People have been toing and froing since time immemorial, and carrying ideas and beliefs with them.
Some of these ideas and beliefs caught on, some didn't.
Then there's Sherlock Holmes who said so aptly, 'What one man can invent, another can discover.'
Point is, a common source in the form of ETs or a Lost Continent is not required.
We need to give peoples of the dim and distant past a lot more credit than we do today; they were just as intelligent and resourceful as us.
It is an insult to our remote ancestors to believe that they needed help from technologically-advanced Aliens to achieve the things they did.
Some say we today could not build the Great Pyramid of Giza.
We could quite easily build it, but, unlike the Ancient Egyptians, we simply lack the will to do it.
So few answers - and not one of them mine.