Cities are sites of intense investment in niche construction, substantially altering ecological dynamics. Although novel in evolutionary terms, cities have distinctive epidemiological and demographic effects on human mortality and phenotype. Cities, however, do not affect all of their inhabitants identically, especially with a trend toward greater inequity with increased urbanization; the urban landscape offers a set of stratified behavioral niches to inhabitants. An examination of meninos de rua (street children) in Brazil highlights the opportunities available in urban landscapes and the demands placed on residents: a radically simplified foraging landscape, availability of energy-dense food resources, decreased activity levels, and challenges to mental health. Considering urbanization as niche construction highlights the embodied and psychological consequences of urban life not just on individuals but also over generations as urbanization creates phenotypic and developmental bias. The niche construction perspective blurs the divide between biological and cultural approaches to human variation and draws attention to the biological consequences of the built environment and socioeconomic structures underpinning urban life. Using street children in Brazil as a case study, this paper outlines the theoretical implications of urban adaptation as an example of niche construction dynamics leading to rapid changes in diet, morphology, and mental health.