The Pak are a species of highly xenophobic humanoids originating from a system near the Galactic Core. They are closely related to other hominid species in the galaxy, including humans and the Morlocks of Wunderland.
The Pak species
Increased radiation levels at the core caused severe mutations that destabilized the Pak evolutionary process. Consequently, the Pak evolved a mechanism to eliminate dangerous mutations from the population. That mechanism is the Protector stage.
Conflicts between bloodlines are a constant reality for the Pak throughout their existence, and have impeded any social and scientific progress for millennia.
Pak go through three stages of development: Child, Breeder and Protector.
Pak Children are sexually immature and are primarily cared for by their Breeder parents. Newborns who have mutated are different in their smell and are destroyed at birth by Protectors.
Pak Breeders are sexually mature, unintelligent and sub-sentient primates known to humans as their ancestor Homo habilis. Towards the end of their reproductive life (about 42 Earth years), Pak Breeders acquire a craving for a root containing a symbiotic virus that triggers the physical and intellectual transformation to the Protector stage where sexual characteristics disappear.
When humans are considered in the Pak context, the Breeder stage is permanent for humans, as the Tree-of-Life is unavailable.
Pak Protectors are highly sensitive to the smell of their close relatives and 'weed out' those that smell wrong, which may indicate a potentially dangerous mutation and thus protect their own bloodline. This weeding also suppressed positive mutations, essentially halting Pak evolution.
Protectors are fully sentient, and are far more physically strong and intellectually intelligent than ordinary humans, possessing "diamond like" clarity of thought. However, this superior intelligence only serves a Pak Protector's hard-wired instincts to protect its bloodline at any costs, including resorting to war, mass murder, or genocide. Their major source of nutrition is the Tree-of-Life, even periodically, to maintain the virus in its body, without which the Protector will weaken and die. A Protector can survive indefinitely on Tree-of-Life root, although it can eat other types of food provided Tree-of-Life root is a regular part of its diet.
Pak Protectors reportedly constructed the Ringworld.
The transformation involves reconfiguration of the anatomy, including the acquisition of a leathery armor or exoskeleton, strong enough to turn a copper 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 movement arm: the Protector can lift 10 times its own weight.
Genitalia vanish, and a second two-chambered heart forms in the groin. Fingernails turn into retractile claws. 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 "unintelligent" as a chimp, becomes far more intelligent than a human mind. The intelligence of the Protector stage of a Pak or Pak offshoot is dependent on the intelligence of the Breeder stage - as such, the Protector form of a sentient Breeder will generally be more intelligent than a Protector of a sub-sentient Breeder species. Humans who convert into Protectors develop intellects that are beyond human comprehension (Louis Wu described the mind of a Protector as having "diamond" precision, insight, and clarity - Breeder level thinking is "fuzzy" in comparison). All the Breeder's hair falls out and the head acquires a bony ridge to protect the newly-expanded cranium. Pak Protectors also acquire an extended lifespan, and can live tens of thousands of earth years. However, most Protectors die as the direct or indirect result of conflict with other Protectors, so death from old age is almost unknown in the Pak culture.
Pak Protectors have an in-built need to look after (or protect, hence the name) close relatives of their family. Protectors recognize their own Breeder family line by scent, and are instinctively compelled to act in the best interests of Breeder relatives. Pak Protectors will fight among themselves to procure land and resources for their respective Breeder populations, often leading to the violent deaths of entire Pak family lines. A Pak Protector with no Breeders to protect will generally stop eating and quickly starve, although some Breederless Protectors have avoided this fate by adopting the entire Pak race, and devoting themselves to activities that benefit the species as a whole.
Because of his vastly increased intelligence, a Protector will always see the best answer to any question for a given set of conditions. If that answer results in an advantage for his Breeder descendants, he will instinctively act upon it. In effect, Protectors have little free will as humans would think of the term. Consequently, Pak Protectors are by nature xenophobic and warlike, inherently incapable of holding abstract moral principles and ruthless beyond measure towards all Paks who are not their own descendants or - in the case of the most "broadminded" Protectors, i.e. those who adopted the entire Pak species - to members of all other species. Pak Protectors from different families will only cooperate in a shared goal until one family sees some advantage in betraying the rest, and thus the Pak homeworld is in a constant state of war.
Humans could transform into Protectors as well, the first being Jack Brennan. A human turned Protector will find himself compelled to perform actions that he would consider morally reprehensible as a Breeder.
The Pak and Humanity
Humans 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. The Tree-of-Life virus in the root requires thallium in the soil to survive and Earth is not adequately thallious; consequently all of Earth's Pak Protectors died out millennia ago, allowing the original Pak Breeder form (Homo habilis) to evolve into modern humanity (Homo sapiens). Other Earth primates (such as gorillas, chimpanzees, and orangutans) are also descended from the original colony of Pak Breeders, and could also transform to Protector form if Tree-of-Life root were available to them.
In 2433 it was discovered that the Morlocks of Wunderland were also the descendants of an ancient Protector expedition, similar to the one that planted the seed of humanity on Earth. Using the Hollow Moon the Pak were able to transport Breeders to Wunderland in hopes of establishing a colony in order to protect that particular lineage. It is unknown why the mission failed, although the most likely hypothesis is that Tree-of-Life could not grow on Wunderland. It is also not known why the Morlocks evolved so differently than humans, although there is documented evidence of Morlocks transforming into Protectors upon exposure to Tree-of-Life. Unfortunately, further research into the Morlock-Pak connection was hampered by the destruction of the cave system where the transformed Morlocks were hiding by Dimity Carmody using nuclear explosives that were buried underground.
It is widely accepted that the Ringworld was built by Pak Protectors. One claimed to be one of the original builders — 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 the Breeders eventually evolved into all the other hominids of the Ringworld.
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.