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People and dogs as bipeds (Re: Speed)

I wrote...

>      A biped needs a means of balncing itself, true.  Theropods 
> accomplished this with a long, fairly flexible tail.  Since apes lack 
> tails, the upper body flexibility of humans in a different approach to the 
> same problem. 
>      Again, human upper body flexibility for the sake of balance is
> a means of coping with a more unstable posture, not the REASON
> humans evolved bipedality.

     I just realized this is probably at least partially wrong.
Vertical posture in humans was the key to being able to balance
bipedaly, lacking a tail, but how vital is upper body flexibility to
balance?  How does upper body flexibility in humans compare with that
in mainly quadrapedal apes like chimps an gorillas?
     Also, regarding the dog frisbee catcher, I have some doubts about 
the purpose of the upright posture being to center all the mass over 
the center of gravity.  If dogs were to stand up on thier hind legs 
BEFORE jumping and turning, I might understand this, but they become 
vertical at the same time they are jumping and turning.  By the time they 
are fully vertical, they are already in position to catch the frisbee 
without additional twisting.  Of course, as they are jumping and 
twisting, they are becoming upright, which would reduce the "lateral 
mass" (I just made that up), giving at least a little bit faster spin 
even if they weren't yet fully upright.  I just though that the point to the 
upright posture was the get the head higher than could be reached if the dog 
were to stay horizontal during the jump.    
     Oh yeah, back to dinosaurs and stuff...

LN Jeff