Charles Darwin was a controversial Englishman, naturalist and geologist who lived between 1809 and 1882. He came up with the idea of evolution. That is to say that humans have evolved over many many years from another species of animal – basically monkeys.
At the risk of over-simplifying all his works and theories here, suffice it to say that he believed that one species does change into another. That is, the species changes slightly every generation in order to survive, and over many generations it becomes, (or looks like), a new species.