It sounds plausible in some aspects; but time to play alien devils advocate, and human's did it at the same time...There are a few anthropological
problems with this aliens did it debunk theory; Every culture in prehistory has gigantic megaliths they are on every continent. First...how did remote
non contact cultures come up with the same technology? Duh alien intervention they can do anything...no need to explain the how and why of this
question on the alien side of things.
If all of this monolith building technology sharing took place before the continents separated using current anthropological data. But if the
continents were separate at the time of building, then we had ocean trade routes established between every continent If this is the case; then a lot
of anthropological evidence is wrong, because only one science experiment showed an ocean trade route possible. The coc aine mummy mystery was
sort of solved when an investigator build an Egyptian reed boat and left the Nile delta in it and arrived easily in South America, that explained that
Egypt and South America could have had ocean trade routes...since coca is only found in South America the mummies testing positive for coc aine
led to this mystery...unless somehow Egyptians made some form of synthetic coc aine. Accepted science said they walked their way around over
frozen connections etc. instead, which is a huge undertaking to just say; Hey you guys moving megalithic stones? No? OK, here's how. The odds of all
cultures stumbling across the same technology is astronomical, if it were that simple to figure out, we would have figured it out again already
hundreds of years ago.
Now for the slightly harder one.
Second...thousands of people would have seen and used this technology while building them. The how to wouldn't have just vanished as we still use
ancient metal casting techniques on up until today, we still use sticks to rub together to make fire sometimes...the only time we lose a a process of
manufacture or it declines is when we discover better technology to do the same job.
If humans did it; some global cataclysm would have to have taken place, to make all cultures on every continent lose or forget this technology...if
this were the case why didn't we lose the ability to make fire or metal castings? It could be explained that we had more immediate use and need of
fire and metal casting than huge structures...but if megalithic building was so easy to do then why abandon it? Stone cutting always has usefulness,
again no one wants to cuts stones in survival mode so a cataclysm had to take place and one long enough and devastating enough, to kill off the many
thousands that knew how and for their descendants trying to survive to just forget about it, on every continent.
But then we have some phenomenal enigmas such as the Biagong pipes, which are iron alloy pipes of various diameters that go through mountains that no
one can currently explain. en.wikipedia.org...
It points to more "lost technology" if aliens didn't do it. The Hopi say we
are about to enter the 5th world; meaning that we have reached a similar technological level 4 other times; but we have ended up being nearly wiped
out each time by either ourselves or from meteorites, super volcanoes etc. and the new World is born out of that to repeat the cycle once
again...after all there are megalithic structures under the oceans like the one off the coast of Japan built who knows exactly how long ago by an
ancient unknown culture.
The planet is well over 6 billion years old after all, and for any Christians etc. that want to argue that point, and say 6 thousand, I have heard the
interpretation of Genesis should be actually translated as In A beginning...not; In THE beginning. Also if a day to god is a 1000 years to man then
god made the earth in 6 thousand years and rested for a thousand if you like to get all literal about it.
In my opinion; humans are like a bad case of fleas that the earth tries to shake off every once and awhile; to borrow a bit from George Carlin, and we
being tenacious bastards run into the earths ears to hide then get up shake it off and go to work screwing it all up again a few thousand years later.
Aliens as a cause and god as a cause are an easy way out, you just have to believe or have faith and your questions are answered; you have reasons to
fight for them to maintain that security, and piece of mind. Whereas digging into things scientifically, there is no peace of mind science is always
restless to uncover it all, yes it lacks and yes it makes mistakes because it is human.