A nonhyperthermophilic common ancestor to extant life forms

The G + C nucleotide content of ribosomal RNA (rRNA) sequences is strongly correlated with the optimal growth temperature of prokaryotes. This propert y 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 linea ges and unequal substitution rates among sites, was devised to estimate anc estral base compositions. This method was applied to rRNA sequences of vari ous species representing the major lineages of life. The inferred C+C conte nt of the common ancestor to extant life forms appears incompatible with su rvival at high temperature. This finding challenges a widely accepted hypot hesis about the origin of life.