Borders can be concrete walls or intercellular membranes. In either case, disgust elicits feelings of maintaining one’s purity, of keeping others out.
Microbes killed millions more of our ancestors than did Tigers and Bears. Whoever can keep themselves and their children from being exposed to illness, wins the evolutionary game. Disgust is judging carefully about people. Is he dangerous? Is she dangerous? That is, our bodily interactions structure how we think about social issues.