Grann's development transforms him from being the type of man who never took risks and enjoyed his creature comforts—to the point that he always took the elevator instead of the stairs—to a man who heads off into the Amazon in search of adventure.
The reason for his trip to the Amazon was to search for evidence of a lost city which he had dubbed "Z". The second part of the rationale behind the trip was to search for Percy Fawcett, his son Jack, and Jack's friend, Raleigh Rimell. In 1925, this trio had headed into the Amazon to search for the Lost City of Z, only to disappear. This led to one of the greatest mysteries of the age. Grann, despite his love of creature comforts, was keen to solve the mystery.
The major change for Grann comes when he manages to get some answers on at least one of the mysteries he is pursuing. In the jungle, he meets up with an American anthropologist named Michael Heckenberger, who proves conclusively to Grann that the Lost City of Z was real. The remains of moats and roads provide evidence, as does pottery that Heckenberger has had carbon dated.
On a personal level, the trip forced Grann out of his comfort zone and gave him an opportunity to bring his passion for history and archaeology out of the pages of books and into real life.