All Exos were once human (and technically still are human). Back in the late Golden Age, scientists were trying to developed ways to help the human mind and body better withstand traveling amongst Vex structures and systems. The result was the Exo. When the Golden Age came to a close and the Collapse began, the Vex started to attack us and claim our worlds, so we retaliated by mass producing what was best at fighting them under, all watched over by a Warmind. Exos are made by inserting a human mind into a mechanical body, and many people volunteered for this, creating the large population of Exos we know of today After the Collapse, all knowledge of how to create Exos was lost, so now those few left amongst us are all that remain of one of the Golden Age's greatest legacies