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the latter too cold. The other is just right, and as a result has evolved into something unique within the known universe: It has come alive. As Craig Venter and his team of synthetic biologists have shown, there is nothing chemically special about life: The same elements that make up our living biosphere exist in abundance on countless other planets, our nearest neighbors included. But on Earth, these common elements—carbon, hydrogen, nitrogen, oxygen, and many more—have arranged themselves into uncommon patterns. In the right conditions they can move, grow, eat, and reproduce. Through natural selection, they are constantly changing, and all are involved in a delicate dance of physics, chemistry, and biology that somehow keeps Earth in its Goldilocks state, allowing life in general to survive and flourish, just as it has done for billions of years.

Why the Earth has become—and has remained—a habitable planet is one of the most extraordinary stories in science. While Venus fried and Mars froze, Earth somehow survived enormous swings in temperature, rebounding back into balance whatever the initial cause of the perturbation. Venus suffered a runaway greenhouse effect: Its oceans boiled away and most of its carbon ended up in the planet’s atmosphere as a suffocatingly heavy blanket of carbon dioxide. Mars, on the other hand, took a different trajectory. It began life warm and wet, with abundant liquid water. Yet something went wrong: Its carbon dioxide ended up trapped forever in carbonate rocks, condemning the planet to an icy future from which there could be no return.1 The water channels and alluvial fans that cover the planet’s surface are now freeze-dried and barren, and will remain so until the end of time.

Part of the Earth’s good fortune obviously lies in its location: It is the right distance from the sun to remain temperate and equable. But the distribution of Earthly chemicals is equally critical: Our greenhouse effect is strong enough to raise the planet’s temperature by more than 30 degrees from what it would otherwise be, from -18°C to about 15°C today on average—perfect for abundant life—while keeping enough carbon locked up underground to avoid a Venusian-style runaway greenhouse. Ideologically motivated climate-change deniers may rant and obfuscate, but geology (not to mention physics) leaves no room for doubt: Greenhouse gases, principally carbon dioxide (with water vapor as a reinforcing feedback), are unquestionably a planet’s main thermostat, determining the energy balance of the whole planetary system.

This astounding four-billion-year track record of self-regulating success makes the Earth unique certainly in the solar system and possibly the entire universe. The only plausible explanation is that self-regulation is somehow an emergent property of the system; negative feedbacks overwhelm positive ones and tend to push the Earth toward stability and balance. This concept is a central plank of systems theory, and seems to apply universally to successful complex systems from the internet to ant colonies. These systems are characterised by near-infinite complexity: All their nodes of interconnectedness cannot possibly be identified, quantified, or centrally planned, yet their product as a whole tends toward balance and self-correction. The Earth that encompasses them is the most complex and bewilderingly successful system of the lot.

One of the pioneers in understanding the critical regulatory role of life within the Earth system was the brilliant scientist and inventor James Lovelock. Lovelock’s original Gaia theory—that living organisms somehow contrive to maintain the Earth in the right conditions for life—was a stunning insight. But his idea of the Earth as being alive, perhaps as a kind of superorganism, only holds good as a metaphor. Self-regulation comes about not for the benefit of any component of the system—living or nonliving—but by dint of the overall system’s long-term survival and innate adaptability.

An important characteristic of the Earth system is that its main elements move around