What is today called Earth System science has its origins in the Gaia hypothesis. The Gaia hypothesis was begun in the mid 1960s when the British chemist James Lovelock was acting chief scientist for the physical life-detection experiments of the NASA mission to Mars originally called Voyager (later renamed Viking). Lovelock's proposed approach to searching for life on Mars, based on chemical analysis of the Martian atmosphere, led to reflections about the utterly different and remarkable atmosphere of our own planet. The stable persistence in the Earth’s atmosphere of gases that quickly react with each other could only be possible with some kind of ‘control system,’ Lovelock reasoned, and this must involve the life of the planet. Later, with the help of American microbiologist Lynn Margulis, the idea was further elaborated such that the Earth was seen as a self-regulating system in which the climate and chemistry of the planet are kept constant, or in homeostasis, in the short term, and evolve over time in response to changing conditions and needs of the total system. An important aspect of Lovelock's idea is that life does not simply adapt to its environment, as in the traditional Darwinian view, but is actively engaged in altering its surroundings for its own needs.

What follows is a tour of Gaia theory, with texts by James Lovelock and accompanying graphics by Sara Mathews, helping to render the material accessible to the layperson. They are reprinted with the kind permission of Gaia Books, and are taken from Gaia: The Practical Science of Planetary Medicine, by James Lovelock, from 1992, with reformatting and editing for online display. The text is now twenty five years old, so some of the data cited is no longer current: for example, on the first page it is noted that atmospheric methane is 1.7ppm (parts per million), but methane has risen since that time to >1.8ppm, largely from human-driven emissions. Despite this, the text remains remarkably contemporary and fresh.