The Gaia hypothesis is a line of thought that arose in the mind of a physiologist, James Lovelock, in the mid-1960s when he contemplated the planet Mars. We were planning the first spacecraft missions to Mars, and Lovelock realized that Mars is probably sterile because its atmosphere is chemically dead, whereas Earth's atmosphere is not. The air of Mars, what little there is, is almost entirely carbon dioxide. But the abundant oxygen gas in our atmosphere would swiftly disappear if living plants didn't constantly manufacture it. Earth's oxygen is balanced between its biological creation by plants during photosynthesis and its chemical destruction by oxidation.
From that notion, it was a short step to the original Gaia hypothesis, which Lovelock laid out in a series of papers over the next decade.
The Gaia Hypothesis
Perhaps the cleanest expression of this idea is in the title of the paper Lovelock and his collaborator, biologist Lynn Margulis, published in 1974: "Atmospheric homeostasis by and for the biosphere: the Gaia Hypothesis." Homeostasis means self-regulation, and it's a fundamental property of living things. Our own bodies, for instance, have mechanisms to maintain a steady temperature, blood chemistry and so on.
Lovelock and Margulis, in effect, hypothesized that Earth itself acts like a living thing. More radically, they posited that "the Earth's atmosphere is more than merely anomalous; it appears to be a contrivance specifically constituted for a set of purposes."
In a limited sense, there are feedback mechanisms between biological and geochemical forces that tend to keep the atmosphere between the oven and the freezer. The idea of studying the Earth by focusing on these mechanisms was novel. Lovelock and Margulis started a lively conversation, which is always a good thing in science. Their radical idea hasn't stood up to scrutiny, but we're better off for it.
One big problem with the Gaia hypothesis is that it isn't well posed. Science has no way to test whether the atmosphere is "a contrivance specifically constituted for a set of purposes." That is, it isn't truly a hypothesis, just a provocative statement. I would say that an argument based on purpose, in fact, is not scientific but philosophical. Lovelock and Margulis weren't misspeaking, either; they constantly repeated the idea that living things exert energy specifically to change the global environment for the better. This line of thinking, once common under the scientific worldview of the 1700s, has long since been abandoned. Only creationists continue to hold that Earth is a purpose-driven planet.
But a larger problem with the Gaia hypothesis is posed by geology: the actual history of planet Earth isn't consistent with it. The Archean Earth was perfectly habitablefor anaerobic microbes. It worked as long as oxygen, the poisonous waste product of photosynthetic algae, could be removed by oxidation of minerals. After that the world was fouled with atmospheric oxygen, forcing life to adapt. The Earth has repeatedly plunged into ice ages, causing mass extinctions, as carbon dioxide was drawn down. Mass extinctions occurred in Ordovician ice ages after plants first colonized the land surface. Oceanic stagnations, megavolcanic episodes and other disruptions caused other mass extinctions. More ice ages occurred in the late Cenozoic, and truly intelligent life is causing a mass extinction today. In the actual history of Earth, life has lurched from crisis to crisis. I would say that life's strongest feature is its ability to adapt to change, not prevent change.
Lovelock did discover something profound: life doesn't just live on this planet, it interacts with it. Earth and life coevolve. His insight was limited to the atmosphere, but the atmosphere in turn affects the ocean and climate. These in turn affect erosion and then vertical tectonics. These in turn interact with plate tectonics, which brings long-term change to the Earth's mantle and possibly deeper. Each of these effects feeds back to the others.
Saying that Gaia is a physiological system, while a nice metaphor, was a nonstarter as a hypothesis. But the conversation that Lovelock and his colleagues started has evolved into a burgeoning field called Earth system science. It's the study of the Earth as a conglomeration of chemical, physical, biological and geological cycles that influence each other in deep and unexpected ways. And life is a physio-bio-geochemical actor, an integral and influential part of the Earth system. It scrambles toward favorable conditions and modulates, to a limited extent, the fundamental cascade of energy from Sun to Earth that gives it existence. To use a different metaphor, life is a group of monkeys on a raft in rough water, always seeking the highest place and thereby counteracting the raft's gyrations.
The Gaia hypothesis has been around for some 40 years, but I've never seen it named in the stream of science research that I monitorjournal articles, review papers and conference proceedings. I do, however, see its effect continually expanding.