WMG / One Froggy Evening

Michigan J. Frog originated in 1892
His little music hall performances are rather dated for 1955 (when the demolition guy finds him) and especially 2056 (when the disintegration guy finds him), but they're perfectly current for 1892 (the date he was hidden in that building before the demolition guy found him). He doesn't update his songs or costume for either of the times we see, so it seems unlikely that he would have done so for the person in 1892. Thus, this person, whoever they were, didn't just find Michigan in yet another building, where he was hidden by yet another unfortunate soul another century earlier. Rather, they were the first person ever to "find" him (in a remote corner of the rainforest? A meteorite impact site? Who knows?), or, my theory: they created him.

Michigan J. Frog was created by the devil to sing to make people who found him miserable.
When the demolition guy found him, he expected to be rich, but the frog wouldn't sing to anyone else no matter what he did. It ends up making him homeless (he bought a theater for the frog to perform in front of everyone but it failed), and when he's on the bench, the frog's singing lands him in a psychiatric hospital. Eventually the man seals him away and he survives the next hundred years where the disintegration guy finds him.

Michigan J. Frog was originally a human.
He was a young and talented performer who loved to bring joy to his audiences. He was so good that eventually a group of music record executives, seeking to exploit the man's talent, offered to let him travel around the world and do what he did best in exchange for writing songs for them. The man accepted the offer, and in a short period of time he was a globally beloved star. People from all over the world endured long journeys just to hear him sing and dance. The man won multiple awards and was even inducted in the Hall of Fame.

But tragedy struck; something happened to the man (an accident or illness) led to him being unable to sing or perform again. When this was discovered, the record executives, now extremely rich from the man's successes, immediately discarded him like a broken toy. The people who watched him perform left, too, now that the star has faded. The man was bereaved over his losses that he died out of sheer despair. On the other hand, he was enraged that people (especially the record executives) took advantage of his skills and abandoned him the moment he wasn't of use anymore.

So moments before he died, the man requested to God that he be given the power to discredit those who will try to use him again. In response, God turned the man into a frog that sings and dances in the presence of people. But when put in front of large crowds, the man acts like a typical frog. When the record executives who exploited the man hear of the frog, they see another opportunity to earn more money, only for it to backfire their faces when time and time again the performing frog will just "ribbit". The numerous attempts to make the frog perform eventually left the bastards poor, homeless, and insane.

However, it wasn't enough for the man; after getting revenge on the record executives, he prayed to God that he stay like this for eternity, discrediting those who will try to use him for money, until he found someone who was willing to let him sing and dance without looking at him with greed. So God put the man (frog) back in the beginning of time, during the creation of the Stonehenge ("Another Froggy Evening"), which eventually leads up to the present day ("One Froggy Evening").

Eventually, the man's wish is granted when he finally does meet someone who genuinely enjoys his music: Marvin the Martian.

Michigan's eventual rise to fame.
After meeting Marvin the Martian, who understands his "ribbit"s and asked for another song, Michigan J. Frog finally agreed to become the famous character he's portrayed as in Sylvester And Tweety Mysteries. The only reason nobody made the big bux of him in the past is that nobody told him Earthlings don't understand Martian. He just thought nobody other than those trying to make him famous liked his singing.