Humans, Animals And Original Sin

Christian apologists claim that God created man in his own image and man initially was "Sin-free."  However, after "The Fall", humans were now capable of "Sin."  Besides the facts of evolution and that humans evolved as a group and not as a pair, science in recent years has showed us that our behavior is not that unlike our closest relatives.  Also consider that human and animal aggression are not that dissimilar, as well as the fact that there were OTHER human species that died out.

Now look at evolution and morality.  Science shows us that humans are more cooperative and altruistic than any of our closest relatives.  If this is the case, what did humanity really "Fall" from?