I was reading more about that left-brain/right-brain spinning dancer thing, and it got me to thinking about the brain. Why do we have two hemispheres at all? Why not just a giant glob? Is it simply an impossibility due to evolutionary inertia? Or is the corpus callosum an indication that mammals (and us in particular) are gradually knitting the two sides together into one mass? Was it a useful redundancy in lower life forms (a la two lungs, two kidneys, etc.)?
Perhaps the same reason we have two arms and two legs instead of three?
Bilateral symmetry.
That’s a description (superficially) of what the brain looks like, but not an answer to the question. We certainly do not have some kind of bilateral evolutionary mandate – much of our guts are quite assymetrical. The digestive system, heart, spleen, pancreas, liver, etc. are all solitary organs and oriented to one side or the other. And we know that the brain is symetrical in appearance but not in function – most higher cognitive functions are localized to one hemisphere or the other.
How far back down the evolutionary path does the split-brain go? Do lower mammals have it? Do reptiles have it? Fish? Arthropods?
I personally don’t know but it seems a good way to determine when and why the brain split as it did.
This site seems to have a decent ‘evolution of the brain’ and describes the merging of the three-sectioned reptilian brain into the two-sided brain we have today.
Much of the asymmetry of our gut is the result of squooshing a long tube into a small cavity. The heart actually develops as two separate, bilateral sections and fuses into a single, central structure during development. The single organs which are part of the gut develop as invaginations of the central gut tube.
I would say that the brain is bilateral because we are bilateral. Brain development is linked to the development of various sensory apparati, which are also paired. Brain hemispheres are almost as old as cephalization itself. Optic and olfactory lobes are paired, so it’s not surprising that hemispheres were paired when they began to show up in early bony fishes.
As to when / how / why brain functions segregated into separate hemispheres…I couldn’t tell ya.