The results of an experimental test for verifying the hypothesis that the common ancestor of all living organisms (universal ancestor, commonote) was a hyperthermophile (Miyazaki, et al., 2001), are explained. In the experiment, mutant enzymes with ancestral aminoacids were made using a gene engineering technique. The mutant enzymes were purified and tested for thermostability. The mutant enzymes with ancestral aminoacids showed higher thermostability than the contemporary hyperthermophile enzyme. The results suggest that the common ancestor of all living organisms was a hyperthermophile. The argument related to the hyperthermophilic common ancestor hypothesis was reviewed with respect to the experimental test.
