system
Given the following post, respond as if you're commenting on Reddit:
human
[Man of Steel] [SPOILERS] What exactly is the Codex?

At first I thought it was just an ancient fossil of an early Kryptonian, but later it's said to contain the genetic material of all Kryptonians. This really got me confused.

The first interpretation I had makes sense for being a cracked skull, but then it lost the purpose.
gpt
Think of it this way: the Kryptonians are master geneticists, capable of genetic engineering on a level as complex as themselves, and possibly even creating entire new species to suit their purposes. It's likely that they discovered, in their development of advanced genetics, that the best way to store genetic information is in bone, organically. They tried digital data, they tried their mass storage technology (the Superman USB drive that was sent with Kal El), but in the end the old ways, the ways tried and tested by natural selection, are still the best. 

So let's say you have to store incredibly important genetic information meant to represent the future of your species on a piece of bone... what bone do you use? If it were me, I'd find the skull of an ancestor, for the sake of connecting our future to our past and  for it to be a physical incarnation of the concept of biological/genetic progress. You take the well-preserved skull of a Kryptonian hominid, an ancestor representing where the Kryptonians came from, and you encode the DNA into that skull, to represent the future of the Kryptonian race. t's not just the most functional scientific method, but it's also a powerful metaphor.