Humanity's Defection
Each of us has indulged an inclination to wrongdoing.
The only plausible explanation for this is that Humanity has abandoned the devotion to God that provides our proper ultimate goal.
Writers about the human condition usually disregard or evade wrongdoing, and of all the classic literature of mankind, only the Bible gives this fact its proper weight. Other old writings may seem to accept it, but in fact they deny the full depth of man's universal wickedness. They consider that a man can redeem himself, usually by good deeds that make amends for the bad ones, or by finding goodness within ourselves. Only the Bible (implicitly in the Old Covenant, explicitly in the New) makes clear that there is no hope in such innate resources.
On the cause of man's defection, this is a pseudo-question. The only cause of such an event is its perpetrator. We can only confess that it happened. The relationship between God and man is not mechanical, it is (more but not less than) personal, and the words that best describe it are personal. Not every event is caused by previous events.
See Satan on the idea that it was caused by a prior supernatural defection.
See Happy Fault on the theory that it was caused by God.
On the characteristics of man's defection, see The Characteristics of Man's Defection.
On the circumstances of man's defection, see Adam and Eve.
On the excuses for man's wickedness, see Excuses for Wrongdoing.
Other Species of Humanity. If in addition to homo sapiens there are other human species, they are involved in the same defection. (And, to anticipate the Restoration section of this Creed, they will be saved by the same restoration. That the man who is God is homo sapiens is of no significance.)
Other Selfkinds. If there is any other selfkind, it is not involved in our defection and I see no way to guess whether it did or will defect. (Either way, to anticipate the Restoration section of this Creed, God will bring them to fulfillment like us.)