Both Paula and James raised the question of why sin couldn't have come into the world without a literal Adam and Eve and a literal fall in Eden -- why it couldn't be the result of evolved humanity exercising a God-given free will. Again, it seems a reasonable idea on the surface: but the moment you start to examine the details the whole thing breaks down. The ability to exercise free will is by no means evil in itself, to be sure, so God could give human beings free will without being the author of sin. But if we propose that sin has come into the world merely as the result of evolved humans choosing to do wrong on an individual basis, with no creatorial Head such as Adam represents in the Biblical narrative, we are left with the following very serious unanswered questions:
1. Why do all human beings, starting at the very youngest age, do wrong things without any coaching whatsoever, but have to be taught and encouraged to do good? Why is selfishness our "default mode", as it were, whereas good and unselfish behaviour requires conscious effort (effort that fails as often as not, or at least doesn't go as far as we would like it to)?
As a mother of two children whom I love dearly, as well as someone who vividly remembers her own childhood, I can readily attest to the fact that it is very easy for "sweet, innocent" children to be rude, unkind, disobedient to authority (even wise and compassionate authority), to harbor evil thoughts and say cruel things to each other -- and to do this knowingly and willfully, with no doubt in their minds that their behaviour is wrong. Even now that I am a well-brought-up and socially adjusted adult who knows better than to throw a tantrum in the street or hurl sharp objects at people who annoy me, I nevertheless still find it far easier to do evil than to do good -- I'm just better at hiding my sins from others.
If God created us (or rather, allowed us to evolve) with no innate tendency toward evil, only the basic power of free will, it should be an easy fifty-fifty proposition as to whether we do evil or good, and we should be readily able to choose good all or at least most of the time. But this is manifestly not the case, as anyone who tries to do only good and no evil, even for just one day, will soon discover. We read many stories in the Bible of noble and godly men such as Noah, Abraham and David, who did great things for God -- but they also committed great sins, with grievous consequences for themselves and those around them. As the psalmist wrote and the apostle Paul echoed in the epistle to the Romans, "There is no one righteous, no, not one."
The idea of human beings having evolved free will and choosing to do good or evil on an individual basis doesn't answer or even address this perennial moral problem. Only if humanity sprang from a single human couple created in a state of innocence (thus allowing God to declare them truly "good" in the beginning, not merely "potentially good if they choose to be") who then used their God-given free will to sin against Him and so corrupted both themselves and all their future offspring in the process, do we have an answer to the question of why we human beings are sinners by default, as it were, and why moral behaviour is such a concentrated (and frequently thwarted) effort by contrast. Only then do we have the necessary guarantee that human beings are not as God created them, and that God Himself is not responsible for nor is He indifferent to our present sinful state.
But there's another very important question left unanswered if "sin" is supposed to be just the result of evolved man exercising his God-given free will for evil instead of good, and it's this:
2. What about evil that is not the direct result of man exercising free will -- natural disasters, disease, "nature red in tooth and claw", etc.?
If we look around this world, we see much that is beautiful, much that is breathtaking, much that gives the appearance (misleading or not) of complex and intelligent design. But there is also much evidence of decay, suffering and death in the natural world. If God set evolution in motion in such a way as to produce the result we see in the world today, then He would be neither loving nor righteous. Only if God created the world in a state of perfection would He be justified in calling it, or Himself, "good".
So then, how did the world get from a state pleasing to God into the obviously imperfect state in which it now exists? Why do we have cancer, tsunamis, schizophrenia, colonies of monkeys who chase down other monkeys and rip them apart for sport? How did death -- something we all instinctively know to be an evil and seek to avoid, however we may try to whitewash it with various philosophies -- become an inescapable part of creation? Again, there is no satisfactory explanation for this if the world as we know it was simply set in motion by God and allowed to evolve to its present state. Only if at some definite point in history something went suddenly and cataclysmically wrong with creation do we have an answer, and Genesis 3 provides us with that answer.
Again, I am not claiming to have a timeline for creation, or to understand all the processes by which God worked and continues to work in bringing life into the world. Where the Bible specifically touches on these subjects I believe it to be factually and historically and scientifically reliable: but it addresses these matters primarily in the context of revealing God's character and expounding on man's relationship to Him, and so does not provide us with an exhaustive scientific explanation. Nevertheless, I can see no means by which the obvious realities of human moral weakness and a corrupted, decaying world can be explained apart from a literal father and mother of all humanity, a literal historical point at which innocent man was presented with the choice to do good or evil, and a literal fall into sin which affected not just human biology and spirituality but the whole of the created world of which humanity was Head. And that means reading and interpreting Genesis 1-3 just as the book itself invites us to do -- as a simple, straightforward historical account of what really happened.
I have more to say in response to Paula's comment in particular, but the reliability of the gospels and the question of whether Jesus actually spoke the words attributed to Him is another subject for another time.