An evolved man does not necessitate no Garden and no original sin. You could still conceive of God taking Adam and Eve and setting them in a literal garden. You could also interpret that as a metaphorical garden in regards to their Pre-Fall innocence. And original sin is not thrown out either because, let’s face it, we know original sin happened. That fruit, literal or metaphorical, was bitten whether man evolved or not.
It does in a very specific way: Original Sin is understood as a perfect creature (or more accurately, without flaw) choosing evil over God. It is not the same species of sin as any sin we can commit, because we are slaves in bondage, whereas the Original Sin was the sin of a free man who willingly chose bondage. The physical reality of this is our concupicence, our passions and emotions are not under our control. In this, we now resemble the animals more than the original ideal.
The sin of Adam requires a knowing sublimation of the perfectly ordered intellect and will to the corruption of sin, and
cannot be the act of an imperfect creature. Adam being born of a proto-human would theoretically possess all the natural qualities of that creature: unruly passions, fear of pain, disordered love of pleasure, a corruptible body, etc. He would have been incapable of the Original Sin.
We know this because the sins of the “modern” father are not imputed to the son. No sin immaginable in the fallen man, no matter how heinous or deliberate, can change the will of another man without his consent. We cannot think of Original Sin as simply " the first instance of sin" but as the
origin of sin itself. It permanently altered the state of will in mankind. It could not be committed by a man of our nature, because our nature is already changed. Therefore, Adam must have originally had a nature different from ours.
Whether the Fruit is exclusively allegorical or exclusively literal (I tend to think of it as both) is irrelevant. The true nature of the sin was not in eating forbidden fruit, but in the sublimation of the will toward a thing that was not God. That requires an unbound will.
So with evolution we have many problems. First we have to reject the dignity of man (he was not made from dust but born in a bloody, painful mess from a screaming monkey; the result of a probably forced copulation), and then we have to assume a totally corrupted, mortal, and irrational creature beget a incorrupt, immortal, and perfectly rational creature. And then we have to assume that once Adam was born, he was either whisked away to a perfect Garden of abundance without death and where all beasts were submitted to him, or that the entire nature of the world was changed upon his birth and then changed back after his Fall.
Or we have to believe that the Perfect God whom cannot decieve created an evil, corrupt thing. Which is so absurd as to border on blasphemy.