Are human beings intrinsically good but corruptible by the forces of evil, or the reverse, innately sinful yet redeemable by the forces of good? […]
Until about three million years ago the ancestors of Homo sapiens were mostly vegetarians, and they most likely wandered in groups from site to site where fruit, tubers, and other vegetable food could be harvested. Their brains were only slightly larger than those of modern chimpanzees. By no later than half a million years ago, however, groups of the ancestral species Homo erectus were maintaining campsites with controlled fire — the equivalent of nests — from which they foraged and returned with food, including a substantial portion of meat. Their brain size had increased to midsize, between that of chimpanzees and modern Homo sapiens. The trend appears to have begun one to two million years previously, when the earlier prehuman ancestor Homo habilis turned increasingly to meat in its diet. With groups crowded together at a single site, and an advantage added by cooperative nest building and hunting, social intelligence grew, along with the centers of memory and reasoning in the prefrontal cortex.
Probably at this point, during the habiline period, a conflict ensued between individual-level selection, with individuals competing with other individuals in the same group, versus group-level selection, with competition among groups. The latter force promoted altruism and cooperation among all the group members. It led to group-wide morality and a sense of conscience and honor. The competitor between the two forces can be succinctly expressed as follows: within groups selfish individuals beat altruistic individuals, but groups of altruists beat groups of selfish individuals. Or, risking oversimplification, individual selection promoted sin, while group selection promoted virtue.