I just did this topic in ethics so I've already been thinking about this a lot
I feel like certain morals are objectively true. For example, killing is wrong, so is stealing, breaking promises, etc.
No matter how you stretch it, these things are never right. Even if you must kill someone in self-defense, the murder is not morally correct. Even if you have to steal to survive, the theft is not morally correct.
From these, less fundamental morals come around, i.e. ideas on homosexuality, abortion, what have you.
Basically the true answer lies somewhere inbetween Objective and Subjective truth
i know i probably fucked this up somehow
Personally, I go with the idea that the fundamental morals you talked about are... "objective," in a way. Not like, set in stone somewhere in the Crab Nebula or something, but well, the Prisoner Dilemma was run through computer simulation of many different strategies, some "evil" (lots of defection) and some "good" (like Tit for Tat, where you are never the first to defect, you just do the same play as your opponent did last time. They defect last, you defect now. They cooperate, you cooperate)
The result was something akin to the top ten had all good strategies? Or at least a 9/10 majority.
So my view is that that idea, where cooperation is mathematically/statistically/computationally(?) shown to be better than selfish behavior, it follows (for me) that the evolution of social creatures like humans, canines, elephants, etc. encouraged cooperation within certain parameters. Like the idea that people tend to treat their in-group better than strangers is also shown in those simulations: When it was a consistent "opponent" (i.e. you play the same person over and over again) cooperation was better, as in family and your tribe, but if it was a one-off play, then the "evil" strategies had a one up (after all, if you only play one time, if your first play is to defect and you're playing against a Tit for Tat, they'll cooperate and you'll come out very far ahead. It's only if you play them again that you get into poor scenarios.)
So uh. That was a long read. Hopefully it made sense?