Invisible wrote:
> Actually I think you'll find it's that all tetrapods are descendants of 
> a single fish ancestor, which just happened to have 5 digits. By now it 
> would be far too difficult to change it.

Sure. But we grew lungs, and new digestive systems, and homeostasis, scales 
and skin and fur and feathers, wings and hooves and talons, but we still all 
have five fingers, including the animals whose fingers are in a stiff and 
unbending mitten of cartilage. Look at the vast range of shapes out there, 
and tell me that it makes sense that none of them got rid of the pinky 
finger, or put on another bone somewhere.

> In short, I suspect that tetrapods all have 5 digits because there are 
> now highly complex, well-developed and extensively inter-dependent 
> systems of gene regulation for building 5 digits. You'd have to change a 
> hell of a lot of stuff to make it, say, 6.

No, that's the point. From what I read, you'd only have to change one gene. 
Except it's a gene in the middle of *another* sequence as well, which 
regulates development of the reproductive system.

> If you look at things that aren't tetrapods, you find that 5 isn't so 
> special.

You may be missing my point. Not that 5 is special, but the *persistence* of 
5 fingers when all else is changing is unusual.

-- 
Darren New, San Diego CA, USA (PST)
   Serving Suggestion:
     "Don't serve this any more. It's awful."

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