The answer is not raw brain processing power, but rather it is our ability to learn from others. At least that is the argument from Joseph Henrich’s book The Secret of our Success. Slate Star Codex has an excellent book review. Consider the following graph comparing human, chimpanzee and orangutan intelligence.
Another interesting quotation from the book:
While hunters reach their peak strength and speed in their twenties, individual hunting success does not peak until around age 30, because success depends more on know-how and refined skills than on physical prowess.
I have not read this book, but certainly some interesting points to consider for your weekend reading.