Pak Protector

Pak Breeders and Pak Protectors are two generic forms of life in Larry Niven's Known Space universe. They are a dichotomy brought about by the Tnuctipun's breeding policies and the plant known as Tree-of-Life.

Spoiler warning: Plot or ending details follow.

Tree-of-Life is a bush whose smell is unnoticeable to breeders until they reach about 25 Pak years (or 42 human years); after that, the smell suddenly becomes irresistible. The breeder eats the root, infecting himself or herself with a symbiotic virus in the root and triggering the transition.

After consuming Tree-of-Life, Pak breeder forms (such as humans) turn into the Pak Protector form, which involves reconfiguration of the anatomy, including the acquisition of a leathery armor or exoskeleton, strong enough to turn a knife. Joints swell until the creature becomes "a parody of the human form done in cantaloupes and coconuts". This expands the leverage available to muscles by increasing the moment arm: the protector can lift 10 times his or her own weight. Genitalia vanish, and a second heart forms in the groin. Teeth fall out, and lips fuse; the resulting structure is a sort of bony beak. The brain grows to an enormous size; the resulting mind, even starting from something as dumb as a chimp, becomes literally too brilliant for human comprehension. All the breeder's hair falls out and the head acquires a bony ridge to protect the newly-expanded cranium.

Pak Protectors have an in-built need to look after (or protect, hence the name) their "parent" breeder species. Part of this protection is to prevent mutations from surviving (thus rendering Darwinian evolution impossible); the reason why humans evolved beyond the original Pak breeder form (Homo habilis) is that the Tree-of-Life virus requires thallium in the soil to survive and Earth is not adequately thallious; all our Pak Protectors died out millennia ago and we evolved into Homo sapiens.

The Ringworld was built by Pak Protectors and populated by Pak breeders. The Pak Protectors dwindled in numbers until they were no longer able to maintain the genetic purity of the breeder forms and they evolved into all the other hominids of the Ringworld that one sees in Larry Niven's novels.