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One of the first to articulate the transcendent view that life directly shaped the physicality of this planet was the Russian geologist Vladimir Vernadsky, writing in 1926. Vernadsky tallied up the billions of organisms on Earth and considered their collective impact upon the material resources of the planet. He called this grand system of resources the "biosphere," (although Eduard Suess had coined the term a few years earlier) and set out to measure it quantitatively in his book The Biosphere, a volume only recently translated into English.

In articulating life as a chameleon on a rocky mirror, Vernadsky committed heresy on two counts. He enraged biologists by considering the biosphere of living creatures as a large chemical factory. Plants and animals were mere temporary chemical storage units for the massive flow of minerals around the world. "Living matter is a specific kind of rock...an ancient and, at the same time, an eternally young rock," Vernadsky wrote. Living creatures were delicate shells to hold these minerals. "The purpose of animals," he once said of their locomotion and movement, "is to assist the wind and waves to stir the brewing biosphere."

At the same time, Vernadsky enraged geologists by considering rocks as if they were half-alive. Since the genesis of every rock was in life, their gradual interaction with living organisms meant that rocks were the part of life that moved the slowest. The mountains, the waters of the ocean, and the gases of the sky were very slow life. Naturally, geologists balked at this apparent mysticism.

The two heresies melded into a beautiful symmetry. Life as ever-renewing mineral, and minerals as slow life. They could only be opposite sides of a single coin. The two sides of this equation cannot be mathematically unraveled; they are one system: lizard-mirror, plant/insect, rock-life, and now in modern times, human/machine. The organism behaves as environment, the environment behaves as organism.

This has been a venerable idea at the edge of science for at least several hundred years. Many evolutionary biologists in the last century such as T. H. Huxley, Herbert Spencer, and Darwin, too, understood it intuitively -- that the physical environment shapes its creatures and the creatures shape their environment, and if considered in the long view, the environment is the organism and the organism is the environment. Alfred Lotka, an early theoretical biologist, wrote in 1925, "It is not so much the organism or the species that evolves, but the entire system, species plus environment. The two are inseparable." The entire system of evolving life and planet was coevolution, the dance of the chameleon on the mirror.

If life were to vanish from Earth, Vernadsky realized, not only would the planet sink back into the "chemical calm" of an equilibrium state, but the clay deposits, limestone caves, ores in mine, chalk cliffs, and the very structure of all that we consider the Earth's landscape would retreat. "Life is not an external and accidental development on the terrestrial surface. Rather, it is intimately related with the constitution of the Earth's crust," Vernadsky wrote in 1929. "Without life, the face of the Earth would become as motionless and inert as the face of the moon."

Three decades later, free-thinker James Lovelock arrived at the same conclusions based on his telescopic analysis of other planets. Lovelock observed, "In no way do organisms simply 'adapt' to a dead world determined by physics and chemistry alone. They live in a world that is the breath and bones of their ancestors and that they are now sustaining." Lovelock had more complete knowledge of early Earth than was available to Vernadsky, and a slightly better understanding of the global patterns of gases and material flows on Earth. All this led him to suggest in complete seriousness that "the air we breathe, the oceans, and the rocks are all either the direct products of living organisms or else have been greatly modified by their presence."

Such a remarkable conclusion was foreshadowed by the French natural philosopher, Jean Baptiste Lamarck, who in 1800 had even less information about planetary dynamics than Vernadsky did. As a biologist, Lamarck was equal to Darwin. He, not Darwin, was the true discoverer of evolution, but Lamarck is stuck with an undeserved reputation as a loser, in part because he relied a little too much on intuition rather than the modern notion of detailed facts. Lamarck made an intuitive guess about the biosphere and again was prescient. Since there wasn't a shred of scientific evidence to support Lamarck's claims at the time, his observations were not influential. He wrote in 1802, "Complex mineral substances of all kinds that constitute the external crust of the Earth occurring in the form of individual accumulations, ore bodies, parallel strata, etc., and forming lowlands, hills, valleys, and mountains, are exclusively products of the animals and plants that existed within these areas of the Earth's surface."

The bold claims of Lamarck, Vernadsky, and Lovelock seem ludicrous at first, but in the calculus of lateral causality make fine sense: that all we can see around us -- the snow-covered Himalayas, the deep oceans east and west, vistas of rolling hills, awesome painted desert canyons, game-filled valleys -- are all as much the product of life as the honeycomb.

Lovelock kept gazing into the mirror and finding that it was nearly bottomless. As he examined the biosphere in succeeding years, he added more complex phenomena to the list of life-made. Some examples: plankton in the oceans release a gas (DMS) which oxidizes to produce submicroscopic aerosols of sulfate salts which form nuclei for the condensation of cloud droplets. Thus perhaps even clouds and rain may be biogenic. Summer thunderstorms may be life raining on itself. Some studies hinted that a majority of nuclei in snow crystals may be decayed vegetation, bacteria, or fungi spores; and so snow may be largely life-triggered. Only very little could escape life's imprint. "It may be that the core of our planet is unchanged as a result of life; but it would be unwise to assume it," Lovelock said.

"Living matter is the most powerful geological force," Vernadsky claimed, "and it is growing with time." The more life, the greater its material force. Humans intensify life further. We harness fossil energy and breathe life into machines. Our entire manufactured infrastructure -- as an extension of our own bodies -- becomes part of a wider, global-scale life. As the carbon dioxide from our industry pours into the air and alters the global air mix, the realm of our artificial machines also becomes part of the planetary life. Jonathan Weiner writing in The Next One Hundred Years then can rightly say, "The Industrial Revolution was an astonishing geological event." If rocks are slow life, then our machines are quicker slow life.

The Earth as mother was an old and comforting notion. But the Earth as mechanical device has been a harder idea to swallow. Vernadsky came very close to Lovelock's epiphany that the Earth's biosphere exhibits a regulation beyond chemical equilibrium. Vernadsky noted that "organisms exhibit a type of self-government" and that the biosphere seemed to be self-governed, but Vernadsky didn't press further because the crucial concept of self-government as a purely mechanical process had not yet been uncovered. How could a mere machine control itself?

We now know that self-control and self-governance are not mystical vital spirits found only in life because we have built machines that contain them. Rather, control and purpose are purely logical processes that can emerge in any sufficiently complex medium, including that of iron gears and levers, or even complex chemical pathways. If a thermostat or a steam engine can own self-governance, the idea of a planet evolving such graceful feedback circuits is not so alien.

Lovelock brought an engineer's sensibilities to the analysis of Mother Earth. He was a tinkerer, inventor, patent holder, and had worked for the biggest engineering firm of all time, NASA. In 1972, Lovelock offered a hypothesis of where the planet's self-government lay. He wrote, "The entire range of living matter on Earth, from whales to viruses, from oaks to algae, could be regarded as constituting a single living entity, capable of manipulating the Earth's atmosphere to suit its overall needs and endowed with faculties and powers far beyond those of its constituent part." Lovelock called this view Gaia. Together with microbiologist Lynn Margulis, the two published the view in 1972 so that it could be critiqued on scientific terms. Lovelock says, "The Gaia theory is a bit stronger than coevolution," at least as biologists use the word.

A pair of coevolutionary creatures chasing each other in an escalating arms race can only seem to veer out of control. Likewise, a pair of cozy coevolutionary symbionts embracing each other can only seem to lead to stagnant solipsism. But Lovelock saw that if you had a vast network of coevolutionary impulses, such that no creatures could escape creating its own substrate and the substrate its own creatures, then the web of coevolution spread around until it closed a circuit of self-making and self-control. The "obligate cooperation" of Ehrlich's coevolution -- whether of mutual enemies or mutual partners -- cannot only raise an emergent cohesion out of the parts, but this cohesion can actively temper its own extremes and thereby seek its own survival. The solidarity produced by a planetary field of creatures mirrored in a coevolving environment and each other is what Lovelock means by Gaia.

Many biologists (including Paul Ehrlich) are unhappy with the idea of Gaia because Lovelock expanded the definition of life without asking their permission. He unilaterally enlarged life's scope to include a predominantly mechanical apparatus. In one easy word, a solid planet became "the largest manifestation of life" that we know. It is an odd beast: 99.9 percent rock, a lot of water, and a little air, wrapped up in the thinnest green film that would stretch around it.

But if Earth is reduced to the size of a bacteria, and inspected under high-powered optics, would it seem stranger than a virus? Gaia hovers there, a blue sphere under the stark light, inhaling energy, regulating its internal states, fending off disturbances, complexifying, and ready to transform another planet if given a chance.

While Lovelock backs off earlier assertions that Gaia is an organism, or acts as if it is one, he maintains that it really is a system that has living characteristics. It is a vivisystem. It is a system that is alive, whether or not it possesses all the attributes needed for an organism.

That Gaia is made up of many purely mechanical circuits shouldn't deter us from applying the label of life. After all, cells are mostly chemical cycles. Some ocean diatoms are mostly inert, crystallized calcium. Trees are mostly dead pulp. But they are still living organisms.

Gaia is a bounded whole. As a living system, its inert, mechanistic parts are part of its life. Lovelock: "There is no clear distinction anywhere on the Earth's surface between living and nonliving matter. There is merely a hierarchy of intensity going from the material environment of the rocks and atmosphere to the living cells." Somewhere at the boundary of Gaia, either in the rarefied airs of the stratosphere or deep in the Earth's molten core, the effects of life fade. No one can say where that line is, if there is a line.