I haven't read anything on it by the theories proponents, but if I understand correctly from the comments of others, Gaia postulates that a planet either has abundant life or none. I think that whole theory results from a sense of wonder at how our whole complicated planet works together. I think it assigns an effect as a cause.

Where energy and resources are scarce the ecosystem may consist soley of a single simple algae Chroococcidiopsis (http://science.nasa.gov/headlines/y2001/ast26jan_1.htm?list88972). Maybe that organism couldn't exist without the rest of the biosphere elsewhere, but that doesn't appear to be the case to me. I think life has simply evolved to take advantage of every energy source available. It only takes basic evolutionary theory to account for that. The result of using all available energy sources is the wondefully complicated and interwoven web of life we have. I also think that Gaia is not accepted by all and possibly even most biologists based on a lot of biological reading over the years and relatively little mention of this theory.

As a follow-up I agree that the chances for intelligence emerging were much higher latter in Earth's history once Mammals and Dinosaurs evolved, but I think it could have evolved over tens of millions of years anytime since about 200 million years ago if the circumstances were right.

Regards,
Mark