Dennis Krebs and Kaleda Denton say some interesting things about the rise and development of morality.*
We can distinguish three broad phases.
Phase One. Primates and archaic humans possessed a primitive sort of morality:
Within groups, dominant members constrained the selfishness of subordinate members, and subordinate members joined forces to constrain the selfishness of dominant members. Mothers sacrificed their interests for the sake of their offspring, and family members also may have helped one another in times of need. In addition, archaic humans probably engaged in primitive forms of reciprocity and mutualism, with some cooperation among members of groups for mutual defense.
Phase Two. Human foragers shifted towards a more cooperative morality involving egalitarianism, sharing meat, suppressing bullies, and punishing cheaters.
They uphold equality and fairness through preaching and practice. Even simple boasting is not tolerated. They punish social deviants, bullies, and free-riders by scolding, shunning, ostracizing, and even killing them.
Why this development of cooperative morality? Several mechanisms could have been involved:
kin selection (which may induce individuals to help non-kin who resemble kin), sexual selection, direct reciprocity, indirect reciprocity, mutualism, group selection, and cultural docility—[and] social selection, including the “double whammy” of selection by reputation and group punishment of free-riders.
“Social selection” means a fitness advantage to the more cooperative:
individuals who inherited the ability to internalize rules and exert self-control would have fared better biologically and genetically than those who did not, mediating the evolution of conscience.
Group selection probably also was involved: groups sharing meat and suppressing bullies and cheaters would have done better than other groups.
Phase Three. In more recent times there has been further evolution of morality, in (at least) three key ways:
most modern societies are less egalitarian and more hierarchical than hunter and gatherer societies are. Second, modern humans do not share their food or resources equitably among members of their groups. Third, contemporary humans behave in significantly kinder and more generous ways than hunters and gatherers do with respect to members of out-groups and other animals. For example, modern humans invest in animal rights movements, donate to charity, and help strangers from distant parts of the world.
Why these changes have happened remains to be explained.
*Dennis L. Krebs and Kaleda K. Denton, How Did Morality Originate? Evolutionary Psychology 2013. 11(1): 9-17.
which is a review of:
Christopher Boehm, Moral Origins: The Evolution of Virtue, Altruism, and Shame. (Basic Books, 2010).