Right... prior to 400 bc. No one had any concept of "right, or wrong". You hold on to that fantasy. Dont let anyone take that from you...
Incorrect, hunter-gatherer tribes such as the Sentinelese, dating as far back as 80,000 BC have a culture and system of values, order, hierarchy, and so forth, such as offering burials of the dead of those they kill in warfare; it's possible that they are more "informal" than "formal" in many cases.
How they would compare to the values within more contemporary legal systems and institutions, I am unsure. (Much as I'm not aware of anything in the Bible, or in the teachings of Jesus and so forth which reduce morality to what is "taught' directly from the Bible, much as moral and immoral behaviors documented in men and women in the Bible itself proceeded the actual writing of the Bible to begin with; this is most likely just an innovation, or archaic "tradition" of man, such as found in 19th century "legalism" and whatnot - I don't believe most serious theologians believe the Bible itself is the "only" source of morality, much as the source of morality mentioned by the individuals in the Bible itself didn't come directly from the Bible; no Bible existed when Adam and Eve had the free will to defy God, for example.
As far as evolutionary biology is concerned, it is observed that "premoral sentiments" in animals may have played a role in the biological basis and evolution of morality in human cultures, such as ant colonies having systems of class, labor division, social roles, and so forth. No modern legal system denies that passions play a role in human behavior, moral, or immoral, just as those thinking men and women who built civilizations and established laws to begin with, obviously had higher moral and rational purposes and faculty than average or ordinary men and women did, or else there would be no civilizations to begin with.
Regardless, morality in human cultures is not "reducible" to biology, but higher notions such as intentions, rational thought, motivations, and self-restraint play a role in human morality and moral philosophy.