Re: Earth without oxygen

On 2008-04-12, Gareth Wilson <wilsong2@[EMAIL PROTECTED]
> wrote:
> It points out that oxygen-producing photosynthesis evolved only once
> and might be one of the less likely steps towards the modern
> biosphere.

This is the first I'd heard that it was definitely known that it
evolved only once.  What evidence is there for that?

If a billion different organisms throughout Earth's span developed a
primitive capability for oxygenic photosynthesis, and would have been
successful if they didn't have the competition from more highly
developed organisms, how would we be able to tell?

For that matter, how do we know that the modern organisms are
descended from the first oxygenic autotrophs?  We don't even know
within hundreds of millions of years when the capability first arose,
let alone the lineage of all such organisms.


- Tim