, but Fragile --> Brain Needs Protection --> Wherever Protection is Best = Where the "head"/"front" has evolved

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    $\begingroup$ I thought about it too, but isn't such an argument mixes cause and effect? Fly shouldn't care where to have its brain, abdomen works as well for protection. I would expect to skull evolve around brain, rather than brain evolve around skull. Otherwise brain should be located throughout insides of thick bones, as protection is best there. $\endgroup$ – aaaaa says reinstate Monica Apr 19 '15 at 18:49
  • $\begingroup$ @aandreev -- They co-evolved. As brains became more complex and bigger, there needed to be a better place to store them. Thick bones aren't very spacious. Hollow bones aren't very protective. The compromise for organisms evolving a larger brain was the skull: A hollow, encapsulating bone that can also be very dense (especially among goats, bovines, etc.). It's also a design that means there's only one area to protect instead of every single thick bone in your body. $\endgroup$ – MCM Apr 19 '15 at 21:03

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