The Gaia Hypothesis is an idea of James Lovelock's that says that life affects Earth's environment to maintain good conditions for itself.
In the 1960s, Lovelock was a scientist contracting with NASA to figure out how to find out if there is life on Mars. Lovelock said he would look for evidence that something was working against entropy. Entropy is the tendency for systems to become less organized. Life is the exception.
Lovelock began to look at our planet the same way. He noticed that the composition of the atmosphere stays within fairly tight limits. Oxygen, for instance, remains in the narrow range between being inadequate and enabling holocaust. In his book, The Gaia Hypothesis, Lovelock cites other planetary material cycles, particularly methane, as evidence.