Probably within just the last ten million years, a plant had a new idea, and instead of spreading its leaf out, it shaped it into a spine, such as those we find today on the cholla cactus. It was this new idea that allowed a new kind of plant to grow preposterously large and live long in a dry place where it was also the only green thing around to eat for miles—an absurdly inconceivable success. One new idea allowed the plant to see a new world and draw sweetness out of a whole new sky.
