"The Garden's First Sinner" is a short story idea I had years ago but never got around to writing, partly because I'm more interested in the idea than the story. Let me be clear up front that the purpose of this story would not have been to advance any new theological notions (I don't believe what I suggest below myself), but simply to make its readers think about what could be possible...
The story would be based on the observation that there is a difference between Satan, the spiritual adversary of the human race, bits of details about whom are visible throughout Scripture, and a serpent, which is described in Genesis as being the most "subtle" beast of the field (in King James English). That is, the serpent is described as the most intelligent of animals, short in smarts only to mankind. Yet Scripture also portrays Satan as a serpent or serpent-like (most famously as a dragon in the book of Revelation) and specifically links him to the temptation of the human race in the Garden of Eden.
Many Christian interpreters of the text over the ages have decreed the serpent was Satan, period. But, as not everything there is to know about God is known (or can be known) in Genesis, not everything is known about Satan...in other words, he who is later known as the "prince of demons" was in disguise as an animal in the Garden. And later we find out the real identity of the "serpent."
Many modern interpreters hold the original text has evolved, so that it really was supposed to be an animal at first, but later Scriptures changed that original idea to something else, a spiritual sense. This is not an interpretation I agree with--I prefer the "not everything is revealed yet" school of thought.
Still, just as the gospels mention that Satan entered Judas before he betrayed Jesus and demons entered a herd of swine (with the permission of Jesus), could it be that the serpent was a real animal of great intelligence, but Satan possessed this animal? So it the tempter was both a very clever animal and the greatest of evil spirits at the same time?
In the "what if" sort of story I imagined, that situation would come about because the serpent became jealous of Adam and Eve's privileged position. So in this hypothetical scenario, the serpent actually sinned first, before the man and woman did, just that event happened "off camera." As a result of that sin, Satan was able to enter the beast and lead it to tempt humans to sin--so both of them were responsible.
Satan's judgment actually comes at the end of the world, when he is cast into the Lake of Fire in Revelation. But the living creature, the serpent, my story would conjecture was punished by God by being forced to crawl on its belly and "eat the dust of the Earth"--which would mean more than simply serpents now being lower to the ground than they apparently used to be--it would also be a reference to the animal becoming unintelligent.
I imagined at the end of this story God making this humiliation of the serpent a long process, while He held the garden itself in suspended animation, so time there would not change. The long process would involve generation after generation of reptilian creatures rising and falling, some becoming enormous and powerful, but all basically unintelligent, until a shock came over the Earth that got rid of all the tall reptiles, leaving only those who "crawl on their belly" left alive. In short, the story would imagine God directed the evolution of life to happen over real time that was not experienced in the garden, as a form of punishment, real time that came about after the original creation of life, in Adam's experience of time, thousands of years ago.
The punishment of evolution would not just touch the serpent, because as the backstory of life was rewritten, suddenly the same creatures Adam already knew would have different backstories. Animals would instantly be more at odds with the human race (though that instant would have taken millions of years to produce)--plants would have produced the thorns and thistles associated with a struggle for survival of a sort that had never been originally intended by the Creator. Evolution would be real, occurring over millions and millions of years--but at the same time, in another timeline, the world would be thousands of years old in its original form, evolution being nothing more than a divine form of punishment...