From C.S. Lewis's The Problem of Pain, chapter V, "The Fall of Man":
What exactly happened when Man fell, we do not know; but if it is legitimate to guess, I offer the following picture — a "myth" in the Socratic sense,1 a not unlikely tale.
1 I.e., an account of what may have been the historical fact. Not to be confused with "myth" in Dr. Niebuhr's sense (i.e., a symbolical representation of non-historical truth).
For long centuries God perfected the animal form which was to become the vehicle of humanity and the image of Himself. He gave it hands whose thumb could be applied to each of the fingers, and jaws and teeth and throat capable of articulation, and a brain sufficiently complex to execute all the material motions whereby rational thought is incarnated. The creature may have existed for ages in this state before it became man: it may even have been clever enough to make things which a modern archaeologist would accept as proof of its humanity. But it was only an animal because all its physical and psychical processes were directed to purely material and natural ends. Then, in the fullness of time, God caused to descend upon this organism, both on its psychology and physiology, a new kind of consciousness which could say "I" and "me", which could look upon itself as an object, which knew God, which could make judgements of truth, beauty, and goodness, and which was so far above time that it could perceive time flowing past. This new consciousness ruled and illuminated the whole organism, flooding every part of it with light, and was not, like ours, limited to a selection of the movements going on in one part of the organism, namely the brain. Man was then all consciousness. The modern Yogi claims — whether falsely or truly — to have under control those functions which to us are almost part of the external world, such as digestion and circulation. This power the first man had in eminence. His organic processes obeyed the law of his own will, not the law of nature. His organs sent up appetites to the judgement seat of will not because they had to, but because he chose. Sleep meant to him not the stupor which we undergo, but willed and conscious repose — he remained awake to enjoy the pleasure and duty of sleep. Since the processes of decay and repair in his tissues were similarly conscious and obedient, it may not be fanciful to suppose that the length of his life was largely at his own discretion. Wholly commanding himself, he commanded all lower lives with which he came into contact. Even now we meet rare individuals who have a mysterious power of taming beasts. This power the Paradisal man enjoyed in eminence. The old picture of the brutes sporting before Adam and fawning upon him may not be wholly symbolical. Even now more animals than you might expect are ready to adore man if they are given a reasonable opportunity: for man was made to be the priest and even, in one sense, the Christ, of the animals — the mediator through whom they apprehend so much of the Divine splendour as their irrational nature allows. And God was to such a man no slippery, inclined plane. The new consciousness had been made to repose on its Creator, and repose it did. However rich and varied man's experience of his fellows (or fellow) in charity and friendship and sexual love, or of the beasts, or of the surrounding world then first recognised as beautiful and awful, God came first in his love and in his thought, and that without painful effort. In perfect cyclic movement, being, power and joy descended from God to man in the form of gift and returned from man to God in the form of obedient love and ecstatic adoration: and in this sense, though not in all, man was then truly the son of God, the prototype of Christ, perfectly enacting in joy and ease of all the faculties and all the sense that filial self-surrender which Our Lord enacted in the agonies of the crucifixion.
Judged by his artefacts, or perhaps even by his language, this blessed creature was, no doubt, a savage. All that experience and practice can teach he had still to learn: if he chipped flints, he doubtless chipped them clumsily enough. He may have been utterly incapable of expressing in conceptual form his paradisal experience. All that is quite irrelevant. From our own childhood we remember that before our elders thought us capable of "understanding" anything, we already had spiritual experiences as pure and as momentous as any we have undergone since, though not, of course, as rich in factual context. From Christianity itself we learn that there is a level — in the long run the only level of importance — on which the learned and the adult have no advantage at all over the simple and the child. I do not doubt that if the Paradisal man could now appear among us, we should regard him as an utter savage, a creature to be exploited or, at best, patronised. Only one or two, and those the holiest among us, would glance a second time at the naked, shaggy-bearded, slow-spoken creature: but they, after a few minutes, would fall at his feet.
We do not know how many of these creatures God made, nor how long they continued in the Paradisal state.
“The Christian answer to the question proposed in the last chapter is contained in the doctrine of the Fall. According to that doctrine, man is now a horror to God and to himself and a creature ill-adapted to the universe not because God made him so but because he has made himself so by the abuse of his free will. To my mind this is the sole function of the doctrine. It exists to guard against two sub-Christian theories of the origin of evil—Monism, according to which God Himself, being “above good and evil”, produces impartially the effects to which we those two names, and Dualism, according to which God produces good, while some equal and independent Power produces evil. Against both these views Christianity asserts that God is good; that He made all things good and for the sake of their goodness; that one of the good things He made, namely, the free will of rational creatures, by its very nature included the possibility of evil; and that creatures, availing themselves of this possibility, have become evil.” (The Problem of Pain, “The Fall of Man”, p. 57)