Interstellar ancestors of humanity (actually Homo habilis) whose life-cycle mimics the stages of human aging.
Pak live through three stages of being: Child, breeder, protector.
A Pak Child is analogous to a human child: sexually immature and dependent upon adults for survival.
A Pak Breeder is analogous to a human adult: sexually mature, self-sufficient (in later writings) and provides 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." When a Pak breeder reaches 30 to 45 years of age he/she will feel an irresistible urge to eat the sweet-potato-like root of a plant, Tree-of-Life, that is found throughout the Pak homeworld. A virus found in tree-of-life initiates the transformation into a protector.
Protectors lose all sense of gender and reproductive desire, and exist solely to defend their clan bloodline. They are xenophobic, violent, hyper-intelligent and driven only by the fierce instinct to protect their descendants. This powerful instinct drove them to commit genocide on several planets where they tried to establish colonies, since every other sentient species was considered a potential threat. It also led to relentless internecine wars whenever two familial lines wound up with goals in conflict.
When the Protectors reappear in The Ringworld Engineers and its sequels, it is strongly indicated that they constructed the Ringworld.
A Pak colony failed on Earth 2.5 million years ago due to the soil's lack of thallium, which allowed Tree-of-Life to flourish but not the virus within its roots. As a result several million Pak breeders spread across the Earth, eventually evolving into Homo sapiens sapiens and all other primate life on the planet. Likewise, every hominid species found on the Ringworld is descended from Pak breeders, and all are susceptible to the virus of Tree-of-life. A protector-stage Homo habilis is more intelligent than a breeder-stage Homo sapiens, and a protector stage Homo sapiens is even more intelligent.