The theory that states that all organisms have a common ancestor. Not clear at all about how the first organisms appeared. If they appeared from nothing, then the theory is contradictory. This is becasue the theory states that all organisms have a common ancestor which has to mean that the common ancestor has that same common anscestor which makes absolutely no sense. It is a theory and has not been proven. It envolves ideas such as natural selection, artificial selection, certain types of predator to prey relations, etc. It is said that the first tetrapod (four legged creature) resembled a lizard with gills. This tetrapod (supposedly) evolved from single cell organizms over time and apparently is a common ancestor shared by all tetrapods today. Personally, I believe in Christianity.
You know what, if you trace your family tree far back enough, your great to the three billionth power grandfather is a lizard with gills and his great to the three billionth power grandfather was a microscopic organism. Shoot, in three bilion more years, us people will probably be bird like mammals with four legs, wings, a beak with built in knife, spoon and fork, and an incredible mind capacity containig over three trillion languages that range from Catterpillar Communication to French. Evolution in a nutshell.