The first step was the “social instincts.” In ancient times, loners were more likely to get picked off by predators than were their more gregarious siblings, who felt a strong need to stay close to the group. The second step was reciprocity. People who helped others were more likely to get help when they needed it most.

Richard Dawkins, The Selfish Gene, explains more about life (including human behavior and myself) than anything else I’ve read.

To tell the truth, if I had been a stag or a Brazilian macaque, the question wouldn’t even have arisen: the first action of a male mammal when he conquers a female is to destroy all her previous offspring to ensure the pre-eminence of his genotype. This attitude had been maintained for a long time in the first human populations.