Pak Protector
[1] 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.
Pak Breeder is analogous to a young-ish human adult: sexually mature, self-sufficient (in later writings) and providing immediate care for the children.
Earlier Niven stories describe the breeder as "just intelligent enough to swing a club or throw a stone".
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.
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.
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.
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.
It is observed several times that this intelligence combined with instinct also compels action so completely that Protectors often have little "free will".
The original Pak Breeder population (known to us as Homo habilis) bred and mutated wildly, evolving into modern humans as well as all other Earth primates.
All Terran primates would transform into the Protector stage if exposed to Tree-of-Life root (or, more accurately, the symbiotic virus it contains).
