We were "born in the Traveller's Light". We know how to use a weapon without any training. We were dead for a very long time. I think that is pretty much all we know/can infer about our guardians as individuals. My interpretation - Light is some variety of energy - some fundamental life force energy. Guardians are individuals who have a high amount of this Light or who have some sort of affinity with it meaning that they generate an excess, gather in the light from dying individuals, and can utilise the excess. Guardians are also able to form light into motes - an exchangeable physical item. This life energy does not always dissipate on death - it is stated about the Guardian outside Crota's temple "nothing remains, not even the light" as if it would be expected for a guardian's light to remain and that this lack of light is unusual. This is how we became Guardians - Our light remained with our body, at least enough that with a little donation of it's own light our Ghost was able to revive us (I personally think that it is able to combine light with a physical transmat template and that is how it manages the act of revival) Who were we before then? Members of our respective orders perhaps? Definitely we come from post Traveller Earth - possibly even Golden Age. More importantly, we know how to use a weapon - so were most likely military. Though knowing humanity, if faced with an excession event, every able bodied individual would become "military" of some variety. being a civilian is a luxury you can't afford when faced with an extra terrestrial threat to planetary survival. As each guardian is unique - each can have a different backstory of your own imagining. Some people reject this concept - but for example I imagine my warlock was an ageing colonel, pulled out of retirement to fight the collapse. At least when she dies and is revived the total body transmat has the effect of resetting the progress of her arthritis.