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 (the plant) 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