Perhaps the Elites were once aquatic animals (IMO, they look like a mix between a shark, an alligator, and a gorilla) that traveled in packs that divided prey equally, forming a sense of unity and brotherhood that evolved later on, forming small tribes and eventually uniting into large "empires". As their empires and their apetites evolved and their numbers grew, they started developing weapons suitable to kill certain types of prey. Similarly to how we discovered fire, they dicovered plasma and used them for their own needs. Throughout time (their time), they established a warrior culture, obssessed with the "hunt" and kept that "pack" mentality of honor and such. They trained elites at an early age in order to become useful to their tribe, probably centered around prophets, who probably "found" plasma and named it "the holy light" and began preaching about their religion, thus easily converting the elites. Most (probably close to none) married but somehow reproduced to make multiple children, just to send them away to fight in their adolescent years as previously stated. They may have been nomadic for a time but eventually settled down in profitable areas and they soon dicovered other tribes and groups and obviously fought them for dominance but instead of taking them as POW's or s*x slaves, they let them join to create a bigger and stronger army. This repeated several times and sooner or later you get the covenant
*sorry it's so long but i tried to include as much as possible without saying too much. (Trust me, it's a lot longer and i only focused on elites. Maybe later i'll talk of brutes and stuff)
"You can take a *** on your neighbor's lawn because it isn't illegal, but that doesn't mean that people still don't hate you for it." -Offensive Bias