A new idea has been emerging recently, of Gaia, the living planet. The Earth is conceived, not as an inanimate habitat for life, but as a single living organism. All plants and animals interact with the elements as part of an evolving co-system. All sorts of mechanisms have been identified, controlled by the actions of living creatures, which maintain conditions on our planet in which life flourishes. Calculations show that life has, over 4 billion years, prevented the water from disappearing and the temperature from reaching 300° Celsius. Life has locked up immense amounts of Carbon, thereby reducing the levels of CO, created and maintained an Oxygen rich atmosphere, found ways of recycling sulphur and iodine, prevented the seas from becoming too saline, and much else. In the process, it has laid down much of the sedimentary rocks that are on the surface of the planet, and probably helped to form the mountains and move the tectonic plates.

From a planetary point of view, the appearance of cities has been bad news. On an unprecedented scale, a life form has evolved, which could throw the delicate systems of the living planet out of balance. Cities are living off the planet, as parasites live off a host. The difference is that other parasites can move to another host when the first is dead, or at least their children can. We, unfortunately, have no such option.