Adaptation to environmental temperature is a major determinant of molecular evolutionary rates in Archaea

Methods to infer the ancestral conditions of life are commonly based on geological and palaeontological analyses. Recently, several studies focused on the use of genomes to gain information about past ecological conditions. Several used the property that the G+C and amino-acid contents of bacterial and archaeal rDNA genes and proteins, respectively, are strongly influenced by the environmental temperature. The adaptation to optimal growth temperature (OGT) since the Last Universal Common Ancestor (LUCA) over the universal tree of life was examined and it was concluded that LUCA was likely to have been a mesophilic organism and that a parallel adaptation to high temperature occurred independently along the two lineages leading to the ancestors of Bacteria on one side and of Archaea+Eukarya on the other side. Here, we focus on Archaea to gain a precise view of the adaptation to OGT over time in this domain. It has been often proposed on the basis of indirect evidence that the last archaea...