The human post-fertile lifespan in comparative evolutionary context

There persist two widely held but mutually inconsistent views on the evolution of post-fertile lifespan of human females. The first, prevalent within anthropology, sees post-fertile lifespan (PFLS) in the light of adaptive processes, focusing on the social and economic habits of humans that selected for a lengthy PFLS. This view rests on the assumption that human PFLS is distinct from that of other species, and focuses on quantifying the selective causes and consequences of that difference. The second view, prevalent within gerontology and comparative biology, emphasizes that PFLS is a phylogenetically widespread trait or that human PFLS is predictable based on life-history allometries. In this view, human PFLS is part of a broad cross-species pattern and its genesis cannot, therefore, rely on human-specific traits. Those who advocate the second view have questioned the "special pleading" for human specific explanations of PFLS, and have argued that human PFLS is quantitatively greater but not qualitatively different than PFLS in many other animals. Papers asking whether human PFLS is explained by the importance of mothers more than grandmothers, whether paternal or maternal grandparents have more of an effect on child survival, or who is providing the excess calories are associated with the first view that assumes the need to explain the existence of human PFLS on the basis of a uniquely human socioecology. Anthropologists largely see human PFLS as derived, while comparative gerontologists point to evidence that it is one instance of a ubiquitous cross-species pattern. The two groups generally occupy non-overlapping research circles, in terms of conferences and journals, and therefore interact little enough to largely avoid the need to reconcile their views, allowing the persistence of misconceptions in each field. Our goal is to identify and address the most important of these misconceptions and thereby make clear that both of these seemingly incongruent views contain valid points. We argue that two distinct but related traits have been lumped together under the same concept of "post-reproductive lifespan," one (post-fertile viability) that is tremendously widespread and another (a post-fertile life stage) that is derived to hominins, and that the differences and connections between these two traits are necessary for understanding human life-history evolution.