Silurian Hypothesis (IMAGE) University of Rochester Caption How do we really know there weren't previous industrial civilizations on Earth that rose and fell long before human beings appeared? That's the question posed in a scientific thought experiment by University of Rochester astrophysicist Adam Frank. Credit University of Rochester illustration/Michael Osadciw Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.