Actually, the Hebrew in Gen. 2:7 says "the man" not "a man". And in Hebrew, as in many other languages, the definite article often indicates a type rather than an individual. It is like saying "The LORD God formed Man."
(English is a notable exception to this rule. We do the opposite and leave out the definite article when speaking of a whole class or type.)
References to people in a story don't mean the story is historical unless you have other reasons to show they are.
Allegory is just one kind of non-historical writing. Or did you mean to include all kinds of non-historical writing in this term?
No one is implying Genesis is not true. Do you think everything non-historical is not true? e.g. Jesus' parables?
We are all trying to listen to what God said. Some of us believe that God gives us some teaching in stories---just as Jesus did. (Jesus said he only did what he saw the Father doing.)
The concept of outer space is not biblical either. Does that mean it's not true?
btw the creationWiki article is meaningless. While all the processes he names produce Light, none of them produce Day. And Genesis is quite clear that the Light created on Day 1 is also Day and that each day from day 1-3 has an evening and a morning. This is quite impossible without the sun.
This was noted by the Church Fathers who took it to mean that Genesis 1 is not intended as ordinary history. Origen (185-254) considered a "historical" view of Genesis "foolish". As he says in his De Principiis:
What intelligent person can imagine that there was a first day, then a second and a third dayevening and morningwithout the sun, the moon, and the stars? And that the first dayif it makes sense to call it suchexisted even without a sky?
Who is foolish enough to believe that, like a human gardener, God planted a garden in Eden in the East and placed in it a tree of life, visible and physical, so that by biting into its fruit one would obtain life? And that by eating from another tree, one would come to know good and evil? And when it is said that God walked in the garden in the evening and that Adam hid himself behind a tree, I cannot imagine that anyone will doubt that these details point symbolically to spiritual meanings, by using an historical narrative which did not literally happen. (p.71)
Some people today suppose that it is only because of modern science that the historical view of the bible is called into question. But this is not the case at all. Early Christians, and also Jewish commentators contemporary with them, often found it more sensible to treat these passages of scripture non-historically. To Origen, it was obvious that Genesis 2-3 was not a literal history just from the content of the story.
As sfs says, where else would you read about a talking snake, a man named Man and a tree that gives life and suppose that it is historical? Why should we have problems thinking that scripture is not being historical here?
The question of literary style and the question of history are two different questions that are sometimes improperly rolled into one----as if history could never be told in poetic form, for example. But in ancient times it was quite common to put history into poetic form (because it had to be remembered orally before it was put in writing and poetry is easier to remember.)
There is a good deal of poetic style in Genesis 1:1-2:4, but that would not mean it is non-historical. Genesis 2:4ff is not poetic, but that doesn't mean it is historical.
So, literary style doesn't really tell us when something is intended as history.
What do you think would be a good clue for knowing when something is intended as history?
This is factually incorrect. Charles Darwin was never an atheist. He teetered a bit between theism. deism and agnosticism after he left the church, but he never embraced atheism. He also never intended his theory to be anti-theist.