In the order Primates the responses to sucrose, alitame and aspartame were ascertained. All primates tested to date like sucrose and prefer this sweet substance to tap water. The artificial dipeptide aspartame was found to be not sweet in Prosimii and Platyrrhini (New World monkeys). Only the Cercopithecoidea (Old World monkeys) and Hominoidea (apes and humans) show the same response to aspartame and to sucrose. In contrast, all primates tested so far prefer alitame, another artificial dipeptide sweetener, which is structurally closely related to aspartame. This phylogenetic difference is consistent with the existence in catarrhine primates of a sweetness receptor containing two differently located hydrophobic recognition sites, one for the hydrophobic binding site of alitame, the other for the hydrophobic binding site of aspartame. On the basis of these results, it is suggested that the alitame-related hydrophobic recognition site, which is found in the sweetness receptor of all primates, could be a requisite for the interaction of the receptor with sucrose, while the aspartame-related hydrophobic recognition site, which is found exclusively in the sweetness receptor of Old World simians, could have been a crucial factor in the improvement in detection or selection of sucrose in foods, so favouring the mental development of these simians and maybe the emergence of humans.