Spoilers for A Quiet Place follow below.

One of the many brilliant aspects of filmmaker John Krasinski’s smash hit horror thriller A Quiet Place is its economy of storytelling. There is zero fat on the film. There’s no needless exposition, no prologue explaining what happened to Earth or how the monsters got there. The film begins on Day 89, in the aftermath of when “they” first arrived, and viewers are dropped into the middle of a story that’s already ongoing. Through Krasinski’s meticulous lens and the cast’s incredible performances we learn all we need to know about this family in order to emotionally attach to them, and to care deeply of their well-being. And as a result, we don’t really care what we do or don’t know about the monsters’ origin story.

But that doesn’t mean Krasinski didn’t still work out what these Quiet Place monsters are and where they came from. And instead of keeping things ambiguous, Krasinski freely admits he mapped out their entire backstory, if only so he could approach the film armed with as much knowledge as possible.

Speaking on the Empire podcast, Krasinski revealed the Quiet Place monsters origins, confirming that they are indeed aliens:

“They are absolutely aliens. They’re from another planet. Where I developed the idea of them and what I wanted them to look like was most alien movies are about takeovers, agendas, they’re a thinking alien creature, and for me this idea of a predator, this idea of a parasite, this idea of something that is introduced into an ecosystem [was interesting]. One of my favorite movies I love to watch is RocknRolla and they tell that whole story about the crawfish in the Thames and that’s what I mean, the introduction of something that can’t be held back.”

In developing the backstory of what happened when the aliens first arrived, Krasinski came up with an appropriate yet disturbing comparison:

“I remember a terrible joke that I said was it would be—it’s disgusting and disturbing but it’s true—it would be like releasing wolves into a day care center. That’s how the world responds.”

Krasinski worked closely with ILM to develop the creature designs, and in doing so came up with why they look the way they do:

“The idea behind all that is they’re definitely aliens and they’re an evolutionarily perfect machine. So the idea is if they grew up on a planet that had no humans and no light then they don’t need eyes, they can only hunt by sound. They also develop a way to protect themselves from everything else so that’s why they’re bulletproof and all these things. I had to make it make sense. I needed the rules of the monster to adhere as tightly to the rules of the family. The family, we had set up all these incredible rules, and I needed the monster to not just be convenient.”

In Krasinski’s mind, the aliens came to Earth after their homeworld was destroyed:

“And the other idea was [the armor is] also the reason why they were able to survive kind of the explosion of their planet and then survive on these meteorites, because they’ve evolved to be bulletproof. Until they open themselves up to be vulnerable, they’re completely invulnerable.”

The filmmaker also explained why he decided to nix any kind of opening exposition in the movie:

“That horrible joke of the wolves in the daycare center is like, it happened so fast there is no—I wanted to break all the rules or the conventions that I had seen in alien movies which is like a speech from the president and people deciding how to survive. There was no deciding, it just happened so fast that you either survived or you didn’t. So it puts these people in a really tense place.”

And it worked! The film is downright brilliant, and while I love the decisions Krasinski made to cut out any kind of unnecessary explanations, it’s also fascinating to hear the backstory he came up with to justify the existence and attributes of the monsters.