The first-century Jewish theologian Philo may not have been the first Jew to reject the literal historicity of the creation stories, as it is my understanding that educated Alexandrian Jews had long understood the creation accounts to be allegories. According to Jean-Louis Ska (The Book of Genesis, page 20), Philo (prior to Josephus and the Talmud) was the first to attribute authorship of Genesis to Moses. In Concerning Noah's Work As A Planter, Philo appears to attribute the story of Adam to Moses alone - implicitly without divine guidance or recourse to more ancient records:
VIII ... For there is absolutely no one at all who is represented as inhabiting the Paradise, since Moses says that God removed the first man who was created out of the earth, by name Adam, from his original place, and placed him here.
In Eve and Adam: Jewish, Christian, and Muslim Readings on Genesis and Gender(page 42), we read that Philo believed that Genesis chapters 1-2 provided two creation stories because there were two creations, and that he explored the allegorical dimensions of chapters 2 and 3 and the symbolism of Adam and Eve.
As to the historicity of the story, Philo says (Concerning Noah's Work As A Planter):
VIII For it is stated, "God planted a Paradise in Eden, towards the east; and there is placed the man whom he has Made." Now, to think that it is here meant that God planted vines, or olive trees, or apple trees, or pomegranates, or any trees of such kinds, is mere incurable folly. For why should he have done so? any one may ask.
IX We must therefore have recourse to allegory, which is a favourite with men capable of seeing through it;
The Church Father Origen appears to have been the first Christian theologian who explicitly taught that Genesis 1-3 tell us nothing about what really happened in history, so that we cannot know if there was a real Adam:
De Principiis, Book 4.1.16: as even these do not contain throughout a pure history of events, which are interwoven indeed according to the letter, but which did not actually occur. Nor even do the law and the commandments wholly convey what is agreeable to reason. For who that has understanding will suppose that the first, and second, and third day, and the evening and the morning, existed without a sun, and moon, and stars? and that the first day was, as it were, also without a sky? And who is so foolish as to suppose that God, after the manner of a husbandman, planted a paradise in Eden, towards the east, and placed in it a tree of life, visible and palpable, so that one tasting of the fruit by the bodily teeth obtained life? and again, that one was a partaker of good and evil by masticating what was taken from the tree? And if God is said to walk in the paradise in the evening, and Adam to hide himself under a tree, I do not suppose that any one doubts that these things figuratively indicate certain mysteries, the history having taken place in appearance, and not literally...
Jean Meslier, a French Catholic priest who lived 1664-1729, was discovered, upon his death, to have written a book-length philosophical essay promoting atheism.
In his 'Testament', Meslier said:
The extravagances recorded in the Bible are no more worthy of credit than the miracles. Hence, to hold with Genesis, the primal paradise, the talking serpent, the story of the apple -- or of the plum, writes Meslier -- the tree of life, of knowledge, a first man and a first woman, an original sin, its transmission to all the descendants of Adam and Eve. Fable, fable, fable . . .