Did the other organs all move together into their location, how did the brain get left out?
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I believe it has to do with sensory organs, mostly. If you look at less evolved creatures, like flatworms, you’ll find that the major ganglia (clumps of neurons that function as primitive brains) are clustered very near their eyes. In flatworms, the eyespots are immediately adjacent to those major ganglia. Crabs? Right near the eyestalks. Octopus? (Spoiler: it’s not in that big head-shaped sac.) Right behind the eyes. What’s fun about octopus anatomy is that they have ganglia controlling *each arm*, which considering how tactile they are, it would make sense to have mini brains at each tentacle.
Our bodies have evolved from very simple structures. Beyond a means of reproduction and ingestion of energy sources, we’ve simply “modded” ourselves to be better at staying alive, and you can’t have brains in with other things that move and shift and stretch. Neurons are delicate – moving them too much can damage them irreparably, as they do not regenerate very quickly at all. Look at squids: they have donut-shaped brains through which their esophagus passes (and is located – you guessed it – right behind the eyes). If they eat something a bit too big, they can suffer permanent brain damage as the food passes by. Given they they’re so sensitive, having brains in with things that move out change shape – lungs, heart, uterus, intestines, stomach – could spell disaster.
So brains get their own little nook right next to the main sensory organs, away from all the squishy organs that might bump up against them. Nicely protected from jostling in an enclosed case, they’re happy!
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