It is not so much understanding which causes the specific distinction of man from all other animals as it is his being a free agent.
Gist of Idea
Humans are less distinguished from other animals by understanding, than by being free agents
Jean-Jacques Rousseau (Discourse on the Origin of Inequality , Part I)
I'm not sure how deep Rousseau takes 'free' to go. Having little enthusiasm for free will, I would say that we are distinguished by the complexity of our decision making. But I attribute that to meta-thought, the mark of humanity.
Rousseau,Jean-Jacques: 'The Basic Political Writings', ed/tr. Cress,Donald A. [Hackett 1987], p.45