What led to darwin to believe that organisms originate from a common ancestor

Darwin believed in evolution. He went to the Galápagos islands to study. The most studied animals on the Galápagos are finches. When Darwin first observed finches on the islands, he did not even realize they were all finches. But when he studied them further, he realized they were related to each other. Each island had its own distinct species of finch. Charles Darwin developed what they say "the theory of evolution by natural selection. Darwin's observations on the Galápagos Islands suggested that animals are well-suited for their specific environments.