As the ancestors of both the great apes and humans began to separate into two lineages, several distinctions emerged and solidified for the separate genera. It is suggested here that the sequelae to sexually transmitted diseases (STDs) and subsequent behavioral tendencies to avoid sexually transmitted diseases played an important role in forging the unique character of the Australopithecine/Homo line. In particular, the advantage of pair-bonding versus promiscuity in avoiding STDs would facilitate the crystallization of both the nascent nuclear family and the man-to-child affiliative bond. In addition, the unexpectedly small sexual dimorphism of Homo is suggested to be a partial consequence of replacing (physical) dominance acquisition as a reproductive strategy with the ability and motivation to form an on-going pair-bond. The capacity of males to send and the capacity of females to receive communication signals of male reliance and competence are suggested to be a key dynamic in the separation of the hominid line from the pongids.