The guests today are co-authors of a new paper in Science Advances titled: "A reassessment of the 'hard-steps' model for the evolution of intelligent life."

Jennifer Macalady is a Professor of Geosciences at the Pennsylvania State University. Her research is focused on microbial ecology and evolution, and interactions between microorganisms and Earth materials like minerals and water through geologic time.

Daniel B. Mills is a postdoctoral researcher in the Orsi Geomicrobiology Lab at The University of Munich in Germany. His research focuses on the co-evolutionary history of Earth's surface environment and eukaryotic life—a topic he approaches by studying modern marine organisms and environments. He also applies this research to the ongoing search for exoplanetary biosignatures and technosignatures.

Shermer, Mills, and Macalady discuss the Hard Steps model of the evolution of intelligent life, the Drake Equation, the Fermi Paradox, the Great Filter, and the Great Silence. They also cover the Rare Earth hypothesis, major transitions in evolution, the uniqueness of humans, and the search for biosignatures and technosignatures.