I think this is very close to what I believe.
What I think is that the traveler has been to all the other aliens' home worlds before. Now bear with me please. I think the traveler has gone to other worlds before Earth. The traveler has been to all the other species home worlds, gave them a golden age but the darkness found them and the traveler left them to the darkness. Essentially abandoning them.
Fallen: here is why I think this for the fallen. The fallen had a golden age once but they used the travelers light to explore the universe. The fallen created ships and nobility, they created armour to survive on the new planets they found, they created the servitors in the shape of the traveler to honor it and gave them the honor of piloting their ships, which was a big deal to them because exploring the universe was everything to the fallen. Then the darkness caught up with the traveller as it always does. The darkness feeds off of lights until it leaves only darkness. It feeds on planets and suns but if isn't enough. It needs a bigger source of energy, a bigger source of light: the traveler. This is why it follows the traveler, it creates its food. The game has mentioned multiple times that the minions of the darkness want to 'consume' your light. More on that later.
Back to what I mentioned before. The darkness caught up with the traveler and the fallen. Then the traveler left. It had no choice but to run because the fallen wasn't strong enough to protect it. They didn't use their technology to create weapons, they used it to explore and discover. They were no match for the darkness. So the fallen hastily tried to create weapons to protect themselves which is why their weapons are so bad. They created the shanks and fallen walkers to try and fend off the darkness but its wasn't enough. The darkness won and corrupted the fallen turning them into its minions of darkness.
Now the fallen long for be traveler and follow the darkness in search for their lost god. They began to worship the servitors as gods similar to how they worshipped the traveler as the game mentions multiple times that the fallen worship the servitors and that they search for what they once lost; the traveler and its light.
I think this is a repeating process that has happened to all of the species. The traveler finds them, gives them a golden age and technology that hopefully will allow it to defend it from the darkness. But the darkness always wins forcing the traveler to abandon them in search for another species of 'guardians' (hence the name guardians). Then the darkness corrupts the travelers 'guardians' turning them into minions of the darkness.
The hive were one of the first, and now the darkness has consumed them wholly. They thirst for nothing but light to feed on as they are now the embodiment of the darkness.
Now it is Earth/humanities's turn for this repeating process but something is different. Mankind used the travelers light to create weapons, advanced machine guns, war machines, space ships to take over other worlds, missiles capable of mass destruction, we created the exo to help us concure and destory. The traveler finally found what it was looking for. A species that militarized its light and lusted for nothing but its own power. One that might be able to defend it against the darkness. One final generation of 'guardians'.
The traveler tired of running and decided to take a final stand. The traveler put its trust in the guardians and did something it had never done before, it stayed when the darkness came. It defended humanity in a last ditch attempt at self-preservation in hopes that humanity was strong enough to beat back the darkness.
The traveler then realized that humanity may need some help so it created the ghosts. Machines powered with enough light to bring one human, awoken or exo from the golden age back to life, one that used the travelers light to create weapons and destruction. This ensured that the guardians would always be the violent ones so that humanity would never change. It brought back to life only the violent ones that thirst for power and war.
Now here we are. An army of the undead created by the traveler to ensure its own survival in hopes that humanity would stay the same, greedy and power hungry species as when it discovered us in order to create weapons that fought back the darkness.
English
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"Where is the great machine? Where is the great machine?" -Chelchis, Kell of Stone. Supposedly the fallen house that Oryx got to, and imprisoned kind of like Alak-Hul, while not turning them into chiton. I think the fallen were [b][i]people[/i][/b]. I think the darkness manipulated them and mutated them, hence the 3rd and 4th arms and the heads. They almost look Human. Like the Covenant, they feast off of scavenging and war. They just have enough military power to get by while their transport and mobility is of high ordeal. "I was there when the first fallen ships arrived. It started with just one skiff. In a few months, the Cosmodrome was theirs." They were for exploration. The Cabal is what the darkness turned into advanced battle-hungry war machines. The hive are like undead. The darkness raised them as the ghosts raised us. They weren't of higher impact, but of different intent. The Vex were constructed to seize and control. The Cabal, command and conquer. The hive, inhabit and ambush. The fallen, scavenge and raid.