[Voronwe]

It is important to distinguish between the Marring (or fall) of Arda, and the Fall of Man. They are quite different things in Tolkien's work (unlike in Christianity where they are equivalent), though both were the work of Morgoth. Possibly Tolkien felt this distinction was required because there were other races in Arda besides men - all races live in a fallen and imperfect world, yet an independent fall of man is necessary for consistency with Tolkien's own beliefs.

The Marring of Arda seems pretty much to be a direct result of Melkor's own Fall during the Music of the Ainur. Arda was marred during its creation (or before, since in the Music only the uncreated vision was marred), and marred further by Melkor dissipating his evil through its substance. This concept of the corruption of Arda by Melkor was present in some of the very earliest work of Tolkien (the pre-1920 'Music of the Ainur') and underlies much of his mythology, though he only seems to refer to it explicitly in later writings.

The Fall of Man is a more complex and less certain issue. A myth of the fall exists - the 'Tale of Adanel' which is part of the Appendix to the Athrabeth Finrod ah Andreth. Tolkien stresses that the legend may or may not be true, though he does say that it is 'no doubt mainly derived from actual lore of the People of Marach'. It is certain that a Fall of Man took place in Middle Earth (from other sources), and this is the only legend of the fall Tolkien gives. It differs from the Biblical story of the fall in a number of respects.

In the 'Tale of Adanel', Men fall because, corrupted by Morgoth, they turn away from Eru, who is to them a 'voice in our hearts', and begin worshiping Morgoth as a god. As I see it, the actual moment at which the fall occurs is this - when Men build Morgoth a temple and bow down before it:

Then in fear we spoke as he commanded, saying: 'Thou art the Lord; Thee only we will serve. The Voice we abjure and will not hearken to it again.'
'So be it!' he said. 'Now build Me a house upon a high place, and call it the House of the Lord. Thither I will come when I will. There ye shall call on Me and make your petitions to Me.'
And when we had built a great house, he came and stood before the high seat, and the house was lit as with fire. 'Now,' he said, 'come forth any who still listen to the Voice.'
There were some, but for fear they remained still and said naught. 'Then bow before Me and acknowledge Me!' he said. And all bowed to the ground before him, saying: 'Thou art the One Great, and we are Thine.'

In a sense Tolkien's Fall of Man is much deeper and more tragic than the Biblical one. The Fall of Man in Genesis is merely, as Milton put it, "man's first disobedience", whereas in the Tale of Adanel the entire race of men reject Eru and take Melkor as their god, though some do this only out of fear.

What then are the consequences of this fall? In both Genesis and the 'Tale of Adanel' the consequence of original sin is death, though, as I see it, there is a difference between the two in what this means. In the 'Tale of Adanel', Eru speaks to the fallen men as follows:

'Ye have abjured Me, but ye remain Mine. I gave you life. Now it shall be shortened, and each of you in a little while shall come to Me, to learn who is your Lord: the one ye worship, or I who made him.'

In brief, I see man's unfallen state in Arda as having free will with regard to death - the original Gift of Eru was for men to live in Arda for as long as they desired, and to choose the time of their leaving it. However, after the fall Eru limited men's lifespan, taking away their freedom of choice concerning death, since having been corrupted by Melkor men feared death rather than saw it as the gift it was intended to be. This was in part a punishment for the fall, and, perhaps, in part an act of mercy, since even men completely under the shadow of Morgoth would eventually die and be released, to go to Eru and learn the truth.

Evidence that men, in Tolkien's work, had free will with regard to death before they fell is provided by the following rather interesting passage (again from the Athrabeth appendix):

The passing 'oversea', therefore, of Mortals after the Catastrophe - which is recorded in the Lord of the Rings - is not quite the same thing [as the passing oversea of the Elves]. It was in any case a special grace. An opportunity for dying according to the original plan for the unfallen: they went to a state in which they could acquire greater knowledge and peace of mind, and being healed of all hurts both of mind and body, could at last surrender themselves: die of free will, and even of desire, in estel. A thing which Aragorn achieved without any such aid.

A little more discussion of what I just mentioned can be found on this rather dusty thread.

To this discussion I would add one more point: as I see it (though there will be many here more knowledgeable on the subject than myself) in Christianity with the fall comes the possibility of redemption, through Christ. So too in Middle Earth: in the Athrabeth Finrod believes that the only way Melkor's Marring of Men and of Arda can be healed is through the incarnation of Eru Iluvatar. Finrod also speaks of 'Arda Remade' - not merely an unfallen world, but something greater still, accomplished through men. This Arda Remade takes Melkor's Marring of Arda by Melkor and turns it back on itself, making good out of evil. From Ainulindalë:

And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.'

Which, I think, sums it up very poetically.