Hmm very interesting! So how could it have been, not as we know it?
IN MY OPINION: we must first establish and apply what we know about Adam & Eve and early creation. Which; although we know very little, there is no easy explanation and the debate could be lengthy. Therefore in my personal study I have formed an opinion of what the 'Garden' really was-and still is; and the nature of "humans" in the Garden before they were 'evicted' so to speak.
Once again: IN MY OPINION, The Garden is a part heaven or heaven itself / the 'new earth'. I come to this conclusion using two key scriptures: Genesis 3:22 & Revelation 22:2. Where is the 'tree of life'? Consider pre-flood earth was much different than post-flood earth due to the devastation of the waters. Consider that we are 'babes' even now in the Spiritual realm before our bodily death on this earth.
IN MY OPINION: we know next to nothing about heaven and 'the new earth' in the next life-but we do have some clues; for example: Matthew 22 & Luke 20:35 imply that there is no 'sex' in heaven-at least no procreation as we know it. Maybe that's because in the spiritual realm it is more exciting to 'create' in other ways we can't imagine? Maybe 'sex' was a lackluster substitute for what they turned down in 'the Garden'?
IN MY OPINION: We are always surrounded by heaven and 'new earth' under construction we just can't see it- UNLESS our eyes are opened by the LORD: Acts 7:55 see also: Daniel, Isaiah, Revelation, etc..2 Kings 6:17 is of particular interest as well as Numbers 22:31 on this matter.
We know that Jesus Christ is our Savior and the 'first' one of the resurrection. Being very careful here because 'resurrection' and being 'raised' from the dead are 2 different phenomena. Being 'raised' from the dead was done by Christ alone in our present physical realm. Being resurrected on the other hand is an unfathomable power of God in all the known universe-physical AND Spiritual. Are the two similar? Yes-but God did not 'raise' the dead on earth to eternal life-only to inhabit their former bodies. Being resurrected is being given a 'new body' devoid of wickedness in the afterlife. Why is this important? Because at some point if Adam & Eve started off in the 'Garden'; their bodies and souls would be transfigured some way or the other. So did they start off in their 'eternal bodies'? I don't know-but Genesis 5:5 says Adam died-so we know that he was in his 'earthly flesh' at that point in time.
Anyway-before I get off on too many human rational tangents...I think that is a good place to start.