Flashbacks are indeed vital to the narrative of Margaret Atwood’s Oryx and Crake. An anonymous narrator tells the story of Snowman in the present tense, but the narrative often switches to Snowman’s previous self, Jimmy. The portions of the novel following Jimmy are told in the past tense.
Essentially, what is happening in this novel is that we are learning about a single character but in two very different modes. Jimmy’s story is highly humanistic and lifelike. It takes place before the contagion-induced apocalypse and follows Jimmy’s childhood, family life, and eventual association with two genetically engineered human beings, Oryx and Crake.
After the apocalypse, Jimmy begins referring to himself as Snowman and sets out to find Crake’s followers, the Crakers. The novel ends with Snowman questioning whether or not he and the Crakers would meet the few remaining human beings as allies or enemies. The novel ends with the future of the human race in question.
Snowman’s flashbacks to his life as Jimmy serve several purposes. On the surface, they serve as a way to retell the events leading to the apocalypse. On a deeper level, the flashbacks allow us to see Jimmy’s evolution into Snowman and analyze themes like the dangers of scientific development, jealousy, and dominance.