This paper theorizes on the constructs of jealousy and infant-caregiver attachment by proposing a synthesis that bridges parent-offspring conflict theory (POCT; Trivers, 1974) with the evolutionary-ethological theory of attachment (Bowlby, 1969). Following Bowlby, we recognize attachment as an adaptation to threat to infant survival in ancestral settings. However, departing from his focus on environmental danger as the source of threat, we draw on Trivers’ understanding of siblings as competitors for parents’ scarce and finite resources, and propose that attachment was compelled by threat posed by the birth of a sibling. In support of this synthesis, we present evidence that illuminates infants’ need for exclusivity in the infant-maternal relationship, and elaborate on the origin of that need as a function of the benefits of exclusivity to infant survival. We also present evidence that infants’ defense against usurpation, jealousy protest, resembles attachment behavior, specifically separation protest, in terms of the specificity of the context in which it is presented, its direction and affective tone, the timing of its onset, and pattern of individual differences. The fact that both jealousy protest and separation protest occur at approximately 9 months of age, which is the juncture when a sibling’s birth is possible, suggests that both forms of protest were compelled by inevitable consequences of a sibling’s birth, namely, sibling competition for parental resources and mother-initiated separation. We argue that as a recurrent and universal event that presaged threat of both usurpation and separation, the birth of a sibling represents the ultimate foundation of attachment. (PsycINFO Database Record (c) 2018 APA, all rights reserved)