The interaction of the genetic program with the environment shapes the development of an individual. Accumulating data from animal models indicate that prenatal and early-postnatal events (collectively called “early-life events”) can initiate long-term changes in the expression of the genetic program which persist, or may only become apparent, much later in the individual's life. Researchers working with humans or animal models of human diseases often view the effects of early-life events through the lens of pathology, with a focus on whether the events increase the risk for a particular disease. Alternatively, comparative biologists often view the effects of early-life events through the lens of evolution and adaptation by natural selection; they investigate the processes by which environmental conditions present early in life may prompt the adoption of different developmental pathways leading to alternative life histories. Examples of both approaches are presented in this article. This article reviews the concepts of phenotypic plasticity, natural selection, and evidence from animal models that early-life events can program the activity of the neuroendocrine system, at times altering life history patterns in an adaptive manner. Data from seasonally breeding rodents are used to illustrate the use of maternally derived information to alter the life history of young. In several species, the maternal system transfers photoperiodic information to the young in utero. This maternally derived information alters the response of young to photoperiods encountered later and life, producing seasonally distinct life histories. Am. J. Hum. Biol. 17:34–43, 2005. © 2004 Wiley-Liss, Inc.