What little I've read of Gaia has left me with a different impression. I don't think Gaia is about the abundance of life, or lack thereof. It appears to be more of a holistic way of looking at the biosphere, of seeing it not as composed of separate species and organisms operating independently of each other.

The Weak Gaia Hypothesis simply states the obvious: complex ecosystems tend to be inherently stable and living things modify their environment. The Strong Gaia Hypothesis sees the biosphere as a single vast self-regulating organism.

Strong Gaia, when taken to its ultimate extreme, leads very naturally to space-travel and the terraforming of planets. The language of the Gaia Hypothesis offers a home for arguments in favor of the human colonization of space.

It is not so much a hypothesis, after all, as a philosophical viewpoint. If we view the biosphere as a single interdependent living system, Earth can be regarded as the nest where this 'organism' has hatched. The Gaia organism knew a time of birth, then infancy, then a series of difficult growing pains, followed by a brief but glorious childhood.

But that childhood is over. The resources of the nest will not last forever, and one day even the nest will fall apart. The Gaia organism will one day die when Sol dies, if not much earlier. It must reproduce by then before the implacable and unforgiving forces of the universe randomly strike to wipe it out. Gaia must scatter her seed as far from the nest as possible, and eventually reproduce herself. New biospheres must be spawned on new worlds. In the context of Gaia, humans fall into the role of spores, fated to fan out from home, carrying with them the information and resources needed to recreate Gaia elsewhere.

As a scientific hypothesis, Gaia may well come up short. The essential points at the heart of the weak interpretation of Gaia merely amount to a restating of some obvious facts. Living things are interdependent, greater diversity leads to a more robust ecosystem, living systems alter their environment. The strong version of Gaia is speculative and not easy to test. How can anyone test the biosphere to determine if it is driven by some greater and more subtle urge to reproduce and expand its territory?

Let us say humans do manage to voyage in person out to distant worlds, establish footholds under alien skies, and create new biospheres elsewhere. If that happens, how much of that will be due to self-fulfilling prophesy, and how much due to some primal omnipresent urge to ensure the continued growth and survival of Earth-life?

There may be something to Gaia, but it may suffer from a terminal case of the losing struggle to separate the observer from the observed. It could be true that the biosphere, from the origin of the first archaea right on up to the evolution of complex life, has been building up its survival capabilities as a whole. It could be true. But it could also turn out to be very hard to prove.

Gaia works better right now as philosophy than as science.

Any statement to the effect that the chances for the emergence of human-type intelligence have been much better later in Earth's history than earlier would seem self-evident. I think the condition 'if the circumstances were right,' while general, is very apt. For, if technological intelligence can simply arise as a matter of pure chance, as a purely random event, then its appearance on any living world would be no more remarkable than the appearance of furry animals. Most of us suspect there is more to it than that, that certain pieces must be in place before a species that builds civilizations can develop. When those pieces are in place, the probability that a technological species might come on the scene goes up. That, after all, is largely the basis of the Rare Earths view, which claims the chances of those pieces all being in place are virtually nil.

The Rare Earths Hypothesis may not be much more testable than the Gaia Hypothesis at this stage of human history. But both provide a useful framework for the discussion of various views on evolution and the development of technological beings.