[notjoedimaggio]

One theory would be that humans have a biological sense of other. In-group, out-group, etc. People fight for the same resources, preferentially for their genetically similar groups.

If there are 10 people at a table about to starve to death, and only 5 of them are related, it helps their genetic lineage to prefer to feed relatives.

Racism, as an outgrowth of us-versus-them, most likely helped with keeping a lineage alive.