Egalitarian Drift
Hypothesis: When there is a social circumstance of injustice within a particular human community, that community will tend toward correcting that particular injustice.
Egalitarianism, the feeling that all people (one feels empathy for) deserve equal treatment under our social contract, is the basis of what we call justice.
A sense of justice would seem to be a core facet of human nature.
A sense of justice for oneself could be easily explained by looking at our own animal nature. The urgency of self-survival is powerful.
But that sense of justice clearly extends beyond just one's self. This sense of justice extends to all beings one feels empathy for. Anyone viewed as lacking this empathetic sense of justice is considered an anomaly and a sociopath.
The egalitarian nature of humanity may run in conflict to the also very strong sense of self-preservation. It is not clear that one is stronger or more important than the other.