Michael Crichton
"Think about oxygen. Necessary for life now, but oxygen is actually a metabolic poison, a corrosive gas, like fluorine. When oxygen was first produced as a waste product by certain plant cells some three billion years ago, it created a crisis for all other life on earth. Those plants were polluting the environment, exhaling a lethal gas. Earth eventually had an atmosphere incompatible with life. Nevertheless, life on earth took care of itself." [Michael Crichton]