"Caldococcus noboribetus" genes have been mutated to understand the characteristics of the isocitrate dehydrogenase ancestral protein. The mutant isocitrate dehydrogenase genes contain inferred ancestral sequences that have been expressed in "E. coli" cells. Expression of these genes resulted in proteins with ancestral amino acid sequences, which provided evidence for relationships with a hyperthermophilic universal ancestor. Thermostabilities of the purified enzymes had greater thermal stabilities than wild-type isocitrate dehydrogenase. This supports a hypothesis that last universal common ancestor was thermophilic or hyperthermophilic. This finding has implications for understanding the evolutionary history of other proteins as well for engineering protein thermostability for experimental or industrial purposes.