Richard Aiken wrote:
 > The original article includes something along these lines, but has
 > the purely silicon early lifeforms beating out both the purely-carbon
 > and the "compromise" (partly carbon, partly silicon) ones, due to
 > better tolerance of the environmental conditions on Uller.

So the silicon-based biochemistry is dominant on Uller, which is one of
the unusual worlds where more than one biochemistry yields life.

The "compromise" forms are likely symbionts. Having one organ be carbon
based while another is silicon based strains my suspenders of disbelief.

 > It's that "better tolerance" that seems to be the sticking point. One
 > of the early posters (I think it was Tim) pointed out that this did
 > not necessarily follow from simply being silicon-based.

No, it does not.

The frequency and intensity of contacts between the two biospheres
determines the rate of evolutionary change.

It can be very difficult for an organism to evolve away traits that were
useful before conditions changed.

There appears to be no biological analogue of quantum tunnelling with
regard to the 'fitness landscape' - organisms can't easily lose a
fitness maximising trait e.g. vision, aerobic metabolism.

Rob O'Connor