The dawn of habitable conditions for complex life in the Universe

The Universe evolved from a hot dense lifeless state to one capable of sustaining life as we know it. Complex life survives on Earth and presumably elsewhere throughout the Universe. Did life precipitate at many places at once, like a universal phase transition or not? Generally, the habitability of exoplanets has improved over time as essential chemical abundances increased and threats such as supernovae, gamma-ray bursts, and AGN decreased in frequency and density. However, habitable conditions for complex life on planets are strongly constrained by host star and host galaxy properties. Environments promoting the emergence of life as a function of cosmic time and location are reviewed. It is suggested that a transition from an uninhabitable Universe to one capable of forming and sustaining complex life took place 5-6 Gy ago. However, many galaxies and stellar systems remain uninhabitable today.