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:
Pak Child is analogous to a human child: sexually immature and dependent upon adults for survival. Pak Breeder is analogous to a young-ish 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 highly sensitive to the smell of their close relatives and "weed out" those that smell wrong, which may indicate a potentially dangerous mutation. This weeding also suppressed positive mutations, essentially halting Pak evolution. Protectors are fully sentient, and are far more intelligent than ordinary humans. This "superior" intelligence, however, serves only a Pak Protector's instincts to protect its bloodline at any costs. The Pak have no drive toward the collection of abstract knowledge, have no concept of art, and do not even possess enough of an artistic impulse to understand the purpose of making sketches and paintings for reasons not directly useful. The change from Breeder to Protector is the result of a peramorphic transformation brought about by the plant known as Tree-of-Life. As humans (and all primates) are descended from the Pak, Tree-of-Life can create a Protector-stage human.
Tree-of-Life
Tree-of-Life is the mechanism by which a Breeder becomes a Protector. The term originally is used to refer to a specific plant which, when consumed, triggers the transformation. The term "Tree-of-Life virus" is used to describe the symbiotic virus which actually governs the transition. Niven took the name Tree-of-Life from the Book of Genesis; specifically to the fruit of the "Tree of Life" that could make Adam and Eve immortal (Genesis 3:22–24), which is quoted as the foreword to the novel Protector, and also mentioned by Brennan within the novel. Tree-of-Life is a bush native to the Pak homeworld. When a Breeder reaches the proper age (early 40s for humans), the smell of the root becomes irresistible; the Breeder gorges on the Tree-of-Life root, infecting itself with the Tree-of-Life virus and transforming into a Protector. The age window for the metamorphosis is relatively narrow (between 42 and 50 Earth years). Tree-of-Life is common on the Pak world, so there is almost no risk of a Breeder living past this window without being exposed to the roots. The transition from Breeder to Protector involves reconfiguration of the anatomy. Skin thickens, becoming similar to leather armor, 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 increases leverage available to muscles by increasing the force of the moment arm, the result being that a protector can lift ten times its own weight. Genitalia and gonads vanish, and a second two-chambered heart forms in the groin at the fusion of the femoral veins. The arms lengthen. Fingernails turn into retractable claws. Teeth fall out and the lips and gums fuse, the mouth forming a horny beak (flat in protectors transformed from humans, non-flat in protectors transformed from Pak). All the breeder's hair falls out and the head acquires a bony ridge to protect the newly expanded cranium. The expanded skull allows the brain to grow to an enormous size; the resulting mind, even starting from something as "unintelligent" as a chimpanzee, becomes far more intelligent than a typical human. Pak Protectors also acquire an extended lifespan, and can live tens of thousands of Earth years (a common element in Niven's stories). Once the transformation is complete, a Pak Protector must periodically consume more Tree-of-Life root to maintain the virus in its body. Without the virus, a Protector will weaken and die as its DNA is degraded; the virus supplies replacement DNA. The Tree-of-Life crop on Earth failed due to there being insufficient thallium oxide (which particular oxide is never explained) in the Earth's soil; the plants grew but did not support the virus. As a result, the Protectors that led the colony to Earth died of starvation when their store of roots ran out.
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