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But this still doesn’t answer the puzzle: Why us? And why were buried stores of carbon the “nutrients” that allowed our species to proliferate so explosively? A satisfactory response requires a brief digression into the evolutionary origins of this remarkable hominid, for it is our past that holds the key to our present and future. This is the story of a species whose biological characteristics combined with an accident of fate to have world-shattering consequences. And it is a story that might shed some light on the central question of this book—whether we are rebel organisms destined to destroy the biosphere, or divine apes sent to manage it intelligently and so save it from ourselves.

Perhaps the environmentalist and futurist Stewart Brand put it best when he wrote these words: “We are as gods and have to get good at it.”5 Amen to that.

THE DESCENT OF MAN

Listening to some environmentalists talk, it is easy to get the feeling that humanity is somehow unnatural, a malign external force acting on the natural biosphere from the outside. They have it wrong. We are as natural as coral reefs or termites; our inherited physiology is entirely the product of selective pressures operating over millions of years within living systems. Our inner ear, for example, was once the jawbone of a reptilian ancestor. Babies in the womb begin life with tails, expressing in the earliest stages of life genes that illustrate our long evolutionary history. Our key biological characteristics—including those that have allowed us to emerge as “sapient” beings—exist only because they conferred on our ancestors some selective advantage as they ate, fought, played, and reproduced over millions of years within the natural biosphere.

The actual origin of life—how animate organisms assembled themselves out of inanimate chemicals without a Dr. Venter to supervise affairs—remains a mystery. Perhaps the first self-replicating amino acids were formed in some primordial soup by a charge of lightning or a volcanic eruption. Or maybe, given the right environment and ingredients, life can spontaneously appear. Some suggest that extraterrestrial microbes may have hitched a lift onto the early Earth from passing meteors or comets. Either way, the first microbes appeared about 3.7 billion years ago, evolving into “eukaryotic” cells—with a proper nucleus, cell walls, and the capacity to metabolize energy—a billion and a half years later. These cells were probably made up of a symbiotic union of several bacteria, which is why mitochondria in our body cells today still have their own DNA. (Symbiosis, by the way, is quite as much part of the story of evolution as red-in-tooth-and-claw competition.)

Some of these early microbes, the cyanobacteria, learned to use photons from the sun to split water and carbon dioxide in photosynthesis. They are probably Earth’s most successful organisms, for cyanobacteria are still prolific today. As eukaryotic cells learned to combine to form multicellular organisms, the stage was set for a major proliferation of life—though still only in the oceans—in an event dubbed the “Cambrian explosion” by paleontologists. During the Cambrian, 540 million years ago, recognizable ancestors of many of today’s animal groups appeared. These include arthropods (insects, spiders, and crustaceans), mollusks (snails, oysters, octopus), and even early vertebrates—the first fish. An evolutionary arms race kicked off, as predators evolved ways to catch, grip, and swallow, while prey developed speed or armor to reduce their chances of being eaten.

Of all the technical novelties evolution called into existence, from scales to jaws, perhaps the most interesting is the development of sight. The eye may have been the innovation sparking this intense burst of Cambrian competition, for both predators and prey would have had an equally powerful reason to evolve vision. The fossil record demonstrates that sight evolved independently in different groups