THE EMERGENCE OF MAJOR CELLULAR PROCESSES IN EVOLUTION

The phylogenetic distribution of divergently related protein families into the three domains of life (archaea, bacteria and eukaryotes) can signify the presence or absence of entire cellular processes in these domains and their ancestors. We can thus study the emergence of the ma jor transitions during cellular evolution, and resolve some of the con troversies surrounding the evolutionary status of archaea and the orig ins of the eukaryotic cell, In view of the ongoing projects that seque nce the complete genomes of several Archaea, this work forms a testabl e prediction when the genome sequences become available, Using the pre sence of the protein families as taxonomic traits, and linking them to biochemical pathways, we are able to reason about the presence of the corresponding cellular processes in the last universal ancestor of co ntemporary cells. The analysis shows that metabolism was already a com plex network of reactions which included amino acid, nucleotide, fatty acid, sugar and coenzyme metabolism, In addition, genetic processes s uch as translation are conserved and close to the original form, Howev er, other processes such as DNA replication and repair or transcriptio n are exceptional and seem to be associated with the structural change s that drove eukaryotes and bacteria away from their common ancestor, There are two major hypotheses in the present work: first, that archae a are probably closer to the last universal ancestor than any other ex tant life form, and second, that the major cellular processes were in place before the major splitting. The last universal ancestor had meta bolism and translation very similar to the contemporary ones, while ha ving an operonic genome organization and archaean-like transcription, Evidently, all cells today contain remnants of the primordial genome o f the last universal ancestor.