IV. Parallelism or Diffusion? The archaeology and ethnography of the past half-century have made it clear that the ancient civilizations of the Old World … derived from a single base, and that this community of origin suffices to explain the homologous forms of their mythological and ritual structures.
Campbell frames diffusion versus parallelism as the central explanatory choice in comparative mythology, arguing that a single neolithic Near Eastern origin sufficiently explains Old World mythological homologies.
, Primitive Mythology (The Masks of God, Volume I), 1959thesis