"Humans are not the only organisms to have affected the live-ability [sic] of Earth. Microbes and plants changed the chemistry of the atmosphere long before we leapt on to [sic] the stage. Bacteria initiated a momentous change 2.3 billion years ago when they began flooding the air with a noxious gas called oxygen. Microorganisms that had been happily ‘breathing’ iron, sulphur and nitrogen for the first million millennia of biology were decimated by this highly reactive, DNA-damaging molecule. As oxygen levels rose, the metal-breathers and their kin retreated to marine muds and other oxygen-free quarters. New life forms evolved to take advantage of the peculiar conditions and found a way to use oxygen to rip more energy from their food, which is why we breathe deeply today."