How Would We Know If Intelligent Life Existed on Earth Before Humans?

From Live Science.com (Apr. 18):

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." [read more]