Protector
Larry Niven
 The novel comprises two phases in the same space that are separated by 220 years of time. Its central conceit is that humans evolved from the juvenile stage of the Pak, a species with a distinct adult form ("protectors") that have superhuman strength and intelligence and care only about younger Pak of their bloodline. A key plot point is that transition to the protector stage is mediated by consumption of the root of a particular plant called Tree-of-Life, which cannot be effectively cultivated on Earth. The species is described in greater detail below. The implication, never explicitly stated, is that the Tree of Life in Paradise, mentioned in the Bible's Book of Genesis, represents a vague memory of that plant. The first half of the book follows the path of a Pak named Phssthpok who has travelled from the Pak homeworld in search of a colony of Pak in the distant system of Sol (our solar system). Upon his arrival, he captures a Belter (a worker from the asteroid belt) named Jack Brennan, who is infected by Phssthpok's store of tree-of-life root and is transformed into a protector (or at least a human variant). They land on Mars where Brennan kills Phssthpok and is rescued by two humans, Nick Sohl and Lucas Garner, who had set out to meet the alien. The first half of the novel ends with Brennan telling his story to the humans before he heads for the outer reaches of the solar system. The second half of the book follows the path of a human named Roy Truesdale who has been abducted with no memory of the event. While searching for his abductor, he befriends a Belter named Alice Jordan who helps him figure out that the man he has sought is none other than Jack Brennan. Truesdale and Jordan find Brennan in the outer solar system on a fabricated world of Brennan's design called Kobold. Brennan discovers that a Pak invasion fleet is headed towards human space and takes Truesdale to a human outpost colony called Home in an effort to divert attention away from Earth. During their journey they battle with scout ships from the Pak fleet. Brennan and Truesdale arrive at Home only to have Truesdale realize that Brennan plans to convert the colony into a defensive Human Protector army. Truesdale kills Brennan and lands on Home, but is himself infected with a mutated strain of the Tree-of-Life virus that quickly spreads to a number of other colonists, thus carrying out Brennan's plan despite Truesdale's initial attempts to thwart it. Upon his conversion to protector form, Truesdale immediately understands the necessity of Brennan's plan and completes it by breaking out of hospital confinement and infecting the entire population of Home. The modified virus either kills or converts the remaining inhabitants, resulting in an army of childless protectors. The new protectors know that they absolutely must act quickly to save the rest of humanity, and start preparing for battle with the Pak invasion fleet. As an aside, it is mentioned that during his sojourn in the outer Solar System Brennan had engineered a genocide on Mars, sending a large ice asteroid to crash into the planet in order to raise the water content of its atmosphere. Water is lethal to the Martians' metabolism, thus this effectively wiped out the species. This incident serves to underscore the Pak Protectors' inherent xenophobia and utter ruthlessness in pursuing their ultimate goal of protecting their descendants. The events which impelled Brennan to this action are those narrated in How the Heroes Die and At the Bottom of a Hole, two short 1966 stories which Niven originally published in Galaxy Science Fiction and later collected in Inconstant Moon.