Full Idea
Men begin to distinguish themselves from animals as soon as they begin to produce their means of subsistence.
Gist of Idea
Producing their own subsistence distinguishes men from animals
Source
K Marx / F Engels (The German Ideology [1846], p.160), quoted by Sydney Shoemaker - Some varieties of functionalism 6
Book Reference
Singer,Peter: 'Marx' [OUP 1996], p.32
A Reaction
At the very least, we must say that there had to be some intrinsic distinctiveness in place before men could do this. I like meta-thought.