FIGURE 1. Environmental temperatures at pivotal points in early evolutionary history.

The tree of life stems from the last universal common ancestor (LUCA), which evolved into bacteria, archaea (microorganisms similar to bacteria) and eukaryotes (organisms with nucleus-containing cells). Extant bacteria are mostly mesophilic (they thrive at temperatures of about 25–40 °C), but some are thermophilic (preferring temperatures greater than about 60 °C). Many extant archaea are thermophilic, but no such eukaryotes exist. Gaucher et al.¹ modelled and reconstructed proteins from ancestral bacteria, and from their heat stability conclude that the earliest bacteria lived in oceans at temperatures of 65–73 °C. Previous studies^{4, 5} based on models of ancient RNA suggest that LUCA was either mesophilic or thermophilic. The temperature at which the earliest archaea flourished has not been estimated.