The fact that the world in which McCarthy has humans live is one in which the struggle for survival is cut throat and one in which humans give little thought to killing one another could be good evidence to reflect a fundamentally selfish nature to human existence. However, I think that in finding the presence of altruism as reflective of human nature, one has to examine the relationship between the father and son. This is testament the basic decency and altruism that human beings possess.
The father looks out for his son in a self- sacrificing manner. He gives whatever he has, including his own life and spirit, to the boy so that he can live. The relationship between father and son represents altruism to an extent that it can serve as a haven in heartless world. While the man grows weaker each day, he does so by giving everything he has to his son, who ends up growing stronger each day. This represents how altruism is a part of human existence. The father's sense of altruism to his son reflects in the son's compassion and care he shows for others that they meeet on the journey. This is further evidence as to how the book argues that the fundamental nature in human existence is a sense of altruism and sacrificing for others.