Hormones do not fossilize. Yet, arguably, they are as important to understanding the evolution of Homo sapiens and other primates as any fossil specimen. The role of hormones in understanding human life history evolution emerges from how genes translate into phenotypes with considerable input from environmental cues. Most hormones are evolutionarily quite conservative, with very similar if not identical chemical structures between species. Many hormones that flow through the veins of humans are identical to those that flow through the most exotic vertebrate. Other hormones and receptors, however, can differ in subtle but important ways between species and even individuals. Hormonal variation, as reflected by circulating levels as well as chemical structure, are of central importance to the evolution of human life histories, both from a macro- and microevolutionary perspective.
The evolutionary significance of hormones is clearly evident in the multitude of functions that are served, including growth, reproduction, metabolism, and senescence, all of which are central to the evolution of human life histories. Hormones are inextricably involved in the optimal allocation of time and energy. Insulin, leptin, and cortisol, for example, initiate and manage the flow and assessment of energetic assets such as glucose and fat. Indeed, hormones are involved in life history trade-offs that influence many aspects of human health (Bribiescas and Ellison,2008). Testosterone, estradiol, and oxytocin affect behavioral patterns that result in differences in how individuals allocate their time, such as in the trade-off between mate seeking and parenting.