well, back in the 90's, bungie made a game for the Mac known as marathon, which pretty much carried most of the same things as halo2. dual wielding, alt fire, even the skull in oddball mode. Bungie's previous owner stated that the protagonist (a mjolnir mark IV cyborg) and master chief are one and the same. A couple of things also point to this
marathon symbol is on the pillar of autumn
master chief wears Mjolnir armor, has cybernetic enhancements
The markings on the rockets (I dont totally get this one tho)
Your main companion is an AI
so on
well, now bungie released the scource and an emulator for the game at
http://source.bungie.orghave fun!