Independent: I have heard that the Gaia theory means that the Earth is alive. What does that mean, exactly? Roger Middleton, Chester The Earth system (Gaia) shares many attributes with a living cell; it metabolises, it responds to changes in its environment, it can die, and it reduces its internal entropy by taking in high quantum energy as sunlight and excreting infra-red radiation to space. It does not reproduce, but something that has lived about 3 billion years hardly needs to reproduce; …