During the 1960s, America was consumed in an era of post-Sputnik competition that was characterized by engagement in a race against the Soviet Union: the Space Race. Humans were launched into space, the surface of the Moon was first explored, and the first of many missions to Mars were planned. It was during the planning stages for the Viking missions that ideas behind the Gaia Hypothesis began to take shape. James Lovelock, an atmospheric scientist, was asked by NASA and the Jet Propulsion Laboratory to help design experiments for the Viking mission for the detection of life on Mars. The experiments conducted during the Viking mission found no evidence for life, but this wasn't of much surprise to Lovelock. He had already predicted as much through his own observations of the Martian atmosphere.
How could Lovelock have been so confident about the prospects for life on the red planet? A simple comparison of Earth's atmosphere to Venus and Mars reveals a stark difference between Earth and its neighbors. While the atmospheres of Venus and Mars are comprised primarily of carbon dioxide with small amounts of oxygen, nitrogen, and other gases, Earth's atmosphere is over 3/4 nitrogen and almost a quarter oxygen. The atmospheres of Venus and Mars, as noted by Lovelock and other scientists, are in equilibrium, a "dead" equilibrium. Earth, on the other hand, has an atmosphere that is far from equilibrium. And what is keeping it out of equilibrium? Life.
Based on his observations of planetary atmospheres, Lovelock proposed his own theory, which has famously become known as the Gaia Hypothesis. His idea was first exposed in his book, Gaia, a New Look at Life on Earth, in 1979. It was here that Lovelock described Earth as "A planet transfigured and transformed by a self-evolving and self-regulating living system. By the nature of its activity it seemed to qualify as a living being." He named this living being Gaia. Within Gaia, the existing biomass is thought to self-regulate physical conditions on the planet to make it more suitable for life. Lovelock's hypothesis was met with much criticism from the scientific community. Although scientists could not deny the interrelatedness of organisms and the environment, they balked at the idea of Earth possessing some form of coherent behavior like that of a living organism. In addition, since Gaia can't reproduce herself, she cannot be considered alive by any conventional definition of life. Finally, scientists argued that there is no way to perform an experiment to show how the proposed feedback systems within the Gaia proposal could have evolved over time.
Several decades have passed since Lovelock published his first provocative book. Since then, the Gaia Hypothesis has evolved as much as Gaia herself. Lovelock later renounced any implications that Gaia is a conscious being by stating, "Nowhere in our writings do we express the idea that planetary self-regulation is purposeful, or involves foresight or planning by the biota." The Gaia Hypothesis has changed from a theory that described Gaia as a homeostatic system (in which homeostatic feedback from living biota influences the abiotic world) to one that is homeorhetic. The difference between the two systems is subtle. In a homeostatic system, there is a trend toward constant values for various parameters (ie: atmosphere, hydrosphere, etc). A system that is homeorhetic will be similarly dynamic, but won't necessarily converge to a constant state. This view of Gaia as a system is more acceptable to scientists as it helps to explain how the system, our Earth, can evolve over time. After all, one concrete example of how living organisms have drastically impacted the physical environment was the oxygen explosion following the activity of photosynthetic bacteria during Precambrian times, a noteworthy mile marker in the evolutionary history of Earth.
With respect to our search for life in the universe, the most important result of the Gaia Hypothesis was the recognition that the biotic and abiotic components of any planet are integrally related. As we extend our searches for life beyond our Solar System, this knowledge will be extremely useful. The vast distances between stars currently make it impossible to directly explore extrasolar planets. However, with the use of new telescope technology, we will soon be able to make measurements of alien atmospheres. If we detect an atmosphere vastly out of equilibrium, we may draw on the lessons and research that were a direct consequence of the controversies surrounding the Gaia Hypothesis.