Why do you think the bible has to tell you if it uses allegory? What makes you think it is intended historically? It doesn't say that either. You realise Jesus spoke in parables, isn't he the one who inspired the writers of Genesis? We are told Jesus often spoke in parables, but if you read the accounts of the parables, Jesus often told them without saying he was telling a parable. He expected his disciples to work it out for themselves.
At the same time there are plenty of indications it is speaking in a parable, as Willtor has pointed out, a literal Tree of Life does not make sense for Christians who believe everlasting life is only to be found in Christ, however it is a beautiful metaphor for the cross, or Christ the true vine. Then we have two contradictory timetables for creation in Genesis 1&2, which is a pretty strong indication the writers or editor did not intend them as literal history. Of course as we have seen Adam means Man or Mankind, another indicator we are looking at a parable or allegory. Adam is not only referred to as 'him' a single individual, but 'them', which is what you would expect from allegory. Then we have the promise of a Redeemer who was is going to bruise the snake's head. Yet the gospel accounts of the crucifixion Jesus never mention stepping on a 6000 year old snake. The promise was fulfilled, but only if you understand it as a metaphor for Jesus defeat of Satan.
Then we have figurative interpretations of Genesis in the NT, Paul interpreting Adam as a figure of Christ Rom 5:14, or interpreting one flesh in Gen 2:24 as a ' profound mystery' speaking of Christ and the church Eph 5:32. Of course you can have allegorical interpretation of historical passages too Gal 4:24, but it certainly tells us the figurative meanings of Genesis are important.