Morals aren't that complicated once you define the metric that is used to seperate moral actions from immoral actions.
As Simmon already brought up, we are a social species. Societies that work together - survive. Societies that don't work together - don't survive. Things like Altruism, Cognitive Empathy, and Theory of Mind are not unique to humans. We also see these in other social speices like dolphins, elephants, chimpanzees, and dogs.
So what is the metric universal metric? Well, things that promote human and animal well being - are good. Things that cause human suffering and harm - are bad. This is the basic metric used to determine the morality of actions (without having to appeal to outside sources like law, holy books, parents, etc.). Of course, all of this begs the question, what is human well being? Well, we can start with some very basic prinicples like; life is generally preferable to death. Pleasure is generally preferable to pain. Fairness is generally preferable to bias. Etc.
One could always argue that there is some outside source that informs ethics but the differnce is the things I just listed are universal and innate (assuming you have the neccissary mirror neurons and a developed pre-frontal cortex).