On Fri, 17 Dec 2004 23:14:03 +0000 (UTC), "Curious in Minneapolis"
 wrote:

>The December 11 issue of The Economist magazine carried an article
>about lefthandedness.  It suggested that the fact that the left side of
>the brain controls the right side of the body, and vice versa arose
>because "long ago in the evolutionary past, an ancestor of humans (and
>all other vertebrate animals) underwent a contortion that twisted its
>head around 180 degrees relative to its body."
>
>I am intrigued by this claim, which I have never heard before.  Could
>someone offer more details about it?  What empirical evidence is there
>for it? What do biologists suggest could have been the reason for this
>"contortion" or was it purely an accident? Is it possible that it never
>really took place, that there is actually some advantage to having this
>left-right crossover in the brain?
>
>I would be grateful if you could direct me to any good sources
>understandable to a layman regarding this phenomenon. Thank you.
>

This "contortion" is the remnant of a very old (and discredited)
hypothesis to explain why many invertebrates have no crossing, between
right and left brain vs right and left sensory/motor function,  a
ventral nervous system, and a dorsal heart whereas vertebrates show
the crossing and have a dorsal nervous system and a ventral heart.
However, it is quite likely that there was an early switch in the
developmental genes that determine body symmetry and differentiate the
left vs. the right sides.  If the brain "chose" one set of criteria to
define which is left vs. which is right but the peripheral system
"chose" the opposite way, then things would cross.  

Whatever the cause, it is not true that somewhere in early evolution
the head of an ancestral vertebrate got twisted around 180 degrees on
the body axis.

I don't know of any reasonable explanation for the crossover as an
adaptation or advantage.  There have been some lame brained
explanations, but nothing that captures the enthusiasm of the majority
of scientists.
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