"The occasion of the disobedience was brought about by the deceiver. He first cast doubt upon the Word of God: Genesis 3:1 "The serpent...said unto the woman, Yea, hath God said...?" He then flatly denies God's Word: Genesis 2:4 "Ye shall not surely die..." Eve was decieved, agreeing with the deceiver, and ate of the tree that God had commanded them not to eat of."
It is the believing of this 'doubt' (i.e. the doubting by mankind in the truthfulness of God's word), cast by the deceiver that was, in my opinion, mankind's 'original sin'. The 'act' of eating the fruit and thus disobeying God's command came only AFTER mankind (Adam and Eve) began to doubt God's word and thus allowed for the deception to occur in the first place.