Who lived on earth before us?

One of my favorite ideas of science fiction writers is that they believe our civilization on Earth - not the first and probably not the last. I first spoke about the creation of the atomic bomb, when humanity has the opportunity several times to destroy all life on the planet. However, this theory has opponents who argue that since there is no evidence of previous advanced civilization, then there was one. However, against this argument can argue as in different parts of the world from time to time miners discovered some rusty parts that have lain buried for at least a few million years. However, these artifacts no one has taken into account, since, according to scientists from advanced civilizations should remain a significant traces. And this thing just is not visible.    But recently a team of researchers from Harvard University failed to answer how the Earth will look like in case of a world war or disaster that could lead to the extinction of Homo sapiens. Using a computer simulation, taking into account all known to modern science, environmental influences on different materials, they found that within 50 years the cities and mega-cities will remain only ruins. After ten thousand years they will become dust. All synthetic materials - concrete, metal or plastic eventually break down and after about two million years there will be no sign of that at one time the earth was people. This work, of course, does not prove the existence of "inhuman" civilizations, but according to the study, if they really are missing a few million years, we can no longer find any trace of them. At the same time, if prachovechestvoto at the time of its destruction was able to fly into space and visit at least moon, then all of it artefacts would remain intact, as our natural satellite is devoid of atmosphere, and therefore of aggressive environment. It turns out that there how and where to look.