How did the nervous system arise in animals?

Question

How did the brain form, what parts and how?

Answer 1

Too broad of a question, really.

The first neurons--and thus the first nervous system--were just variants of regular cells. We sometimes think of neurons as "wires," but all their wire-like properties are actually very regular-cell like, with chemical concentrations changing, passing through ion channels, just like in regular cells.

The first animals with nervous systems didn't have brains, per se. E.g., radially symmetric animals have no head, and their nervous systems are not bunched up disproportionately up in one spot (the head), but distributed all around in a more uniform net.

When bilateralism was invented, there became a head with most of the senses up front. To keep "wire" costs down, one wants to do the processing nearer to all these sensors (i.e., near the eyes, nose, and ears), and so the brain is put up there in the head, rather than, say, in the midsection. This was the first "brain."

And too many things happened since then, but roughly speaking smarter animals have bigger brain-body ratios.

Answer 2

It was necessary to have a nervous system in animals as there are organ system coming up,then the movement of limbs and different parts of the body needs co-ordination and control over them.

Answer 3

Too much caffeine.