Conclusion
Because the costs of reproduction for females are so high and human infants require extended care and provisioning, increasingly during human evolution successful reproduction (i.e. producing children that live long enough to produce children of their own) came to depend on parental investment by both the mother and the father. Females without committed, caring male partners would have been at an enormous disadvantage when it came to childbearing and child-rearing. We theorize that because of the growing importance of paternal investment, mechanisms may have evolved to terminate pregnancies under conditions in which support and provisioning by the child’s father were doubtful. One reliable means of indexing paternal commitment would have been frequent and recurrent insemination of the female by the child’s father. Subtle differences between males in semen chemistry could have been the basis for the evolution of an ensemble of pregnancy termination mechanisms triggered by impregnation as a byproduct of exposure to unfamiliar semen.