The concept of a tree of life is a widespread mytheme or archetype in the world's mythologies, related to the concept of sacred tree more generally, and hence in religious and philosophical tradition.

Pak Breeders and Pak Protectors are two developmental stages of fictional life in Larry Niven's Known Space universe. The Pak first appeared in "The Adults," which appeared in Galaxy in 1967; this story was expanded into the novel Protector  by Larry Niven (1973). The Pak also appear in several of Niven's later novels, notably those set in the Ringworld.