On 10/01/2011 06:32 PM, Darren New wrote:
> 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,

Just about every living organism has homeostasis. I presume you mean 
*temperature* 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.

Right. So what you're saying is that it's unusual that 5 digits is a 
conserved feature?

Well, horses and similar have finger/toe bones fused together. But then, 
actually the anatomy of a horse leg is quite interesting. A human arm 
has two straight bits, a wrist and then a bunch of fingers. In a horse, 
the first straight bit is actually inside the body, the second straight 
bit is the top of the visible leg, and the remaining bones of the leg 
are actually what would be knuckle and finger bones in a human. Weird.

I suspect what it boils down so is that there's no specific reason why 
some number other than 5 would be an advantage, so it hasn't changed. 
(This of course doesn't rule out random "neutral" changes I suppose...)

> 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.

Well, if that's the case, I haven't read about it myself. I would 
suggest though that something like this would require changing many, 
many genes, all at the same time, which is why it doesn't happen.

(Oh, you can probably make it happen by changing one of the cellular 
differentiation genes, which tell cells "where they are". But that would 
probably break stuff all over the place. Indeed, these genes tend to be 
extremely highly conserved.)

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