James Lovelock is one of the most influential thinkers on the environment of the last half century. His grand theory of planet earth, Gaia, the idea that from the bottom of the earth's crust to the upper reaches of the atmosphere, planet earth is one giant inter-connected and self-regulating system, has had an impact way beyond the world of science. As James Lovelock, celebrates his hundredth birthday (he was born on 26th July 1919) he talks to science writer Gaia Vince about the freedom and frustrations of fifty years spent working outside the scientific establishment. While working at the National Institute for Medical Research he invented the Electron Capture Detector - an exquisitely sensitive device for detecting the presence of the tiniest quantities of gases in the atmosphere and led to a global ban on CFCs. It also took him to NASA and via designing a detector to look for life on Mars gave him the idea of Gaia. Public interest in Gaia proliferated after the publication of his first book Gaia: a new look at life on earth in 1979; but the scientific community remained highly sceptical. For decades Gaia was ignored, dismissed and even ridiculed as a scientific theory. To this day, evolutionary biologists, in particular, take issue with the notion of a self-regulating planet though some are coming round to the idea. Gaia Vince talks to earth system scientists Professor Andrew Watson and Professor Tim Lenton of Exeter University who have both championed the Gaia theory, and to Professor Ford Doolittle of Dalhousie University, an evolutionary biologist who has changed his mind about the theory. Producer: Deborah Cohen Picture: British scientist James Lovelock poses on March 17, 2009 in Paris. Credit: Jacques Demarthon / AFP / Getty Images.