Abstract Each amino acid in a protein is considered to be an individual, mutable characteristic of the species from which the protein is extracted. For a branching tree representing the evolutionary history of the known sequences in different species, our computer programs use majority logic and parsimony of mutations to determine the most likely ancestral amino acid for each position of the protein at each node of the tree. The number of mutations necessary between the ancestral and present species is summed for each branch and the entire tree. The programs then move branches to make many different configurations, from which we select the one with the minimum number of mutations as the most likely evolutionary history. We used this method to elucidate primate phylogeny from sequences of fibrinopeptides, carbonic anhydrase, and the hemoglobin beta, delta and alpha chains. All available sequences indicate that the early Pongidae had diverged into two lines before the divergence of an ancestor for the human line alone. We have constructed some probable ancestral sequences at major points during primate evolution and have developed tentative trees showing the order of divergences and evolutionary distances among primate groups. Further questions on primate evolution could be answered in the future by the detemination of the appropriate sequences.