"In Protector, Niven explains that humans (and all of Earth's primates) are descended from a colony of Pak breeders that were stranded on Earth 2.5 million years ago. The protectors that built the colony ship died when their Tree-of-Life crops failed (due to a lack of thallium in the soil). The original Pak Breeder population (called Homo Habilis) bred and mutated wildly, evolving into modern humans as well as all other Earth primates (such as gorillas, chimpanzees, and orangutans). All Terran descendents of the Pak could transform into the Protector stage if exposed to Tree-of-Life root (or the symbiotic virus it contains).

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Niven has stated in other writings 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 human's long lives after females have passed reproductive age. Accordingly, most of the positive attributes of Protectors are based on negative human aging effects: sore joints, poor circulation, wrinkled skin, lack of sex drive, and rotting teeth are all turned to advantage during the shift from Breeder to Protector.