the root of the Tree-of-Life plant begins to smell delicious to all Breeders, and they eat it and metamorphose into the Protector stage via a virus that lives in the plant. A Protector's joints expand to give its muscles a greater moment arm, its skin wrinkles into a tough armor, and fingernails turn into retractile claws. A second heart develops near the groin, the mouth fuses into a beak, and all sexual characteristics disappear. The most important change is that the brain expands, giving a Protector a tremendous intellect. A Protector has no motivation other than the preservation of its bloodline, and to that end the Pak homeworld is never free from war, since all Protectors are constantly trying to ensure their progeny's survival at the expense of everyone else. If a Protector has no children left, it no longer feels the urge to eat and dies unless it can alter its outlook to adopt the entire Pak race and work towards their benefit. Origin of humanity Two and a half million years ago, a group of Protectors hollowed out an asteroid, turned it into a ship, and set sail for the galactic arms. They eventually settled on Earth, but discovered that the Tree-of-Life root would not grow, meaning that no Breeders would turn into Protectors, and the Protectors would die without the root. The Protectors spent their time building a laser strong enough to send a message for help back to the Pak homeworld. Eventually, the Breeders, known to us as Homo habilis, evolved into present-day forms, including humans. Phssthpok Phssthpok had lost all of his fertile descendants in a war on the Pak homeworld, and needed to find meaning for his life, or he would die. After spending much time in the homeworld's library, he found the story of the Pak expedition to find a habitable planet in the galactic arms. He worked out that the soil of Earth lacked adequate thallium oxide, and mounted a rescue mission aboard a ramship 33,000 years ago, arriving in the Solar System in 2125 AD. Persephone According to the novel, Persephone is the tenth planet (a Planet X) of the Solar System. While actually proposed in the 1970s to account for perturbations for the orbit of Neptune, Niven gives it additional qualities such as retrograde motion, an orbit tilted 61 degrees to the ecliptic, and a mass slightly less than that