Summary Complex life cycles are ancient and widely distributed, particularly so in the marine environment. Generally, the marine biphasic life cycle consists of pre-reproductive stages that exist in the plankton for various periods of time before settling and transforming into a benthic reproductive stage. Pre-reproductive stages are frequently phenotypically distinct from the reproductive stage, and the life cycle transition (metamorphosis) linking the larval and juvenile stages varies in extent of change but is usually rapid. Selection of suitable adult sites apparently involves the capacity to retain the larval state after metamorphic competence is reached. Thus two perennial and related questions arise: How are environmentally dependent rapid transitions between two differentiated functional life history stages regulated (a physiological issue) and how does biphasy arise (a developmental issue)? Two species of solitary ascidian, a sea urchin and a gastropod, share a nitric oxide (NO)-dependent signaling pathway as a repressive regulator of metamorphosis. NO also regulates life history transitions among several simple eukaryotes. We review the unique properties of inhibitory NO signaling and propose that (a) NO is an ancient and widely used regulator of biphasic life histories, (b) the evolution of biphasy in the metazoa involved repression of juvenile development, (c) functional reasons why NO-based signaling is well suited as an inhibitory regulator of metamorphosis after competence is reached, and (d) signaling pathways that regulate metamorphosis of extant marine animals may have participated in the evolution of larvae.