Evolution simply means "change" and there are various theories that try to explain how change has occurred on Earth regarding life. The Theory of Natural Selection is an important work by Charles Darwin. In this theory, he points out that living things overpopulate. This means they reproduce more individuals than can possibly survive. Next is the idea of variations. No two individuals are alike and these differences can either help or harm an organism in its struggle for survival. Therefore, in life, there is competition. Those that do survive and reproduce are deemed "fit" and hopefully pass along these adaptations to their offspring. Over the centuries, as more of these variations are inherited, eventually, a new species might emerge that is different from the ancestral population. More modern theories rely on more evidence than Darwin did--he mainly looked at the fossil record and structural evidence. Today, there is comparative embryology, comparative cytology, DNA evidence as well as others evidences to show that life indeed has evolved on Earth since the beginning of time. And, based on the evidence, most scientists are agreed that evolution did indeed occur!