One can think of the possibilities: 1) If thinking was mainly a biological phenomenon, we wouldn't have any significant differences, 2) If thinking was mainly a cultural phenomenon, we would have to explain how different cultures emerge, transformations and even abandonments of a culture.
In Plato's Phaedrus, you will find a tale about how the souls are in heaven with Gods, wherefrom they fall, each one choosing the sort of life one wants to have on earth. In Christian terms (not very far away from Plato) a man becomes whatever one becomes, by personal will. Thinking and all cultures are based on each and everyone's personal will or character. In the roots of thinking there exists pure Freedom. A person prefers and approaches a similar person, so that bigger entities are formed - friendships, cities, nations..., together with the respective culture and ways of thinking.
This is a very schematic answer, but I hope it doesn't betray Plato or Christianity very much.