> Morality applied to humans wasn't universal either.

Yes it was. Always. It's just that morality, being a human invention/illusion, varied from culture to culture and from era to era. Your examples ( slaves, caste system, etc ) isn't a lack of morality or non-application of morality, it's just different morality applied in different cultures. India has/had morality and the south has/had morality, just different morality than we do today in the US.

Regardless, morals can only be applied to humans. Try to apply it to animals and you get nonsense. I asked basic questions in my comment. If an elephant has 'moral weight' then killing it is wrong. So must we arrest and prosecute lions who kill elephants? Do we arrest everyone working for national geographics for profiting off of snuff films?

The key argument is not that we ought to expect non-human animals to abide by human morals (although animals do seem to have their own moral codes that they abide by) but that human moral codes (which we hold humans to) ought to assign non-human animals a value much closer to the value that we assign humans than they often do.

> Regardless, morals can only be applied to humans. Try to apply it to animals and you get nonsense.

This seems overly simplistic. For the sake of argument, I'll accept that moral judgments can only be made for human actions. But when those actions involve animals, they can still be made. People clearly make moral judgments about human cruelty to animals - every US state makes some version of animal cruelty a felony offense. The distinction between the presumed agency of the actor and the acted upon seems important here, and you are conflating them.