In the late 1960s, James Lovelock presented a hypothesis which he called the Gaia hypothesis, named after the Greek word for the Earth Goddess.

Lovelock and his colleagues suggested that the Earth is a self- regulating system, one able to maintain the climate, atmosphere, soil, and ocean composition in a stable balance favourable to life. The inputs and outputs are delicately balanced and controlled by feedback mechanisms. The Gaia hypothesis proposes that life itself produced and kept the very gases essential for its own existence at a constant level, and that life itself kept the natural systems of the Earth in a state fit for life.

The Gaia hypothesis also explained the extinction of species as a result of those species failing to contribute to maintaining the natural systems and therefore the balance of Gaia. The hypothesis also helped to explain why the surface temperature of the Earth has remained relatively constant over the last 4 billion years, since life first emerged from the primordial organic soups and gases of our planet, despite the fact that the Sun's heat has increased by 25 per cent. Over the same period, the overall carbon dioxide level has dropped, reducing the heat- holding potential of the Earth. These were all consequences of botanical activity, the fixation of carbon dioxide from the atmosphere by photosynthesis. The amount of oxygen has remained constant over the past 200 million years, the result of the balance of the complex interactions of organisms and the inorganic components of Earth.

Lovelock and his colleagues believe that if human activity continues to disturb the geosphere, by disturbing the natural balance of Gaia, and if human activities are not harmonised with the natural processes of Gaia, then this life-support machine will no longer sustain us and we will become extinct, to be replaced by new species.

To help illustrate the Gaia hypothesis, Watson and Lovelock developed the 'daisy world model'. They imagined a world inhabited only by black and white daisies. In their model the Sun began to warm up, a process that actually happened during the early history of the Earth. A lifeless world also warmed up because of the greater heat energy being emitted from the Sun. In the daisy world, however, the black daisies absorbed more of the incoming radiation, and were thus favoured because of their more suitable survival strategy, at least during the early days of the faint Sun. As the Sun continued to heat up, however, the black daisies became unsuited to the warmer world, and then the white daisies began to compete more successfully since they reflected more sunlight, which also provided a negative feedback by helping to cool the planet's surface. In such a changed world, the white daisies could become more abundant than the black daisies. Eventually the Sun would become so bright that all the daisies would die, unable to reflect the large amounts of solar radiation reaching the surface of the planet.

This simple model shows how evolving life on Earth could modify climate through both negative and positive feedback. Lovelock and his fellow workers suggest that similar processes took place on Earth throughout geological time.  The Earth will continue to self-regulate itself, if human activity does not cause changes in global climate, by affecting the biosphere and atmosphere-ocean system, which will change faster than any negative feedback that might otherwise moderate the system.

Not all scientists agree with the Gaia hypothesis. A contrary view is that the planet's atmosphere has evolved by chance chemical reactions and degassing from the mantle. Lovelock argues, however, that this and the traditional evolutionary theories are inadequate as theories because they invoke a passive role for biota throughout Earth's history. Lovelock believes that living organisms played, and continue to play, an active role in controlling their environment. This perspective led Lovelock to propose that life itself is the major control on the atmosphere-ocean system on Earth, maintaining a habitable planet through complex feedback.

Today, the consensus of scientific opinion lies somewhere between a Gaian perspective and an appreciation of non-biological, often random processes that contribute to creating and maintaining the living world. Many scientists criticize the Gaian hypothesis because it is untestable. Though parts of the hypothesis can be tested by observation, it is generally impossible to prove that an organism can produce its own environment rather than be conditioned by it.