Determination of sequence alterations unique to one ancestral line. (a) Sequences from the same hypothetical gene in humans and chimpanzees are compared. The sequence of this gene in their last common ancestor is unknown. (b) The orangutan genome is used as an outgroup. The sequence of the orangutan gene is found to be identical to that of the chimpanzee gene. This means that the mutation causing the difference between humans and chimpanzees almost certainly occurred in the line leading to modern humans, and the common ancestor of humans and chimpanzees (and orangutans) had the variant now found in chimpanzees.