James Lovelock's controversial [http://en.wikipedia.org/wiki/Gaia_theory Gaia theory] states that in some respects, all [[life]] on Earth acts as a single organism, named for the Greek Earth goddess Gaia, which regulates Earth's environmental conditions in much the same way a human body regulates its internal environment. To pick a simple example: as warm-blooded creatures, humans automatically regulate our body temperatures to keep them in a narrow range somewhere around 37.5 degrees Celsius. Similarly, Lovelock believes that by regulating the levels of greenhouse gases in the air, Gaia has kept the range of temperatures on Earth's surface within the spread of less than 100 degrees Celsius necessary to support [[life]], even as [[Sol]] has grown gradually hotter over the past four billion years.