"Who has the authority to establish moral bounaries?"
It has always been those in power. Likely it always will be. Now if the question is "Who SHOULD have the authority. . ."
I would argue that there never has been a divine standard, simply because there has never been one religion, or one faith, or one set of morals.
Morality is an extension of the survival instinct and is a product of empathy. If we are going to establish moral boundries, the only just way to do so is on the basis of what we can agree on. Clearly the divine is not one of those things.
So, what can we agree on?
Causing suffering is normally bad. Aleviating suffering is normally good. Why don't we start there?
Humans need sustenance. (air, food, shelter). Depriving a human of these things will cause suffering. It is therefore immoral to deny sustenance.
Humans need freedom. Depriving a human of freedom will cause suffering. It is therefore immoral to deny freedom.
Sure, you can think of times when depriving someone of food is not immoral. (night before surgery for example), and depriving someone of freedom because they committed a crime might also be considered just, but I think any moral code has to start from the needs of humans, and the reasons that we justify depriving those needs.
I need to make money, therefore I will deprive humanity of clean air. . . this would be immoral to my mind.
No divine intervention required.