I vote for fire as the key transition point for intelligence. Cooking makes many foods that would be poisonous safe to eat. But more importantly, fire is very hard - it is not just one skill but at least 5. Further you have to plan to make fire and collect ingredients and know how to prepare them for the attempt to start a fire, and when things go wrong, you need to know alternate ways of using your ingredients to make the fire a different way. Try it, it's really hard without matches and gasoline.