Sandra L Wegert <sandraw at U.Arizona.EDU> wrote:
>> @ucl.ac.uk wrote:
>> : A hypothesis I came across last night, attributed to Marcel Kinsbourne by
>> : Steven Pinker in his book "Language as Instinct", runs as follows:
>> : bilaterally symmetrical invertebrates have an uncrossed nervous system,
>> : their spinal cords (or the invert. equivalent) ventrally and their hearts
>> : etc. dorsally, whereas vertebrates have a crossed nervous system, their
>> : spinal cords dorsally and their internal organs ventrally. Kinsbourne
>> : speculates that sometime during invert-vert evolution, the head was
>> : twisted around 180 degrees so that it points the other way (sort of like
>> : what happened to flatfish only worse). I thought that was a kind of cute
>> : idea. I have no idea whether it has any merit or not.
>And we're talking about left/right crossing, not dorsal/ventral
>crossing. Also the mammalian brain DOES bulge ventrally, but the spinal
>cord (where things are crossed) is still the same. Nice ideas though.
No, what I meant was, if sometime during evolution the head turned 180 deg.
around its rostro-caudal axis, then left would become right and right left (as
well as dorsal become ventral).
I agree it doesn't seem quite the right explanation though. It doesn't explain
why the cerebellar projections *aren't* crossed (unless it happened before the
cerebellum evolved), nor why different vertebrates have different degrees of
crossing - you would expect it to be all or nothing. Anyway I still think it's
an interesting idea.
k.jeffery at ucl.ac.uk