 Shepard's books have become landmark texts among ecologists and helped pave the way for the modern primitivist train of thought, the essential elements being that "civilization" itself runs counter to human nature - that human nature is a consciousness shaped by our evolution and our environment.We are, essentially, "beings of the Paleolithic".Based on his early study of modern ethnographic literature examining contemporary nature-based peoples, Shepard created a developmental model for understanding the role of sustained contact with nature in healthy human psychological development, positing that humans, having spent 99% of their social history in hunting and gathering environments, are therefore evolutionarily dependent on nature for proper emotional and psychological growth and development.