Excerpt From Chapter 2 Air has shaped life's evolutionary path in countless ways. Long ago birds exchanged forelimbs for wings and mastered a new realm. Many insects seem almost to be made of air-a bloom of mayflies hatching on a stream, butterflies drifting on invisible air currents, a buzzing, dancing cloud of microscopic gnats. Countless life-forms exploit the air for the crucial ceremonies of their existence-broadcasting sounds, scents and molecules such pheromones to attract mates, warn of danger or locate their young. Plants fling their seeds into the wind and attract pollinators by perfuming the air. But most of all, the need for air has minutely shaped the physiology of all aerobic creatures. If you needed reminding, simply try to stop breathing. You will quickly find that you have no choice in the matter. Within a few seconds, your body will demand air; within a minute, blood vessels in your head will bulge, your heart will pound, and your chest will heave with silent screams for air. We are more than just air breathers; we are creatures made for and by the substance we need every minute of our lives. And just as air has shaped and sustained living beings, so living beings created and still sustain the air.