The Original Sin comes back to the Genesis Creation Myth in which Adam and Eve in the Garden of Eden ate an apple from an evil tree (the reason why it was there in the first place isn't fully explained). Anyway the tree had - guess what - the devil in disguise as a talking snake (again, reason for him being there in the first place isn't explained). The snake convinced Eve to eat an apple because all she wanted was to know right and wrong. Since God told them not to, he had them clothed and kicked out of the Garden of Eden as well as giving Eve (the representation of all women) the pain of birth. Well wanting to find out what is right and what is wrong is terribly sinful and that one sin then reverberated and affected all of humanity. That one sin caused God to commit global genocide years later, flooding the world and killing every man, woman, child and animal except for Noah, his extended family and what could fit onto the ark.
Plus, what exactly did Adam and Eve do? They didn't shoot the snake and steal his skin. They ate fruit off a tree that God told them not to eat from. Then it made them more intelligent, which made God angry (didn't he create everybody in his own image?). But still, all they did was disobeying God, because of their curiosity. Even the most careful Christian is disobeying God at some point, because the Bible has so many contradictions that it's impossible to keep track of them. Curiosity isn't a sin. It's a quality of being human God should have thought about before he made Adam and Eve.
Since the existence of Adam and the whole Genesis event was disproven by the impressively intelligent Theory of evolution, the whole concept of Original Sin is wrong and the story doesn’t make much sense anyway. Many modern mainstream-Christians who believe in evolution claim that Genesis was simply symbolic, which pretty much makes the sacrifice of Jesus pointless (though probably either way Christianity doesn’t make much sense).