We hardly have any idea what our own planet was like fifteen thousand years ago and we haven't even gone anywhere.
Granted, but we do have archaeological evidence of human beings having existed for a very long time on this planet and potentially enough pieces of evidence to trace an evolutionary history of our species going back much farther than that. That evidence tells us that we most likely didn't come to Earth from somewhere else and probably were not fashioned out of nothing at the whim of a passing deity. In New Eden, there would be no equivalent archaeological record.
To some, the absence thereof and the presence of other advanced human societies at approximately the same level of technological development spread throughout the cluster would certainly lead many to conclude that humans in New Eden came from somewhere other than the planets they consider to be their homeworlds. That place having yet to be conclusively found (although you can imagine a great parade of explorers who have throughout history claimed to have found it), what is so absurd in thinking that place may have been a single planet on the other side of the EVE gate?
I have a hard time suspending my disbelief when anyone claims to have any sort of hard, detailed knowledge of what human life was like 15,000 years ago, but that doesn't mean I won't appreciate the evidence of what life
might have been like then and reason out a probable scenario. In the case of New Eden, one such probable scenario is what they might call the "Earth theory" or the "Terran mythology": that everyone alive in the cluster is descended from people who came from somewhere else (probably from the same place or at least through the same route) a long time ago. That notion does not necessarily suggest someone who believes it to be true has or could have details about what happened in that incredibly distant past.
A compelling counter theory that would likely be quite popular in Amarrian circles is that a passing deity did just happen to create all the peoples of New Eden out of whole cloth and plop them on their respective planets at the same time. All the left over relic technology from way way back would then be gifts left by said deity to be discovered when his creations were ready for them. I can imagine this idea, based on the same evidence as the "Earth theory" but resulting in a different conclusion, having a significant following and competing intellectually with the several other theories, the Earth one among them.