I'd like to argue that not only is there no conception of original sin given the fact of human evolution, but moreover, that the cards were stacked against humans from the get-go, by simply looking at the conditions we all evolved in throughout the last 13.7 billion years since the rapid inflationary period (commonly referred to as the "Big Bang"). This is inconsistent with observations about nature, and hence is a contradiction.
The Genesis mythology has a few core elements. Firstly, that God created everything in a perfect and pure state. Secondly, that humans deliberately spoiled it by disobedience of God's will. This is a critical element of Christianity. Without some form of "sin", then there is absolutely no reason for Jesus to exist to begin with, since the existence of Jesus is to remove sin.
Now, let us come back to our reality. The universe underwent rapid inflation 13.7 billion years ago. Since then, it's a pretty inhospitable place. Collisions of huge objects, explosions, deep cold, extreme heat, almost nothing in the middle. Despite these stacked cards against the creation of an environment susceptible to evolution of life, it did manage to evolve at least once (obviously). However, remnants of the dangerous place we live in are everywhere. Living a few miles above our heads, we would die (freezing to death or suffocating). Living a few thousand miles below our feet, we would die (burned to a crisp or being crushed by enormous pressures). We've been struck by countless asteroids that, to say the least, are really good at killing just about everything on the planet at the time.
Into this little pocket of habitability (in both time and space), we humans evolved. The theists among us will claim that this was an act of God. However, think of the enormity of that statement. That 13.7 billion years after the apparent "creation" (according to theists), after millions and billions and trillions of catastrophic collisions between massive objects, supernovae, pulsars, neutron stars, etc, in a geological blink of an eye we humans, the pinnacle of all creation (according to theists), the actors in the divine comedy, were created.
Then, 95% of human existence proceeds without any further information from God that is ever recorded. After 95% of human existence is already done, this God reveals himself to a select tribe of woe-begotten goatherders in ancient Palestine. Another few percent of human existence goes by, and he comes down to earth himself to spread the news, and leaves no first-hand evidence whatsoever, but several secondary-source books are propagated throughout human civilization in the remaining few percent of our existence.
Not only that, but presumably, before we go extinct, humans will then actually be introduced, in the flesh, to the incarnation of God yet again. Not only that, but he's going to be PISSED OFF at the fact that we haven't been following his dictates since his last visit, and will separate us based on... well... SOMETHING (varies by Christian). But whatever it is, it's supposed to be bad, so we're supposed to just gamble that it's better to be Christian than not. Or something.
This all sounds monumentally implausible already. But let's presume that every word of it is true, for the time being. *cough*.
Why on earth would an all-powerful, benevolent deity put us in such a state to begin with? This contradicts one of the central tenants of the Christian religion, that God created a perfected environment for us that we screwed up. There never WAS such a perfect environment. In the rough-and-tumble, shit-assed universe that we've evolved in, for most of our history we were not even the top of the food chain, much less part of some divinely inspired "plan". Observations simply contradict that the universe was perfect before we screwed it all up.
Now what about those Christians who believe that there was never such a perfect state, but our subsequent sins are why Jesus came about? To be honest, while it may seem that these are very forward-thinking theists, it turns out that it's even worse of a cessation of reason than a literal reading of the Bible. If the world was never perfect, then is it realistically to be expected that human beings, evolved from a common ancestor with chimpanzees (and fish, and algae, etc), have any capacity at all about determining what is "right" and "wrong" according to an uncommunicative invisible deity? The genes that this God allegedly gave us make us do one thing and one thing only: reproduce them. That's it. We are only as "good" or "evil" as our genetic makeup predisposes us to be.
Now, in the course of human history, we have evolved into a societal structure. In such a group, it turns out that the message of reciprocal altruism is well-received by our genes. They have successfully reproduced for millenia according to this strategy. From this principle of reciprocal altruism, most of the morality we consider follows directly. It is simply more beneficial for us, as a society, to cooperate than it is to compete. Far from being "invented" by Jesus in the Golden Rule, it is an evolutionary dictate in our brains to do this (and not only OUR brains, also the brains of other animals!). We are even starting to understand the mechanism (for instance, see the developments on mirror neurons).
So let's recap the score: the idea of even more "progressive-thinking" Christians is that God created the universe, let the rough-and-tumble go for 13.7 billion years, finally managed to get a spot for us to evolve, evolved us from the animals that already existed, somehow communicated that despite all of our evolutionary impulses for one thing or another, we should instead follow the dictates of him instead. Those dictates were written down in the last few thousand years of history, and then somehow if we don't follow these, we're going to experience "something bad".
This is a direct contradiction to the idea of a benevolent deity. There is no conception of "benevolent" where this makes any remote sense whatsoever. We are then back to Epicurus:
Is God willing to prevent evil, but not able? Then he is not omnipotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Then whence cometh evil? Is he neither able nor willing? Then why call him God?