What if humans weren't the first civilization on Earth?

Is it possible that modern humans aren't the first civilization on Earth?

This is the insanely interesting question probed by "The Silurian Hypothesis", a new paper authored by Gavin A. Schmidt and Adam Frank, two NASA scientists.

As they point out, if an industrialized civilization existed in the deep past, it's not clear there'd be easily recognizable traces of it. Our geologic record doesn't go back any further than the Quaternary period of about 2.6 million years ago. "Go back much farther than the Quaternary," as Frank writes in an essay about the paper in the Atlantic, "and everything has been turned over and crushed to dust."

It's not even clear we'd find fossilized remains of a previous civilization, because while museumgoers might think that fossils are reasonably common, they're actually incredibly rare. A near-zero percentage of life on earth has ever been fossilized. A civilization could last what seems -- to us -- like a super-long time and still not produce any fossils, as Frank notes:

So, could researchers find clear evidence that an ancient species built a relatively short-lived industrial civilization long before our own? Perhaps, for example, some early mammal rose briefly to civilization building during the Paleocene epoch about 60 million years ago. There are fossils, of course. But the fraction of life that gets fossilized is always minuscule and varies a lot depending on time and habitat. It would be easy, therefore, to miss an industrial civilization that only lasted 100,000 years—which would be 500 times longer than our industrial civilization has made it so far.

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