I won’t beat about the bush…
Why is the brain of humans/animals located in the bit of the body that seems most vulnerable to the perils of the outside world (such as attack from other animals, or accidental damage)
“Why” is not a question you can answer with evolution. Evolution doesn’t have goals; the brain is where it is essentially because that’s where the first successful creatures with a central nervous system had it. If it had been located in the posterior instead, today we’d all have our heads up our asses.
Because that is where the sense organs are, and the wiring was easier if the processing centre is kept close to the inputs.
The sense organs are there because that is the end that the worm’s mouth was, and even worms were helped by being able to see and taste what they were eating.
What Q.E.D. said. There isn’t something called “Evolution” that sits around and thinks about the best solution to every problem. Mutations happen randomly, and natural selection eliminates the unworkable ones. All the others get to stick around, whether they’re optimum or not. The only requirement is that it doesn’t kill you before you can reproduce. Apparently, enough of us survive to reproduce, with our brains where they are. That’s all that’s necessary.
Eyes as high as possible are good, because you can see more that way.
Brain near the eyes is good, because info gets to the brain from the eyes faster.
Evolutionairily* speaking, creatures with a better field of vision and faster response to same survived better (produced more offspring) than those with better-protected brains.
- Yes, I like making up my words as I go along, thank you.
Not just easier, faster. Nerve impulses are relatively slow, and getting sense data upon which to act is more time critical than just firing muscles, which pretty much run themselves once started. So (wording things so as to not personify Evolution and further distract us from the actual OP question) mutations that separate the brain from the sense organs are more likely to kill the mutant.
Likewise, mutations that make it hard to check six are more likely to kill the mutant, so critters with bendy necks are more likely to have offspring.
I’ve heard it said (sorry, no cite as I can’t remember where) that the cheap B-grade sci-fi films we all know and love actually have it more or less right with regards to intelligent alien life. The films did it because it was cheaper just to put some makeup on, but the theory goes that an intelligent extraterrestrial life form is likely going to be bipedal, between four and seven feet tall, and have forward-facing binocular vision with eyes situated as high as possible.
I think “why” is a question you can ask of the evolutionary process, but more in terms of “Why was this feature evolutionarily successful?” as DrFidelius and NoCoolUserName demonstrated.
Another angle: if all of the key sense organs are there and the essence of where “you” are, then this is a body part that will probably receive the highest degree of attention and care from its owner.
In other words, don’t you spend more effort and care protecting your face then, say, your backside?
If your ass had eyes it would be your face. And then we would all be talking about how much sense it made to have them there.
Simon Conway Morris is a big proponent of this idea. It certainly makes sense to me.
Another thought:
The brain produces a lot of heat. If too well isulated, it would probably overheat.
I think the brain is quite well-protected versus other vital organs. The skull is a pretty hard helmet. Rib cage protects the heart and lungs…it’s those damn intestines we can’t cover with bone!
'Course, you might shoot your eye out…
Seems to me that the important bit only seems the most vulnerable because it’s the important bit.
Tell that to the turtle. 
In general, the brain is going to be somewhere near the eyes. The eyes (more specifically, the retinas) are part of the central nervous system, not the peripheral. They are direct outgrowths of the optic nerve, which plugs directly into the brain. Pretty much every organism I know of that has eyes and a brain has the eyes really close to the brain; even grasshoppers and other invertebrates typically have the brain right between the eyes, or at least the eyestalks. The other special senses - taste, smell, hearing - might have transducers elsewhere, from the thorax all the way down to the feet, but I don’t know of any organism that moves the eyes far from the brain. Those optic nerves are awfully thick and need to be pretty high-bandwidth, and it’s hard to run that sort of thing just anywhere, and for a long distance. In the CNS, the myelination is provided by oligodendrocytes, not Schwann cells. Oligodendrocytes myelinate multiple cells, forming a very dense and difficult-to-remodel structure. This is one of the reasons we have so much trouble regenerating CNS lesions, by the way - you need to snake the regenerating axon through that thick web of myelin, and without a carefully prepared path, both spatial and chemical (trophic factors), it won’t work that well.
Now, vertebrates have the spinal cord, of course, which is CNS, but that’s a very specialized structure. To the best of my knowledge (and this is out of my field, so caveat lector) the notochord only evolved once. So, you want the eyes to be close to the brain. On the other hand, you want to be able to move the eyes in any number of directions, so you want them to be on the end of a rather dextrous appendage. There’s your neck/skull compromise right there.
I think this response begs the question. There are measures of fit other than “what came first”, and they are not “the first that works”, but rather the thing that works is the thing that satisfies the measures of fit. In the case of the brain - it’s in the head because that’s where the sense organs are, placing the brain nearby both minimizes wiring costs, as well as lowers response time to/from said organs (since neurons don’t react instantaneously). The question of why the sense organs are on the end of a thin vulnerable bit is a related question, but not the one asked, so I won’t approach it.
What’s the “thin vulnerable bit”?
This is only true in the male, of course.
The neck. Sicko. :smack:
