The plane crash is crucial. It's what starts the whole adventure for Eric. Then, his first encounters with the cannibals are important as it shows the danger of the island. Discovering the underground facilities is another big event as it starts to unravel the mystery of the island's mutants.
The plane crash is the initial event that sets the story in motion. As Eric begins his exploration, the first time he discovers a tool or weapon that can be used against the cannibals is important. It could be a simple axe or a crafted spear. When he stumbles upon a hidden entrance to the underground areas, that's a major turning point. Inside, he finds evidence of the experiments and the cause of the mutants. And of course, every small clue he finds about his son's location is vital in the overall story as it drives his continued search and survival on the island.