My understanding is that many exo's were human. Clovis Bray began the exo experiment in an attempt to expand exploration into horribly harsh environments that humans could not survive. They were made and had actual human being cryo frozen and then their consciousness was placed into the exo "container" using golden age/traveler technology. Then the collapse happened and... whoops... all those "humans" now are stuck in exo bodies. Now look, I am not claiming in any way to be some lore expert. I'm not. I'm just stating that on the few occasions I have researched the golden age, exos, Cayde, and the collapse, that was my understanding of it.