The late stage of genetic code structuring took place at a high temperature.

The correlation between the optimal growth temperature of organisms and a thermophily index based on the propensity of amino acids to enter more frequently into (hyper)thermophile proteins is used to conduct an analysis aiming to establish whether genetic code structuring took place at a low or a high temperature. If the number of codons attributed to the various amino acids in the genetic code constitutes an estimate of the mean amino acid composition of proteins produced when the genetic code was definitively structured, then the thermophily index can also be associated to the genetic code. This value and the sampling of the variable thermophily index of different alignments of protein sequences from mesophile, thermophile and hyperthermophile species make it possible to establish, with an extremely high statistical confidence, that the late stage of genetic code structuring took place in a hyperthermophile (or thermophile) 'organism'. Moreover the 95% confidence interval of the temperature at which the genetic code was fixed turned out to be 91+/-24 degrees C. These observations seem to support the hypothesis that the origin of life might have taken place at a high temperature.