We review the thermal history of the Universe. We obtain a new estimate for the thermal history of the Earth over the past four billion years using a biological thermometer based on (1) a rough estimate of the absolute time calibration of the genetic distances in the 16S rRNA-based tree of terrestrial life and (2) the maximum growth temperatures of the extant organisms used to construct the tree. The only other thermometer on comparable time scales is based on a controversial interpretation of the oxygen 18 abundances in chert. These two thermometers are consistent with each other and with the idea of a roughly linear decrease in the surface temperature of the Earth from ~ 90 C four billion years ago to ~ 10 C today. Based on the hyperthermophilic domination of the roots of the tree of terrestrial life, and the probably universal nature of the thermal constraints on terrestrial biogenesis, we speculate that hyperthermophilic organisms dominate the roots of life emerging on any terrestrial planet.