Patterns and processes of human life history evolution

Abstract

Phenotypic plasticity, or the ability to alter one’s morphological, physiological, and behavior phenotype in response to environmental change ( Pigliucci 2001 ), is central to organismal life histories. Plasticity in reproductive physiologies and behaviors in response to stochastic environmental signals (i.e., diet, activity, stress, disease, availability of mates) is hypothesized to represent a suite of complex adaptations, reaction norms produced by natural and sexual selection and constrained by trade-offs under conditions of resource restriction ( Schlichting and Pigliucci 1998 , Sinervo and Svensson 1998 ). This is to be expected given the central role of reproduction in life history evolution. Like other organisms described throughout this volume, humans are required to allocate physiological resources between reproduction and a number of competing functions, particularly growth and survivorship ( Stearns 1992 ). Humans are capital breeding, iteroparous organisms that budget time and stored energy over a number of reproductive events within a lifetime. Humans have unusual life history characteristics: we are born helpless, take a long time to mature, and can live for 70 years or more, including a female post-reproductive period following menopause ( Mace 2000 ). These life history traits may be linked with our remarkable cognitive and social abilities compared to other species: humans have very large brains that have complex and lengthy patterns of psychological development ( Geary 2005 ). The interactive development of human life history traits may be understood by considering the context in which they could have evolved. Here we review both theory and mechanisms of human life history evolution. We emphasize the key endocrinological mediators of life history trade-offs, particularly those involving reproduction. Specifi c topics include reproductive maturation, male and female reproductive ecologies, mating/parental behaviors, and reproductive senescence.