Reason for crossed projections in the nervous system?

[k.jeffery at ucl.ac.uk]

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. 

<deleted>

>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.
>Sandra

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. 

Cheers,

Kate
k.jeffery at ucl.ac.uk