Protector (novel)
Protector is a 1973 science fiction novel by Larry Niven, set in his Known Space universe. It was nominated for the Hugo in 1974, and placed fourth in the annual Locus poll for that year.
The work fleshes out a species called the Pak, originally introduced in a 1967 story called The Adults, which forms the first half of the novel (there titled Phssthpok); the second half is titled Vandervecken. The Pak also appear in several of Niven's later works, notably the later volumes of the Ringworld series and the novel Destroyer of Worlds which serves as a semi-sequel to Protector.
Plot summary
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 immense 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.