Made famous by Charles Darwin, the theory of evolution holds that a one-celled organism spontaneously emerged from steaming, nutrient-rich seas about 3.5 billion years ago, and that increasingly varied and complex organisms developed through such processes as genetic mutation. Over time, an unforgiving environment killed the weaker members of each species, and sometimes the entire species, in favor of better-suited ones. Scientists say evidence supporting the theory of evolution includes fossil records, the existence of similar structures in different animals, and the fact that all living things share similar biochemistry. They say the theory of evolution is not only scientifically valid—it is the unifying theory of biology.