I am still pondering as to whether it exists now.

I find these types of articles fascinating and unlike those who are bored or simply fire off, "God did it!" why not use those so called skeptical brains once in a while?

"How Would We Know If Intelligent Life Existed on Earth Before Humans?
How do we really know there weren't previous industrial civilizations on Earth that rose and fell long before human beings appeared?

Reptilian menaces called Silurians evolved on Earth before humankind — at least in the "Doctor Who" rendition of the universe. But science fiction aside, how would we know if some advanced civilization existed on our home planet millions of years before brainy humans showed up?

This is a serious question, and serious scientists are speculating about what traces these potential predecessors might have left behind. And they're calling this possibility the Silurian hypothesis.

When it comes to the hunt for advanced extraterrestrial civilizations that might exist across the cosmos, one must reckon with the knowledge that the universe is about 13.8 billion years old. In contrast, complex life has existed on Earth's surface for only about 400 million years, and humans have only developed industrial civilizations in the last 300 years. This raises the possibility that industrial civilizations might have been around long before human ones ever existed — not just around other stars, but even on Earth itself.

"Now, I don't believe an industrial civilization existed on Earth before our own — I don't think there was a dinosaur civilization or a giant tree sloth civilization," said study co-author Adam Frank, an astrophysicist at the University of Rochester in New York. "But the question of what one would look like if it did [exist] is important. How do you know there hasn't been one? The whole point of science is to ask a question and see where it leads. That's the essence of what makes science so exciting."

As the article linked to in the description states, rarely does any fossil trace exist after millions of years. So the emphasis is on the more subtle evidence in geological records of earth or another planet.

"One may also find it difficult to unearth fossils of any beings who might have lived in industrial civilizations, the scientists added. The fraction of life that gets fossilized is always extremely small: Of all the many dinosaurs that ever lived, for example, only a few thousand nearly complete fossil specimens of the "terrible lizards" have been discovered. Given that the oldest known fossils of Homo sapiens are only about 300,000 years old, there is no certainty that our species might even appear in the fossil record in the long run, they added."

They left messages ...."Kilroy was here""

If intelligent life existed livign in harmony with nature, which probably woudl leavevery very little or no archeological record, we probbly woud never know.

You wouldn't, so why waste any time on it?

Well, according to the documentary "Unacknowledged," found on Netflix and YouTube, humans have always had contact with aliens, and there are videos of interviews with US presidents, leaders, NSA agents, and astronauts talking about it.

The Sumerian texts, written 2000 years before the Bible, has the same stories as the Bible, but told from the perspective of the Sumerian race, who described how they genetically engineered humans by mixing their DNA with that of human apes, in the garden of Aiden.

One would hope they'd have left behind physical evidence; however, depending on their physiology, building materials, etc, and the time involved, such evidence could have long turned to dust.

Or maybe they left in Spaceships - anything is possible (even if not probable)