Tree-of-Life is a perennial bush native to the Pak homeworld. The bush is found on Earth as yam, but the virus did not survive. When grown with Thallium oxide the Tree-of-Life root is infected with a symbiotic virus.
Its smell is unnoticeable or unpleasant to breeders until they reach about 35 Pak years (or 42 human years, i.e. early middle age); after that, the smell suddenly becomes irresistible. Even if too old, hominid breeders are still irresistibly attracted to the root's smell.
The breeder gorges on the root, infecting himself with a symbiotic virus in the root and triggering the transition; an enzyme works profound changes at the cellular level of the Pak causing apparent, emotional and intellectual transformations.
The age window for the metamorphosis is relatively narrow. If a breeder is just a few years past early middle age, the virus in the root will kill him instead. (This is not a problem on the Pak homeworld, where Tree-of-Life is readily available to any breeder that reaches the age of transition.)
After consuming Tree-of-Life root, Pak breeder forms (and their offshoots, such as humans) turn into the Pak Protector form. 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, 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.