Let’s all get on the same page before going any further: this is not canon. This is not hidden lore, endorsed by Nintendo, that you should arm yourself with and take with you on an official basis every time you play a Mario game from now on. OK?
OK good. So, here’s a panel from a 1996 manga (Super Mario 64 4Koma Gag Battle), which is suggesting that maybe 1-UP mushrooms grow out of the bodies of the Marios who have died before you.
As the ever-wonderful Supper Mario Broth point out, this is from a manga that is “filled with jokes and eccentric theories”, and that it “should be taken more as an interesting thought experiment than anything that would actually be endorsed by Nintendo!”
Let’s do that, then. Let’s take this as a thought experiment. Imagine this is really how this all works. What it’s suggesting for the way the Mushroom Kingdom operates, and Mario’s relation to it, is fascinating. It’s some Returnal, Groundhog Day type shit. Mario isn’t just here to save the Princess. He’s trapped here, in a place outside of not just Brooklyn, but space and time.
Every 1-UP mushroom you see is a former, failed attempt, where a Mario—you, just like you, but also not you—has died, his heroic spirit extracted and preserved for the next Mario. Who is also you, until his demise, when the mushroom will be collected by...you.
As replies to Supper Mario Broth’s tweet have pointed out, it’s an idea more recently explored in this classic Extra Fabulous comic from 2019:
I love the idea that in 2023 a lot of people’s first thoughts—mine included—weren’t about the timeline stuff but drawing fungus analogies with The Last Of Us. And that this would be such a Nintendo take on it. In Naughty Dog’s universe, the mushrooms growing out of dead people are a threat to all of humanity, and create ravenous zombies. In Nintendo’s, they’re just a cute little thing that helps the player by giving them special powers. Well, “cute” provided you don’t think about where they came from...