Engagement with and study of nature is increasingly important for science literacy and civic engagement. Spurred on by challenges of the Anthropocene, many informal learning institutions are exploring how their collections, programs, and scientific expertise can be mobilized to create new naturalist learning pathways for children and youth. In this paper, we explore retrospective life histories of 18 adult naturalists to examine experiences that they recall supporting their interest development in the natural world. Drawing on interest and informal learning literature, our analysis reveals how elements across the learning ecology, including school, family, and out-of-school learning, work together to support the development of naturalist practices and identities. We found that interest development in nature occurred across the learning ecology and that expression of situational or individual interest depended on the participants' age and the type of learning experience. A closer examination of three individual cases--a serious amateur naturalist, an environmental educator, and an ecologist--reveals some of the nuanced ways that interest in nature arises, is maintained, and can eventually develop into a deep, lifelong naturalist identity. We consider implications for how one might conceptualize and support informal learning pathways that involve deep engagement with and connections to nature.