Evidence from proteins - Proteins - E.g., things like blood proteins (the things that give us our A, B, O blood typing and the Rh factor (the plus/minus thing) which incidentally stands for 'rhesus monkey'); the exact structure of the insulin molecule; and my favorite, the proteins responsible for color vision. The specific proteins found in human color vision are exactly the same as those found in Old World primates (the great apes and the monkeys found in Africa and Asia). These proteins are absent in New World primates (the Central and South American monkeys), and from all other mammals. In fact among the New World primates, only the howler monkey has color vision ... but these use slightly *different* proteins, coded on different locations and chromosomes, than humans and the OW primates. This is yet more evidence of a closer link between humans and the OW primates.