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technological life really hasn’t increased that much,” Drake replied. “That suggests to me there are probably significant barriers to the development of widespread, powerful technology. To surpass them, you might need a planet quite a lot like Earth. That may sound discouraging, until you realize just how many stars there are. Their sheer number suggests the equivalent of Earth and its life has probably happened many times before and will occur many, many times again. They’re out there.”
    He chuckled, coughed, and creakily unfolded himself from the couch, clearly weary of sitting. We went outside to breathe fresh air.
    Afternoon sunlight warmed our faces, and a cool breeze sighed through the towering redwoods to tousle Drake’s silver hair. The air smelled of green, growing things. Drake pointed out the Moon’s thin waxing crescent, faintly visible high in the cloudless sky. It was adjacent to the passing silver needle of a high-flying passenger jet. As we walked down into the yard, I gingerly stepped over the pale blue remnants of a robin’s egg cracked open on the front steps, fallen from a nest in an overhanging tree. The tide was rolling in far below us, down past the forested hills and beachfront suburbs, and surfers rode big waves toward the shore of Monterey Bay.
    The scene from Drake’s front door encapsulated many of the essential facts of life on Earth. Fueled by raw sunlight, plants broke the chemical bonds of water and carbon dioxide, spinning together sugars and other hydrocarbons from the hydrogen and carbon and venting oxygen into the air. Sunlight scattering off all those airborne oxygen molecules made the sky appear blue. Animals breathed the oxygen and nourished their bodies with the hydrocarbons, utterly dependent upon these photosynthetic gifts from the plants. In death, plants and animals alike gave their Sun-spun carbon back to the Earth, where tremendous heat, pressure, and time transformed it into coal, oil, and natural gas. Mechanically extracted from the planet’s crust and burned in engines, generators, and furnaces, that fossilized energy powered most ofhumanity’s technological dominion over the globe. Built up and locked away for hundreds of millions of years, the carbon stockpile was gushing back into the planet’s atmosphere in a geological instant.
    Our experience at Monterey Bay was a product of our planet’s physical characteristics—and the unlikely events that led to them. Earth’s abnormally large Moon, which stabilizes our planet’s axial tilt and bestows it with tides, was born when a Mars-size body collided with the proto-Earth early in our solar system’s history. Another impactor, a six-mile-wide asteroid, struck the Earth 66 million years ago and sparked a global mass extinction, ending the age of dinosaurs. Humanity’s small mammalian ancestors began their slow progress toward biospheric dominance, and the saurians that didn’t die out gradually gave rise to birds. Billions of years before the dinosaurs, the life-giving liquid we recognize as Earth’s ocean was mostly delivered by impactors, too, in a shower of water-rich asteroids and comets from the outer solar system. Earth’s aquatic abundance, it is thought, lubricates the planet’s fractured crustal plates and allows them to drift and slide in the geological process we call plate tectonics, a climate-regulating mechanism unique to our world out of all those in the solar system.
    Turning away from the bay, Drake walked over to the center of his driveway, where the weathered stump of a giant redwood rose like a long-extinct volcano. He stooped and placed his hands upon the ancient wood. Years ago, he said, he had spread a thin layer of chalk on a section of the stump’s surface, allowing the growth rings to be easily seen, and set his young children to the task of counting them as an informal science project. They counted more than 2,000, one for each year of the tree’s life, which apparently began