In my opinion, the argument that opposable thumbs are what separates mankind from most beasts is totes flurft (which is a word I just made up that means exaggerated, sweaty and dull ALL AT THE SAME TIME).
My qualm is, why doesn't anyone ever talk about this:
I think what really sets mankind apart from most beasts is that mankind can grow a mean moustache. Beasts can not. Most beasts just throw their faeces around.