At the top of this post, you will find a plot of the Vostok Ice Core Data (source link below). This represents a history of the atmospheric carbon dioxide and methane concentrations for the past approximately 400,000 years. Let's make a reasonable hypothesis that these concentrations are roughly proportional to the population of life on Earth - breathing, burning, and pooping. One can notice that there is a saw-tooth pattern. Maybe we found the poo?

There is an old saying, "Nothing is new under the Sun." The homo sapien species is arguably around 500,000 years old - maybe older... maybe a little younger. Thus, the Vostok data encompasses a large window of time that our ancestors have inhabited the Earth. If the above hypothesis is correct, we could safely assume that about every 100,000 years there is a population boom of life - likely including humans - followed by a seemingly cyclical event that destroys life on a large scale. Oh my.

The hubris of the human species generally concludes that everyone that came before was less advanced than we are now. It's a very haughty conclusion. Constant geological processes do not guarantee that all ancient civilizations would be detectable. For example, of our modern trash only glass is likely to persist longer than a thousand years. The mostly forgotten work of Georges Cuvier comes to mind. Cuvier would be interested to see the Vostok Ice Core Data above. Have we been here before?

Of course, this is just educated speculation, but the probability is substantially above zero that it only takes our species a few thousand years - according to modern existence - to figure out a lot. Has the human species escaped this planet previously? Maybe and maybe not. Have our ancestors discovered this cycle and tried to preserve knowledge through to the next cycle? Maybe. Do we occasionally seed the Earth after everything gets destroyed? Possibly. Are we the aliens? Who knows?