A leech (Hirudinea) has 32 brains throughout its segmented body. An Octopus (Octopoda) has 9 brains, one for its head and 8 for its arms (legs?).
How do these animals process tasks using multiple brains? It would suck if 3 out of your 8 arms wanted to go find a place to hide while your brain and other 5 arms are actively hunting.
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The fact that there are 9 separate lobes in an octopus does not mean there are 9 separate consciousness. Humans have two lobes which happen to be next to each other. If we evolved to have our lobes further separated, we would think a bit slower in some ways, since it takes time for signals to move across nerves. But we wouldn’t necessarily think in a different way.
Furthermore, there’s a difference between making decisions (which we call executive functions), and figuring out how to perform those decisions. If you decide to take a sip from a cup of water, a relatively small part of the brain is making that decision. A larger part of your brain and nervous system is involved in pulling it off:
Your visual centers are keeping track of the glass and its position relative to you and the floor.
Your sensory centers are processing the feeling of the glass in your hand, the water on your lips, the position of your arms etc
Your motor centers are controlling the movement of your arm, and fine tuning it based on feedback from your visual and sensory centers
And so much more. None of this is conscious. A lot of this doesn’t even happen in our brains, but in our spinal chord. We still have *executive* control over what our bodies are doing, we just automate a lot of the fine detail.
So why have many brains in the arms instead of one centralised brain? Well it depends what you’re optimising for.
If we had tiny brains in our arms, we could have less brain in our heads, and thus smaller heads. This would have many advantages – childbirth in particular would be a lot easier. Our arm-brains would be closer to the muscles they were moving, allowing them to react faster to touch stimulus. But they’d be further from our eyes, so we’d react slower to visual stimulus. We are *very* visual creatures, so that’s probably a big problem.
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