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Thirty years ago, biologists asked NASA to shoot a couple of unmanned probes towards the two likeliest candidates for extraterrestrial life, Mars and Venus, and poke a dipstick into their soil to check for vital signs.

The life-meter that NASA came up with was a complicated, delicate (and expensive) contraption that would, upon landing, be sprinkled with a planet's soil and check for evidence of bacterial life. One of the consultants hired by NASA was a soft-spoken British biochemist, James Lovelock, who found that he had a better way of checking for life on planets, a method that did not require a multimillion-dollar gadget, or even a rocket at all.

Lovelock was very rare breed in modern science. He practiced science as a maverick, working out of a stone barn among the rural hedgerows in Cornwall, England. He maintained a spotless scientific reputation, yet he had no formal institutional affiliation, a rarity in the heavily funded world of science. His stark independence both nurtured and demanded free thinking. In the early 1960s Lovelock came up with a radical proposal that irked the rest of the folks on the NASA probe team. They really wanted to land a meter on a another planet. He said they didn't have to bother.

Lovelock told them he could determine whether there was life on a planet by looking through a telescope. He could measure the spectrum of a planet's atmosphere, and thereby determine its composition. The makeup of the bubble of gases surrounding a planet would yield the secret of whether life inhabited the sphere. You therefore didn't need to hurl an expensive canister across the solar system to find out. He already knew the answer.

In 1967, Lovelock wrote two papers predicting that Mars would be lifeless based on his interpretation of its atmosphere. The NASA orbiters that circled Mars later in the decade, and the spectacular Mars soft landings the decade following made it clear to everyone that Mars was indeed as dead as Lovelock had forecasted. Equivalent probes to Venus brought back the same bad news: the solar system was barren outside of Earth.

How did Lovelock know?

Chemistry and coevolution. When the compounds in the Martian atmosphere and soil were energized by the sun's rays, and heated by the planetary core, and then contained by the Martian gravity, they settled into a dynamic equilibrium after millions of years. The ordinary laws of chemistry permit a scientist to make calculations of their reactions as if the planet were a large flask of matter. When a chemist derives the approximate formulas for Mars, Venus, and the other planets, the equations roughly balance: energy, compounds in; energy, compounds out. The measurements from the telescopes, and later the probes, matched the results predicted by the equations.

Not so the Earth. The mixture of gases in the atmosphere of the Earth are way out of whack. And they are out of whack, Lovelock was to find out, because of the curious accumulative effects of coevolution.

Oxygen in particular, at 21 percent, makes the Earth's atmosphere unstable. Oxygen is a highly reactive gas, combining with many elements in a fierce explosive union we call fire or burning. Thermodynamically, the high oxygen content of Earth's atmosphere should fall quickly as the gas oxidizes surface solids. Other reactive trace gases such as nitrous oxide and methyl iodide also remain at elevated and aberrant levels. Both oxygen and methane coexist, yet they are profoundly incompatible, or rather too compatible since they should burn each other up. Carbon dioxide is inexplicably a mere trace gas when it should be the bulk of the air, as it is on other planets. In addition to its atmosphere, the temperature and alkalinity of the Earth's surface also exhibits a queer level. The entire surface of the Earth seems to be a vast unstable chemical anomaly.

It seemed to Lovelock as if an invisible power, an invisible hand, pushed the interacting chemical reactions into a raised state that should at any minute swing back to a balanced rest. The chemistry of Mars and Venus was as balanced as the periodic table, and as dead. The chemistry of the Earth was out of kilter, wholly unbalanced by the periodic table, and alive. From this, Lovelock concluded that any planet that has life would reveal a chemistry that held odd imbalances. A life-friendly atmosphere might not be oxygen-rich, but it should buck textbook equilibria.

That invisible hand was coevolutionary life.

Life in coevolution, which has the remarkable knack of generating stable instability, moved the chemical circuitry of the Earth's atmosphere into what Lovelock calls a "persistent state of disequilibrium." At any moment, the atmosphere should fall, but for millions of years it doesn't. Since high oxygen levels are needed for most microbial life, and since microbial fossils are billions of years old, this odd state of discordant harmony has been quite persistent and stable.

The Earth's atmosphere seeks a steady oxygen level much as a thermostat hones in on a steady temperature. The uniform 20 percent oxygen level it has found turns out to be "fortuitous" as one scientist put it. Lower oxygen would be anemic, while greater oxygen would be too flammable. George R. Williams at the University of Toronto writes: "An O2 content of about 20 percent seems to ensure a balance between almost complete ventilation of the oceans without incurring greater risks of toxicity or increased combustibility of organic material." But where are the sensors and the thermostatic control mechanisms? For that matter, where is the furnace?

Dead planets find equilibrium by geological circuits. Gases, such as carbon dioxide, dissolve in liquids and can precipitate out as solids. Only so much gas will dissolve before it reaches a natural saturation. Solids can release gases back into the atmosphere when heated and pressed by volcanic activity. Sedimentation, weathering, uplift -- all the grand geological forces -- also act as strong chemical agents, breaking and making the bonds of materials. Thermodynamic entropy draws all chemical reactions down to their minimal energy level. The furnace metaphor breaks down. Equilibrium on a dead planet is less like a thermostat and more like the uniform level of water in a bowl; it simply levels out when it can't get any lower.

But the Earth has the self of a thermostat. A spontaneous circuit, provided by the coevolutionary tangle of life, which guides the chemicals of the planet toward some elevated potential. Presumably if all life on Earth were extinguished, the Earth's atmosphere would fall back to a persistent equilibrium, and become as boringly predictable as Mars and Venus. But as long as the distributed hand of life dominates, it will keep the chemicals of Earth off key.

Yet the off-balance is itself balanced. The persistent disequilibrium that coevolutionary life generates, and that Lovelock seeks as an acid test for its presence, is stable in its own way. As far as we can tell Earth's atmospheric oxygen has remained at about 20 percent for hundreds of millions of years. The atmosphere acts not merely as an acrobat on a tightrope pitched far from the vertical, but as an acrobat teetering between tilting and falling, and poised there for millions of years. She never falls, but never gets out of falling. It's a state of permanent almost-fell.

Lovelock recognized that persistent almost-fell is a hallmark of life. Recently complexity investigators have recognized that persistent almost-fell is a hallmark of any vivisystem: an economy, a natural ecosystem, a deep computer simulation, an immune system, or an evolutionary system. All share that paradoxical quality of working best when they remain poised in an Escher-like state of forever descending without ever being lowered. They remain poised in the act of collapsing.

David Layzer, writing in his semiscientific book Cosmogenesis, argues that "the central property of life is not reproductive invariance, but reproductive instability." The key of life is its ability to reproduce slightly out of kilter rather than with exactitude. This almost-falling into chaos keeps life proliferating.

A little noticed but central character of such vivisystems is that this paradoxical essence is contagious. Vivisystems spread their poised instability into whatever they touch, and they reach for everything. On Earth, life elbows its way into solid, liquid, gas. No rocks, to our knowledge, are untouched by life in former times. Tiny oceanic microorganisms solidify carbon and oxygen gases dissolved in sea water to produce a salt which settles on the sea floor. The deposits eventually become pressed under sedimentary weight into stone. Tiny plant organisms transport carbon from the air into soil and lower into the sea bottom, to be submerged and fossilized into oil. Life generates methane, ammonia, oxygen, hydrogen, carbon dioxide, and many other gases. Iron -- and metal-concentrating bacteria create metallic ores. (Iron, the very emblem of nonlife, born of life!) Upon close inspection, geologists have concluded that all rocks residing on the Earth's surface (except perhaps volcanic lava) are recycled sediments, and therefore all rocks are biogenic in nature, that is, in some way affected by life. The relentless push and pull of coevolutionary life eventually brings into its game the abiotic stuff of the universe. It makes even the rocks part of its dancing mirror.