V. The Genesis myth
Any doctrine based on assumptions is inherently on shaky ground. Indeed, errors in the traditional Roman Catholic doctrine of original sin arise from each of the three assumptions (Section I above) on which it is based. We will consider here the first two assumptions, namely, regarding as historically and literally factual the stories of creation and the fall in Genesis 1-3. The consequence of the third assumption, that Adam and Eve were punished in a way that would affect all their descendants, is discussed in Section VII below.
The doctrinal status of original sin has been propped up in the Catholic Church by recent Vatican sanctions against non-traditional theologians. That status seems increasingly curious today because the story of Adam and Eve, in which occurs the first sin, progenitor of original sin, is widely accepted as allegorical, not historical. The Catholic Study Bible, for example, asserts:
All of Genesis 1-11 treats the creation of the world and the first events of human existence without any historical concreteness. ... Genesis 1-11 is almost entirely based on the myth genre. 
Myths are stories of our search for meaning; and the meaning in myths is allegorical or symbolic. Campbell points out that, to get the symbolic meaning, we should read other people's myths, not those of our own religion, because we tend to interpret our own religion in terms of facts. 
When we read the creation story of the Bassari people of West Africa, which has great similarity to the Genesis story, we can more readily accept it as myth:
One day Snake said, "We too should eat these fruits. Why must we go hungry?" Antelope said, "But we don't know anything about this fruit." Then Man and his wife took some of the fruit and ate it. Unumbotte came down from the sky and asked, "Who ate the fruit?" They answered, "We did." Unumbotte asked, "Who told you that you could eat that fruit?" They replied, "Snake did." 
Yet we regard as gospel truth our own story that the human race began "with two full-grown, Hebrew-speaking adults and a talking snake." 
Already in the first centuries of the Church, Gnostic Christians pointed out that a literal reading of the creation story in Genesis made no sense. For example, did Adam and Eve actually hear God's footsteps in the garden? (Genesis 3:8) And since they lived long after eating the fruit of the tree, did God lie when he said they would die? (Genesis 2:17) 
Augustine knew the value of interpreting Scripture allegorically -- something he learned from St. Ambrose.  In his discussion of Genesis 1,  Augustine acknowledged ignorance of the meaning intended by Moses and proceeded to do an allegorical exegesis himself.  However, his boundaries on allegorical interpretation extended only to the phrasing of the text and did not encompass the creation story itself. In spite of the illogic of literal interpretation, the Gnostics view of the creation story as allegorical was rejected along with other of their views which were deemed heretical.
The Catechism, while acknowledging that "The account of the fall in Genesis 3 uses figurative language," maintains that it "affirms a primeval event, a deed that took place at the beginning of the history of man." 
Schmaus raises the question of Genesis' historical accuracy in light of contemporary evolutionary theory:
Above all there is the question of whether Adam is an actual historical figure ... or whether he represents only a literary construction. Connected with this question there is the problem of whether the human race originated with a single pair (monogenism) or a multiplicity (polygenism) of first parents. It is rightly emphasized that the fact of original sin can be preached to modern man as worthy of belief only if what one must understand by original sin is expressed in a manner which he finds worthy of belief. It may justly be maintained that the Council of Trent, in the passage on the transmission of Adam's sin to all men, did not answer -- indeed, it did not even treat -- the question with which we are here concerned. This view is supported by the fact that ... Trent could hardly have taken a formal position on a question which did not belong to the problematic of that time but has emerged only lately, in connection with the idea of evolution. 
Scientific discoveries and scientific study of the Bible, primarily during the past century, have made a literal understanding of the early chapters of Genesis critically impossible. Much of the historical controversy about interpretation of Genesis arises directly from regarding the creation stories as historically factual and disappears when we see them as allegorical.
Various thinkers have attempted to reconcile the traditional doctrine of original sin with modern scientific understanding of evolution. For example, Trooster contends that the Genesis authors intended to convey only the doctrinal matter and never intended the texts to be taken as historical.  This reversal of affairs in the history of original-sin theology is without foundation. From Augustine to Aquinas to Trent to Pius XII, it was the assumed historicity of the texts that served as the basis for development and defense of the doctrine.
Such attempts -- to finesse the traditional doctrine of original sin to mean something else not so obviously untenable in light of the contemporary evolutionist view of man and the world -- are futile. An erroneous theology based on an erroneous worldview is not reconcilable with the reality evidenced by modern science.
A problem historically with theological development around Genesis 1-3 is that there has been too much theologizing. Great minds of the Christian era have taken hold of the Genesis story and invested it with complex theological imaginings that could never have occurred to the unsophisticated minds that originated the story. Aquinas observed:
[I]n the account of the Creation, no mention is made of fire and air, which are not perceived by senses of uncultured men such as those to whom the Scripture was immediately addressed. 
Looking at Genesis 1-3 through the eyes and minds of its unknown ancient authors, the underlying theological content has to be simple -- profound, but simple: God exists; God created the universe; God created man with intellect and free will; man knows good and evil; man is tempted and sins; man suffers and dies.
The authors of the myths that were appropriated for Genesis 1-3 originated them as their best primitive ways of conceptualizing the origin of the universe, of man, and of the human condition. They created myths which they believed to be true. Campbell has observed that the great civilizations everywhere have been prone to interpret their symbolic figures literally.  In like manner today, Native Americans regard their creation story as factual and consider evolutionary science and studies of human origins in the Americas as hostile to their traditional religion. 
Although the stories in the first 11 chapters of Genesis are now widely understood among contemporary Scripture scholars to be myths, the full import of that status is difficult to establish because of the heavy investment in the traditional doctrines developed and taught over the past two millennia. In particular, the acts of the mythical -- that is, never-existing -- Adam and Eve, the acts which constitute the fall, have established a life of their own. That life is invested most heavily in the Catholic doctrine of original sin and all its associated doctrines on salvation, grace, and baptism.
Adam and Eve's story can be no more historically true than the creation of the universe in six days, according to Genesis, or in the year 4004 B.C., according to James Ussher's Genesis-based calculation in 1650 A.D. Augustine put the age of the world at less than 6,000 years.  If Adam and Eve are themselves fictional characters in a mythical allegory, the events of their lives -- including their fall from grace -- are equally fictional. Thus, there cannot be a hereditary original sin because there never was, in reality, a "fall" by any human person on which to predicate such a punishment. The reality of sin in the human condition does not authenticate the fanciful myth of disobedience to an explicit command personally delivered by God to the first humans.
This necessary conclusion presents an interesting challenge, namely, discerning the significance of the inspired -- but mythical -- Genesis account. The challenge seems not so much to understand the theology as it is to understand the myth.