From Novel to Extreme: Contemporary Environments and Physiologic Dysfunction

Abstract

This chapter presents a new conceptual framework to characterize the interactions between modern environments and ancestral physiology that can influence health over the life course. Although the human body was not designed by natural selection to maximize health, it was nonetheless designed to function within certain environmental parameters. Physiology serves as the essential interface between genes and environment not only during life history transitions but also during short-term responses needed to regulate the impact of environmental variation on the internal state. Here, I argue that the contemporary habitat has become an “extreme environment” in the sense that, much like climbing Mount Everest, it requires the human body (and psyche) to function beyond the limits of its adaptive capacity, with potential dire consequences to health. Adaptive mechanisms that have become dysfunctional in an extreme environment show three distinct features: (1) gradient effects, often without overt signs of dysfunction, whereby (2) compensatory mechanisms themselves become the source of illness, and (3) involve systemic repercussions. I present an analysis of altitude hypoxia to illustrate an extreme environmental condition and then apply the same framework to a consideration of metabolic disorders. A conceptual framework that identifies lifestyle factors as being equivalent to entering an extreme environment conveys an immediate, intuitive sense of urgency and an implicit recognition that the human organism has strayed into a habitat for which it is ill equipped.