Could some other species have built a civilization on Earth long before we evolved? Gavin Schmidt, director of NASA's Goddard Institute for Space Studies, considers that possibility:Are We Earth's Only Civilization?
If a society of intelligent, nonhuman beings existed before the Quaternary period, 2.6 million years ago, mainstream geology tells us no material evidence of them would remain. "Go back much farther than the Quaternary and everything has been turned over and crushed to dust." Then how would we know about their civilization? The preservation of fossils and artifacts, even if that hypothetical nonhuman society had flourished recently enough to possibly leave such relics, depends on sheer chance. Schmidt speculates about how we could know they existed, as a thought experiment exploring what evidence, if any, from our own society would survive millions of years in the future. He suggests plastics, changes in sedimentary nitrogen patterns (from using so much of it as fertilizer to feed our population), and the appearance in sedimentary layers of "rare-Earth elements used in electronic gizmos." Above all, our intensive burning of fossil fuels should leave evidence in the form of shifts in the balances of carbon and oxygen isotopes. Schmidt wonders, if our own Anthropocene epoch is in the process of depositing traces in the Earth's bedrock, "might the same 'signals' exist right now in rocks just waiting to tell us of civilizations long gone?"
The article concludes, "By asking about civilizations lost in deep time, we’re also asking about the possibility for universal rules guiding the evolution of all biospheres in all their creative potential, including the emergence of civilizations." Could guidelines for such "universal rules" help us predict what we may find on alien worlds?
While Schmidt and the author of the article don't believe such a nonhuman culture actually preceded us on this planet, the possibility is interesting to consider. And since it's hard if not impossible to prove a negative, especially regarding events so unimaginably far in the past, we can't be sure one didn't exist. Unless time travel were invented, we would never have any contact with the builders of such a civilization or even know what they were like. That is, unless we somehow found long-buried structures such as the vast city of the extinct Elder Things in Antarctica in H. P. Lovecraft's "At the Mountains of Madness." These creatures arrived on Earth when the moon was young and became extinct long before advanced terrestrial life evolved. The Elder Things also coexisted with giant penguins, and interestingly, the fossilized bones of penguins about the size of human adults have been found in New Zealand. They came along much too late to be alive at the same period as the Elder Things, though:Giant Penguin in New Zealand
Suppose we discovered an abandoned city like that, miraculously having avoided being "crushed to dust," inhabited only by monstrous, amorphous shoggoths that survived and continued to reproduce after their creators died off? Hmm, I wonder what we could do with tame shoggoths. . . .
Margaret L. CarterCarter's Crypt