Abstract 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. Introduction When humans and their nearest simian relatives are examined, two statements seem without serious challenge. First, humans and the great apes-the chimpanzees and the gorilla-have a recent common ancestry,3 and second, humans and these great apes behave differently. This article attempts to complement the various extant theories that link the first and the second statements. That is, as humans and the great apes diverged from each other, they each developed their own behavioral repertoires, which resulted in descendants-over the millennia-to the present time. This article attempts to discuss the evolutionary effect of sexually transmitted diseases (STDs) upon (a) pair-bonding, (b) adult male provisioning, (c) the man-to-child affiliative bond, and (d) minimal sexual dimorphism. Homo sapiens has two unique features which would be unexpected from a large, terrestrial primate: (i) sexual dimorphism is minimal and (ii) men systematically and actively provision particular women and their own children. In addition, the species harbors a wide array of sexually transmitted diseases (STDs) which can severely and adversely affect reproductive health of individuals and, by extension, of the group. This article attempts to lend insight into how these three features may have interacted with each other in our evolutionary history. The putative linkage of STDs with our phylogeny will be examined first. For a discussion and simulations/ models of the relationship of STDs in humans and bio-cultural evolution, see Immerman (1986a, 1986b, 1988, 1992a, 1992b) and Immerman and Mackey (1997, 1999b, 1999c). STDs and Human Evolution STDs represent unique phenomena in that the best prevention from individual infection-abstinence-is a formula for societal extinction. If everyone in a tribe or group were to engage in abstinence, then the tribe or group lasts only one additional generation. Even though all individuals who are born are guaranteed to have ancestors and an individual mortality, they are not equally guaranteed descendants. Hence, there is no guarantee for the perpetuity of the commonweal. If there is universal abstinence within a group, then there is no spread of infection; however, there would also be no children. If there are no children, then there is no survival of the tribe or community over generations. When the last childless person dies, the commonweal is extinct; e.g., the Shakers.4 Accordingly, sexual intercourse among its members is mandated for the survival of an intact, coherent social group. For all extant groups, there must have been continuous sexual interaction across the millennia. With this highly commonsensical mandate comes the potential of STDs. Problems with STDs Given that all organisms are subjected to diseases and parasites, why would STDs generate any unique problems? The source of the unique sequelae is not only the death of the host, but is also the dual threat (i) to the fertility of the host and (ii) to the viability of infants that are born. …