Describe the theme of animalism in The Road by Cormac McCarthy.

One of the key themes in Cormac McCarthy's postapocalyptic novel is the thin line that divides human beings from other animals. The man repeatedly assures the boy that they will remain on the right side of this line: "we're the good guys."
Because the unidentified catastrophe has wiped out flora and fauna except for humans, the remaining people eat almost exclusively canned food when they can find it. Other surviving humans, however, have resorted to cannibalism to survive. To the man, this behavior would be worse than death. The boy agrees—if the situation arises and their gun has a bullet—to kill himself rather than eat another person.
Although they escape a close call, escaping from a strange man who grabs the boy, they are elsewhere confronted with gruesome evidence that other people have descended to cannibalistic behaviors. Entering a house to scavenge inside, they discover that cannibals have been using it as a kind of human livestock pen. Rather than kill and eat a whole person, the captors have kept their victims imprisoned and alive, but they have cut off and eaten some of their limbs.
This sort of premeditation and planning show that these predators are all too human, not simply another type of animal that hunts and kills.