The proposition that complexity generally increases with evolution seems indisputable. Both developmental and generational changes often display a rise in the number and diversity of properties describing a wide spectrum of ordered systems, whether physical, biological, or cultural. This article explores a quantitative metric that can help to explain the emergence and evolution of galaxies, stars, planets, and life throughout the history of the Universe. Energy rate density is a single, measurable, and unambiguous quantity uniformly characterizing Nature's many varied complex systems, potentially dictating their natural selection on vast spatial and temporal scales. © 2010 Wiley Periodicals, Inc. Complexity 16: 27–40, 2011