On Mon, May 12, 2014 at 4:03 AM, Rob O'Connor <robocon@ozemail.com.au> wrote:

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.

The material (it's the introduction to Uller Uprising, written by a 1940s scientist but aimed at laymen), seems to be saying that the carbon-based and compromise forms were relegated to small ecological niches because most of the habitable area of Uller experiences such extreme climate variations as to leave only those small niches available. Unlike on Earth, early Uller had a whole class of (silicon-based) life for which the "hostile" conditions weren't limiting factors.