Other, weaker answers have been proposed to that age-old question; all have been proven wrong.
I have an answer to propose, assuming the question itself is even valid--What separates humanity from animals? The ability and the desire to deny reality. I'm not saying that humans are the only animals that lie or deceive--any predator tries to deceive its prey, up to a certain point anyway--but I don't know of any other animal that wants to believe the lie.
There was a perfect example of this just this week--the story of the 2 wolves, one healthy, one suffering from a gunshot wound, who (we are told) killed 120 rams (and only rams from the mixed herd, because the wolves knew that the rams were the more valuable animals in the herd), and then the two wolves piled the bodies in a corner of the pasture. This utterly fantastical story is being used as justification for the wolf hunts in Idaho and Montana. Those who want justification for their actions have no desire to think about this story or question it. They don't ask what sort of atomic-powered wolves these must be to have achieved a kill like that. They don't wonder what method the wolves used to pile the bodies--did the wolves drive a forklift, or did they use their atomic power to just casually toss the bodies through the air into the corner? An unquestioned lie is perfectly good justification for what they want to do.
This desire to deny reality is in every aspect of our society. Recently, it seemed like every web site in the country featured the picture of the man who cheated on his wife and had to stand on a street corner wearing a sign as his punishment. But it turns out that the whole thing was a hoax, a publicity stunt by a radio station. But how many web sites featured the truth about the story just as prominently as they had featured the lie? After all, an attractive lie brings in more people than a boring truth.
And now I return to what I referred to above: Is the question of what separates man from animal even a valid question? Every person I know of who works closely with animals and observes them carefully will say that it is not a valid question. One of my favorite quotes is from Garreth Patterson, sometimes called the Lion Man of Africa:
and it is because of subtle human conditioning
-- not the actual facts --
that we are raised to believe there is a wide gap between
what is human and what is animal.