National Geographic: Ruth DeFries, chair of the department of ecology, evolution, and environmental biology at Columbia University, in New York, and author of The Big Ratchet: How Humanity Thrives in the Face of Natural Crisis, has spent much of her life looking down at Earth from a great height, using satellite images to track human development. But to understand how we went from being hunter-gatherers to a species that so completely dominates the planet, she had to go much further back in time.
Speaking from her...