  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.
