Charles Darwin left home at age 22 aboard the ship The HMS Beagle. When he got back, he had made a discovery that would change the path of modern science. He just didn’t know it yet.
On his journey sailing around the world, Darwin was amazed by the natural diversity he observed. The Galapagos Islands, in particular, had a fantastic range of species. Even the little finches seemed to be different on almost every island. Years later, Darwin wrote about the finches: “Seeing this diversity in one small group of birds, one might think that from an original group of birds on these islands, one species had been taken and modified for different ends.”
Years later, this observation led Darwin to publish his theory of natural selection. The theory, now accepted by 99.9% of scientists, states that when species have variety, they reproduce at different rates. For example, one finch might be born with a thinner beak, allowing her to eat worms more easily. Because of this advantage, she would be able to obtain more food, she would be healthier, and she would be better able to care for her young. Over time, the genetics for that thin beak could be passed down and spread throughout the entire finch population.
Of course, this would not happen overnight. Evolution like that could take thousands of years. But any time there is (a) a competing population, (b) genetic variation, and (c) inherited traits, evolution will indeed happen. And over millions of years, you could turn a fish into a rodent, a rodent into an ape, and an ape into a human!