Testing Gaia: The effect of life on Earth's habitability and regulation

The Gaia theory proposes that the Earth system self-regulates in a habitable state. Here the effect of life on the state of the Earth and its response to forcing and perturbation is considered. It is argued that life has not survived for >3.8 billion years purely by chance, rather the Earth system possesses regulatory mechanisms. The Earth system is more resistant and resilient to many (but not all) perturbations with life present and the Earth is predicted to remain inhabitable for longer with life present than it would without. With only one case to study, it is difficult to test whether the Earth exhibits such regulation by chance or because regulatory outcomes of life-environment interaction are more probable. The search for life on extra-solar planets and `artificial life' simulations of virtual worlds may increase the sample size. Theoretical principles suggest that regulation is a probable outcome of life-environment coupling, and on Earth it appears that there may be an innate tendency for regulatory properties to accumulate and strengthen as the biota evolves.