Some folks argue that the morality we see in the world is a result of humans evolving from lower life. And as they have been able to communicate with each other they have made clear what is right and wrong in their relationships. [What I find amazing about all this is that the randomness as to how the world came to be (as many atheists assert) could form any kind of coherent logic for morality and agreement between individuals]. Here’s another quote from Lewis:
Now I do not deny that we may have a herd instinct: but that is not what I mean by the Moral Law. We all know what it feels like to be prompted by instinct – by mother love, or sexual instinct, or the instinct for food. It means that you feel a strong want or desire to act in a certain way. And, of course, we sometimes do feel just that sort of desire to help another person: and no doubt that desire is due to the herd instinct. But feeling a desire to help is quite different from feeling that you ought to help whether you want to or not. Supposing you hear a cry for help from a man in danger. You will probably feel two desires – one a desire to give help (due to your herd instinct), the other a desire to keep out of danger (due to the instinct for self-preservation). But you will find inside you, in addition to these two impulses, a third thing which tells you that you ought to follow the impulse to help, and suppress the impulse to runaway. Now this thing that judges between two instincts, that decides which should be encouraged, cannot itself be either of them. You might as well say the sheet of music which tells you, at a given moment, to play one note on the piano and not another, is itself one of the notes on the keyboard. The Moral Law tells us the tune we have to play: our instincts are merely keys (C.S. Lewis, Mere Christianity, 1952, p. 22).