Also referred to as the Darwinian theory (as it was developed by Charles Darwin), it basically states that every single organism came from one common ancestor, and everything is related, from the birds to the bees, and the cats and the dogs. In a nutshell, as one common ancestor produced offspring, and that offspring produced more, every generation underwent some mutation, and as the beneficial mutations or properties were passed on (called "natural selection"), time went on and on until an entirely different organism was created.