The DC Universe may be moving away from Zack Snyder’s original vision, but there are still plenty of superhero fans who have a soft spot for Man of Steel (2013). The film not only laid the groundwork for the next decade of releases that followed, but it was revisited in one of the most notable set pieces of The Flash (2023).

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It’s obvious that director Snyder and screenwriter David S. Goyer put a lot of thought into Superman’s origin story, and how Krypton operated before it was destroyed. The first twenty minutes of the film take place there, after all. One element of the story that has confused some fans, however, is the importance of the Codex. What is it? How does it factor into the story?

There’s a simple and a complicated way to break it down. Let’s do simple first. The Kryptonian Growth Codex, or simply the Codex, are the remains of an ancient Kryptonian that contained the genetic information for all future Kryptonians. The Codex allows Kryptonians to manipulate their DNA, and can be used (along with the Genesis Chamber) to artificially create life. It ensures that said Kryptonian life is designated to serve a specific purpose. Zod (Michael Shannon), for example, was created using the Codex, meaning he was born and bred to protect Krypton at all costs.

The Codex has a secondary use, which is where things get complicated. According to the DC Movies Wiki, a Codex (or Growth Codex) can be combined with genetic matter to terraform other planets. It’s something Kryptonians tried to do when they realized their planet was expanding, but it was a concept that Zod took too far. The general wanted to erase some of the bloodlines from the Codex, as he deemed some of them unworthy of a second chance, and partially to blame for the crumbling state of the planet.

Jor-El (Russell Crowe) was opposed to this eugenic application of the Codex, and stole it from General Zod before putting it into his son, Kal-El, and sending him away in an escape pod. Yes, the Codex is actually made to be part of the baby’s being.

Kal-El, of course, becomes Superman (Henry Cavill). It’s important to note that Superman is the first Kryptonian in centuries to be born naturally, without the use of the Codex. This means that he has the gift of free will. He arrives on Earth, but so too does Zod, who was sent away to the Phantom Zone before Krypton was destroyed. When the general arrives on our humble planet, he decides to pick up where he left off, and use Superman to revive the Kryptonian species.

Snyder talked about the original concept behind the Codex during a Q&A with Kevin Smith in 2013. “My idea for the Codex was that the skull was the first Kryptonian that they could identify in their evolution,” he explained. “That skull had been fought over and different factions and many people had died, so a group had carved different Kryptonian sayings, and finally, when it was wrestled away by the people who created the Genesis Chamber.”

Snyder went on to say that other parts of Krypton may have different parts of the remains, like a femur, and that they would be treated with the reverence of a religious object in our world. The Codex is actually mentioned during a brief scene in The Flash. Zod reveals that he intercepted the pod carrying Kal-El and the Codex in an alternate timeline, and he killed the infant in trying to extract the Codex from him. It’s unclear whether it will resurface in future DC releases.