The height that adult humans achieve results from a complex interplay between genetic endowment and environmental exposures during development. We hypothesize that exposure to microbes—both exogenous pathogens and endogenous biota—are critical environmental determinants of the expression of human height in a community. Both experimental studies and historical changes in height in relation to presumed microbial transmission support this hypothesis.

In developed countries, humans are growing taller, and have been for at least a century (Eveleth and Tanner 1990). Although this secular increase is well documented in many localities throughout the world (van Wieringen 1986), the causative factors and the reasons for local variations are less clear. Barring long-term selective factors, human height appears largely determined by environmental influences (Malina 1979), and human history has occurred in changing environments. We postulate that the microbial environment of humans, involving both exogenous microbes and indigenous biota, plays a substantial role in determining human height. Thus, we postulate that temporal changes in microbial transmission help explain the fluctuations in human height.[End Page 475]