Las Vegas functions as a complex urban system composed of interrelated subsystems like population, water, transportation, and climate. Cities first developed during the agricultural revolution when people moved into cities for the first time, and modern cities gained prominence during the industrial revolution as commerce replaced farming. Today's third urban revolution involves continued urban growth. While rural areas rely on natural resource-based jobs, urban areas have jobs not directly connected to resources. Urbanization affects land use and wildlife habitat, and can create issues like urban heat islands, air and water pollution from sprawl. Smart growth aims to develop cities compactly and preserve open spaces through mixed land uses and public transportation.