I see your point, after all it is just my personal opinion which is not supported by any historical facts; so, truly debatable!

But then, how do you explain their knowledge?
How could they possibly know what they knew and I am not speaking about the year 2012 but the precessional motion of the planets for example? Why were they able to calculate in such a marvellous and greatly accurate way all the movements of the 12 constellations? How could they know how to calculate the lunar and solar eclipse until our days?

Another theory speaks about they were human indeed but were visited by "others" who gave them the knowledge. Not only to them, there are many other occurrences in the historical development of our race.