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. Destroyer of Worlds depicts a confrontation between the Pak and the Puppeteers. Narrative purpose Niven has written that he invented the Protectors as a thought experiment to explain the common effects of aging on humans and to create a fictional evolutionary explanation for humans' long lives after females have passed reproductive age. Accordingly, most of the positive attributes of Protectors are based on negative human aging effects: swollen joints, decreased muscle-fat ratio, weakening heart, invariant diet, decreasing height, facial atrophy, leathery skin, hair loss, lack of sex drive, and tooth loss are all turned to advantage during the shift from Breeder to Protector. The Pak species In Protector, the audience learns that humans are descended from the Pak. Pak Children and Breeders appear in Earth's fossil record as Homo habilis; the few Pak Protectors to make it to Earth apparently are not found in the fossil record. The Pak species goes through three stages of development: Child, Breeder and Protector. Pak Child is analogous to a human child: sexually immature and dependent upon adults for survival. Pak Breeder is analogous to a human adult: sexually mature, self-sufficient (in later writings) and providing immediate care for the children. To the Pak, the Breeder stage, though capable of space travel, is not deemed fully sentient; Breeders, to a large extent, rely on Protectors for long-term survival. Earlier Niven stories describe the breeder as "just intelligent enough to swing a club or throw a stone." Pak Protector is not analogous to any human form. It is described as a 'fighting machine,' with armor-like skin, super-human strength and super-human intelligence. Niven's stories that focus on the Pak mostly concentrate on the unique Protector-stage. Niven explained the evolution of the Pak as resulting from high radiation levels on their home world near the core of the galaxy. The high radiation near the star-dense core caused severe mutations that can destabilize the evolutionary process. As a result, the Pak evolved a mechanism to eliminate dangerous mutations from the population. That mechanism is the Protector stage. Protectors are