The G + C nucleotide content of ribosomal RNA (rRNA) sequences is strongly correlated with the optimal growth temperature of prokaryotes. This property allows inference of the environmental temperature of the common ancestor to all life forms from knowledge of the C + C content of its rRNA sequences. A model of sequence evolution, assuming varying C + C content among lineages and unequal substitution rates among sites, was devised to estimate ancestral base compositions. This method was applied to rRNA sequences of various species representing the major lineages of life. The inferred C+C content of the common ancestor to extant life forms appears incompatible with survival at high temperature. This finding challenges a widely accepted hypothesis about the origin of life.