what makes human beings distinctive amongst animals?

What makes human beings distinctive amongst animals? This is an argument I found myself having a few times last week. I just came across a great passage by Martha Nussbaum, quoted on Brain Pickings, reflecting my own views on this. When I say ‘reflexivity is a defining characteristic of the human’, it’s a short hand for this broader proposition, expressed in this case more elegantly than I am able to:

Human beings appear to be the only mortal finite beings who wish to transcend their finitude. Thus they are the only emotional beings who wish not to be emotional, who wish to withhold these acknowledgments of neediness and to design for themselves a life in which these acknowledgments have no place. This means that they frequently learn to reject their own vulnerability and to suppress awareness of the attachments that entail it. We might also say … that they are the only animals for whom neediness is a source of shame, and who take pride in themselves to the extent to which they have allegedly gotten clear of vulnerability.