I have my preliminary dissertation tomorrow, so here we go! I'd argue @cybereality there is a case that morality is constructed in a similar way for all intellectual life and would largely be similar each time intelligent life evolves. Dolphins poop out baby dolphins, that's why they exist.
We exist because we do that but also don't run around killing and burning food (usually). In small tribes it was easy to punish those who hurt the group's chances for survival. In the first cities religion helped to standardize those rules. Religions that exist do so because they aided in survival and increased group homogeneity (homogeneity helpful when not everyone knows each other). I'd suggest that just surviving the world and successively mating breeds objectively the stem of all modern morality.
I'd go further to suggest that all the little things that seem subjective were actually tools to just separate/distinguish one group of people from another, especially those who are out of reach of traditional punishments. Those subjective branches must inevitably be tied to procreation and furthering our survival.
Edit: To touch on @fire7side slavery, agency is critical in economic growth, especially in the absence of an all powerful super computer which could assign us tasks and spark exponential growth. There was just no way any slaving economy was going to compete with the free ones. Early on the North was unable to really punish the South, forcing the population to splinter on moral grounds. I'm suggesting slavery and other morals, which could be argued subjective, objectively grew out of a need to divide, self-distinguish (we are social animals after all), and survive.
