Well, we've heard from Rosseau (man in nature is basically good), from Hume (life is nasty brutish and short, competition and dominance determining all), from the Yin and Yang of eastern philosophy, and maybe a hint of Judeo-Chrisitian tradition of original sin. (Note to Skuzzy - talking about the philosophical tradition, not the religion - staying clear of rules!)
There is a very long tradition originally called "Natural Law" but now morphed into many related ideas - like for example "human rights." The basic idea is that inside each of s there is a sense of right and wrong that is separate from the bare instinct that makes the lion behave the way he does. This sense of right and wrong is universal, understandable, and intrinsic.
Those who disagree have to address some problems with the alternatives
1. If there is no intrinsic sense of right and wrong, if everything is relative:
a. What exactly is missing in true psychopaths, who do not seem to have the ability to tell right from wrong
b. If morality is cultural convention, how can we have grounds to criticize cultures that we would like to think are immoral? "Immoral" behavior that is agreed on by an entire culture would have to be by definition moral for them, and thus every bit as valid as our current system.
2. If humanity is basically good, well, please explain the world in general!
a. People on their own do not behave in moral fashion. Cue "Lord of the Flies" and "Heart of Darkness". Ignoring literature, look at Somalia and other failed states. Without cultural restraints, well, it's not pretty. If you disagree, lets see some counterexamples.
b. Man in nature is not idyllic - Margaret Meade's studies in the early part of last century are now understood to have been distorted by expectations. Deeper, repeated studies of "unspoiled" cultures show all the greed, selfishnes, and incest that was supposed to be the result of horrible modernity