Raven, thanks very much for commenting on the last post and for bringing up those key points - I appreciate it!
The split-brain patient's actions in the video were not representative of what happens in daily life, and yes, it is true that motor functioning is not impaired. Here's a quote from a paper that addresses this:
"[M]ost of the changes are subtle and require special laboratory tests to emerge. But it is true that the daily lives of 'split-brain' patients stand in sharp contrast with their performance in laboratory tests and defy some simple, unitary understanding of how the mind is organized in the brain...[I]n daily life, the patients appear to behave as if there was no evolutionary purpose to this major forebrain neuronal connection [corpus callosum] between the hemispheres. Certain functions considered by some to be specialized in the right hemisphere such as voice modulation or prosody appear unimpaired. Left hemisphere functions such as speech and language comprehension also appear unimpaired. Previously learned functions that require bilateral interaction such as, cooking, cycling, swimming, or piano playing appear unchanged, and have remained so until now, as long as 30 years post-surgery in some cases. Neither have there been major changes in personality or mannerisms, or in general intelligence. There are no psychiatric symptoms such as hallucinations, dellusions, fugue states, or multiple personalities. Each patient behaves as one with a single personality and unified consciousness."
So in short, the split-brain experiments are an oversimplification of brain functioning within a controlled environment and shouldn't be taken as representative of what those patients go through every day. But they are an interesting example of hemispheric functioning in that specific setting.
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Is it possible that the split hemispheres are able to collectively achieve most normal brain functions as a evolutionary safeguard against brain damage? Also, does that mean that, in some manner, the almost independent and self contained functioning of the brain originated from the same evolutionary mechanism that promotes the existence of multiple organs (two kidneys, two lungs, two eyes etc...)?
I think that's possible -- the evolutionary argument has also been proposed for why we developed parallel processing. But the brain's plasticity (ability to reorganize neural pathways) isn't the same in adults vs. young kids (about 8 yrs. old or so). Young kids with brain damage to one hemisphere are better able to compensate using the other, non-damaged hemisphere. And in cases where one hemisphere is completely removed in young kids, they are even able to compensate for that.
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