Sometimes, unraveling a mystery doesn't mean everything becomes clear. In this fucked-up reality, most mysteries are followed by a bunch of even more bewildering things. I feel trying to find out the truth from back then is like traversing mountains and rivers; when you finally cross a great river, you look up and see the South Pacific...
Regarding the soldier who went astray to Earth back then, we currently have the following clues: He arrived on Earth at least during the early Uruk period, which is more than five thousand years ago (this is the earliest evidence we have). Based on the tomb found in the Taklamakan Desert, he died over a thousand years ago, spanning three to four thousand years in between.
