When a novel isn't written as a novel — that is, when it borrows its form from another genre altogether, like letters, newspaper articles or even an auction catalog — the experienced reader sets out to find the reason why. What is gained by rendering fiction as nonfiction? In the case of Hanya Yanagihara's engrossing, unsettling first novel, "The People in the Trees," the full implications of this choice only become apparent in the final pages.

"The People in the Trees" purports to be the memoirs of Nobel Prize-winning scientist A. Norton Perina, edited by his colleague Ronald Kubodera — memoirs the truth of which is doubly suspect. For Perina writes from prison, convicted of molesting his adopted son (one of a brood of 43 adoptees); and while Kubodera staunchly defends his mentor, he admits in his introduction that he has "judiciously [cut] passages that I felt did not enrich the narrative or were not otherwise of any particular relevance," adding a layer of uncertainty to what one already suspects may be a self-serving tale.

Both Perina's fame and his unusual family result from a 1950 expedition to the Micronesian nation of U'ivu. Aimless after medical school, he jumps at the chance to travel with renowned anthropologist Paul Tallent, who's searching for a lost tribe on U'ivu's "forbidden island" of Ivu'ivu — a tribe, U'ivuans say, that lives forever, though at terrible cost. Tallent half-hopes the myths are true, and Perina finds himself caught up in the quest.

In the green jungles of Ivu'ivu — "so many shades and tones of green — serpent, aphid, pear, emerald, sea, grass, jade, spinach, bile, pine, caterpillar, cucumber, steeped tea, raw tea" — they find a band of near-feral immortals, centenarians whose bodies are healthy but whose minds continue to decay. Perina believes their remarkable longevity may stem from the ritual consumption of a rare turtle, and when he returns to the United States, he brings a specimen, and three of the immortals themselves, to study. His findings have dire cultural and ecological consequences for all the people of U'ivu.

Perina's story elicits questions about the practice of science, its responsibilities and its goals; in a footnote, Kubodera writes, "To be a scientist is to learn to live all one's life with questions that will never be answered … with the anguish of not having been able to guess at the solution that, once presented, seems so obvious." Too, Yanagihara asks what we want the scientist to be. Can Perina be a great mind without being a great — or even a good — man? What, in the end, are we willing to forgive for progress?

Anna Perleberg Andersen lives in Wichita, Kan., and blogs at http://museathighwayspeeds.blogspot.com.