That was not explained in the anime, but it was kind of explained in OVA. When people crave por power to change the reality, one day they are starting to have that power. It is granted by collective cousciousness (in OVA), or how we call it in real world: Prima Materia or Chaos. It orginated around that time when first abstract inteligence evolved (so long LONG time ago, before written history). I think that my answer is quite accurate, even if it is not based solely on the Code Geass. Note, that Geass comes from Geis, which comes from Arianrhod, which is not in anime anymore, but in celtic religion. Still, Chaos or Prima Materia was first described in writings (hieroglph "kh") by ancient egyptians long long time ago.

That was not explained in the anime, but it was kind of explained in the OVA. 

When people crave for power to change the reality, one day they are starting to have that power. It is granted by collective consciousness (in OVA), or how we call it in the real world: Prima Materia or Chaos. It originated around that time when first abstract intelligence evolved (so long LONG time ago, before written history). 

I think that my answer is quite accurate, even if it is not based solely on the Code Geass. Note, that Geass comes from Geis, which comes from Arianrhod, which is not in anime anymore, but in Celtic religion. Still, Chaos or Prima Materia was first described in writings (hieroglyph "kh") by ancient Egyptians long, long time ago.