This may just be speculation but I think we are the zombies from cod that actually got kills on other players and each guardians story is different and we'll get to see back story for each of us before we became zombies. The golden age refers to when cod was historically noted to be the largest online shooter ever with what is now the largest easter egg ever. All the zombies that lived each game of survival mode are now guardians. Those zombies were being controlled by alpha alpha testers secretly testing destiny for back story. Thoughts?