Some thoughts relating to pre-history, in a separate thread to avoid hijacking anyone else's....

If our civilization were to end, would there still be clear evidence of it having existed, 10,000, 100,000, a million, ten million, or a hundred million years in the future? Yes definitely:

1. List of man-made objects on the Moon - Wikipedia, the free encyclopedia. There will still be a lot of junk in orbit indefinitely also.
2. Quarries, mines, roads, and home foundations cut into rock will still be obvious. There are millions of such features all over the world, and it doesn't go away even in hundreds of millions of years except where it gets scoured away or buried. Such features don't scour away easily or everywhere, and they don't get buried indefinitely everywhere, they get buried in some places and exposed in others. And they can be dated with a fair degree of accuracy using a variety of techniques.
3. Its not as if we built our whole society on a single island or small continent, while somehow never affecting the rest of the globe. And its not as if it makes geologic sense for anything that big to disappear quickly, as if Newton's laws of force, friction, and inertia don't apply to rock.
4. We exist in a biological context, and not in some other context. The ecosystem we are a part of will still be preserved in fossils. This contrasts, for instance, with the 400 million year old rock that was dug up when my house was built in Ohio. Theose rocks were packed with coral and various shelled animals, so well preserved that at first I thouht I was looking at something only a few thousand years old. But the types of that age are all primative, with the highest animal being a nautilus that doesn't have a modern equivalent. So we can say with some confidence that there were no civilizations when those rocks were formed.

In regards to past civilizations, the first point doesn't say much, other than that it appears that previous civilizations didn't launch satellites into high orbits. The second and third points pretty much completely eliminate the possibility of past technological civilizations, in my opinion, aside from relatively recent ones such as recognized by mainstream archeologists and paleontologists. The fourth point is more problematic I think, and is what is driving so much of this speculation to start with.

A dynamical system tends to stay in one characteristic region for a long time before bifurcating and moving quickly into a very different region. Biological forms must be like this also - they stay mostly the same for tens of millions of years, accumulating small changes, then appear to change a lot very quickly once they cross a threshold after where further changes become more advantageous. Then they reach a relatively optimal design where additional changes become less advantageous, and don't appear to change much for a while. With a sparse random sampling of fossils, you mostly only see the long, steady states of species that survive. The other more temporary states go by too fast, hundreds of thousands of years instead of tens of millions.

Man seems to be an enigma. There are differences between other animals, but the gap between any other animal and a man is huge in some regards. People often make a big deal, for instance, over the intelligence of dogs or dolphins. But it appears to me that in many ways a dolphin is much more like a dog than a man. Both animals learn, can be conditioned though rewards, and can understand dozens or even hundreds of words, in whatever language. Both have emotional qualities which are similar to those of men, like empathy for instance. And both animals are sentimentally appealing to men. Dogs are compassionate to men and suck up to men as alpha leaders, while dolphins have that permanent friendly smile-like face shape even when they're angry. But a man's capacity to learn is orders of magnitude greater. A man can learn hundreds of thousands of patterns and ideas, and logically coordinate abstractions in a manner that other animals do not even remotely approach. With this ability, technology changes rapidly, in contrast to the situation with other animals, where technology changes almost not at all and culture changes little. And man seems to have acquired this distinguishing characteristic a mere tens of thousands of years ago, with nothing before that.

Does this make sense in terms of what I said about dynamical systems? A chimpanzee has some man-like characteristics. Is it close enough to a man, does the picture make sense? Or is man just too different from anything else, and some part of the picture must be missing? It seems that many people feel that there is a piece missing, which is a big part of where the 'ancient astronauts' and other ideas come from. My view is that if there is a piece missing, its another piece that we haven't thought of yet. Maybe it has something in common with other ideas, such as cycles of earth, water, air and fire ages for instance. But none of those ideas work in their present form, they don't actually match the evidence that we have, and believing in them requires ignorance, misunderstanding, or outright denial of much of that evidence. On top of that, for an average person the real evidence is buried in a mountain of fabrication, brought to us for the sake of History or Discovery Channel ad revenue. Or Disney before that. It amazes me that people who have so little regard for big corporations or capitalism in general will also put so much faith in the output of profit-seeking media organizations. I guess it all depends on what they're selling.

But I do share a big part of the motivation, that it seems there is a piece missing.

Based on my own experience, its clear to me that there is 'synchronicity' in life which has some kind of supernatural or paranormal nature. This must apply to biology as much as to anything else. If anything, in my experience, such forces are stronger in relation to reproductive direction than almost anything else. And so evolution is directed, it isn't blind. That must be at least a part of the missing piece. Things really do evolve faster and more abruptly than they would otherwise, and they do so according to some kind of plan. Though who knows what that plan is, how it works, or what its aim is.

In addition to this, I think we also feel conditions in other civilizations, and we try to make sense of those feelings. For the first half of my life, I could fly or levitate in all of my dreams. (The primary reason I don't fly in dreams now is I rarely dream of having a body. And I think part of the reason for that is I'm more aware of my sleeping body, which limits the freedom of my dream body.) I think that part of the reason for dreams of levitation and flying, is that my sense of weight, buoyancy, and inertia is no longer being driven by my senses when I'm asleep, and so it just floats. And I think part of it is an instinct for swimming, which I imagine as flying, since I don't spend a lot of time under water. But I think there's somehow more to it than that. Other people feel this too, and it appears to me that this motivates a lot of the speculation about other times where various forms of flying are possible. Likewise for other powers like mind reading or manifesting objects. We feel these to be real, and compelling, so we try to build those feelings into a picture that makes sense.