Yoshi's Island took 5 years to make, Baby Mario almost transformed into adult Mario

This info comes from an interview all the way back in 1995 with Shigeru Miyamoto. The interview focuses on Yoshi's Island...

Q: Still, from a sales point-of-view, a game where you control Mario would likely sell more, wouldn’t it?

Miyamoto: Yes, and that’s why we added the ability to transform into Super Baby Mario when you pick up a star. In fact, our first idea was that the baby would transform into fully-grown, bearded Mario when you got the star, but another staff member pointed out how that would be weird with the story, so we kept him as Super Baby Mario. Personally, I still think the idea of adult Mario running around is better. (laughs)

Q: How long did the development take?

Miyamoto: The staff members who have been working on it the longest started around the time Super Mario World was released, so about 5 years. The first two years were mainly spent experimenting with different ideas. We had one idea where Yoshi would move around freely, and he’d support Baby Mario and lead him through the stage. Almost all of those early ideas came to naught, but the one that really stuck and bore fruit for us was the idea of a game where even though you hit enemies, you don’t die.