| This dissertation utilizes life history theory to describe traits that are derived in humans through comparisons with other primate species. Modern human life histories are unique in that they are slower, exhibiting distinctly long postmenopausal life spans and later ages at sexual maturity as a result of a reduction in adult mortality since the evolutionary split the last Pan-Homo ancestor. Faster reproduction with shorter than expected interbirth intervals and earlier weaning ages are likely the result of cooperative breeding featuring postmenopausal grandmothers. Life history traits are distinguished from life history related variables (LHRVs) which are used to makes inferences about life history variables in extinct taxa. Body mass LHRV is a strong predictive life history proxy, but brain size and dental development are only weakly associated and inferences using them should be made with caution. Age at first birth is a central variable in demographic life history models as it identifies the beginning of fertility. For most mammals, age at first birth is closely aligned with the timing of physiological maturity. Humans live in varying ecologies that influence maturation rates and have marriage institutions that can constrain sexual access to fecund females. With few exceptions, the floor of the range of human age at first birth is remarkably consistent at about 17--18 years. Women who experience their first births before this age suffer maternal and infant costs. Heterogeneity, the inherent variation in individual quality, may have an important impact on the timing of life history events. Individuals of lower quality in severe conditions are prone to culling, leaving a subset of robust individuals who thrive in measurable ways. A test of this heterogeneity hypothesis is conducted using a subset of historic vital records from the Utah Population Database. Results show that mothers of twins have a more robust phenotype with lower postmenopausal mortality, shorter average interbirth intervals, later ages at last birth, and higher lifetime fertility than their singleton-only bearing counterparts. Thus, bearing twins may be a useful index of maternal heterogeneity. |
