Although records of intense spiritual experiences in nature exist throughout history, the phenomenon remains a little-investigated question. This article is the first in a series describing empirical findings on intense spiritual experiences in nature. Three data points were established including: 1) a cognitive analysis of forest attitude research interviews; 2) a cognitive analysis of nature authors who write about forests; and 3) a broad review of literature drawn primarily from research in neuroscience, psychology, medicine, consciousness studies, and philosophy. The findings suggest that intense spiritual experience in nature has two variations: mystical and traumatic experience. The positive (mystical) and negative (traumatic) variation share seven physiological and psychological characteristics, with each characteristic providing adaptive, evolutionary advantages. Although partial and preliminary, the data offers compelling evidence demonstrating the existence of certain basic properties of the role of nature in intense spiritual experience. The findings suggest that natural selection may favor intense spiritual experiences in nature.