>>created to prove the existence of God

If you look at it more generically, Baysean reasoning was a systemic way of dealing with new information that either reinforces or contradicts an already-held belief. If you run into people talking about 'priors', that's what they're talking about. Personally, my first contact with it was in building anti-spam filters using multiple inputs. Medium cutting-edge for the mid 80s. Now, it's just how a lot of what we take for granted is done. It's a really good fit for the build-a-model-of-the-physical-world that science is all about.

As far as the nervous system being considered part of the brain, well, considering it all to be nervous system, the brain as part of it is a handy way to split things up, which seems to be the way most complicated things get figured out. I dealt with complex problems in software by first breaking the problem down into pieces, then figuring those out (or sometimes with recursion which is essentially the same thing, but in opposite order). A common saying around this methodology is: "How do you eat an elephant? One bite at a time."

If you look at the protocols now commonly in use as underpinnings of the Internet, you'll note that there are multiple layers. Those layers represent splitting a complex problem into separate smaller problems, then solving those (and as someone who watched it happen I can tell you there was a one-to-one mapping between the original number of layers and the number of conference rooms in the hotels where it was first addressed — an arbitrary number.) Biology is the definition of complex, so divisions have to be made, even if the divisions are arbitrary. Speaking of biology and complex, it’s worth tracking what’s going on in synthetic biology, which even though it sounds like synthesizing life, it’s more a matter of reducing genes in a model organism so biologists have something simple to work with, paralleling hydrogen in physics.