Darwinian threshold

The Last Universal Common Ancestor is often considered to be an already complex organism with a DNA-based genome, a complex informational flow and an efficient metabolism, but some authors, like Carl Woese, believe instead that the LUCA was not a discrete entity but rather a diverse community of cells that survived and evolved as a biological unit. RNA is capable of both catalysis and replication and could have been central to the origins of heredity and life itself. It has been proposed that the initial molecular events were carried out by transfer RNAs (tRNAs). It is hypothesized that structured tRNAs could have provided amino acids during a process called self-translation of a single extended tRNA strand. Compartmentalization with membranes was not yet completed and translation of proteins was not precise. Not every progenote had its own metabolism; different metabolic steps were present in different progenotes. Therefore, it is assumed that there existed a community of sub-systems that started to cooperate collectively and culminated in the LUCA. == After the Darwinian threshold ==

Most scientists place the LUCA at the root of the tree of life. From this root depart two Prokaryotic Domains: the Bacteria and the Archaea. Just after this first split, one of the branches, going towards the Archaea, splits again and gives rise to a third branch which is that of the Eukaryotes so that now there are three Domains of life. Carl Woese thought that even during the era around the origin of the LUCA, the root and the first branches were very blurred since the cells were not very well defined yet and HGT was still quite important. According to these authors the Domains that derived from LUCA through a process of reductive evolution or "streamlining" were Prokaryotes; mesophilic and thermophilic Bacteria and thermophilic Archaea. The term "prokaryote" should therefore be abandoned, since it suggests that "prokaryotes" preceded "eukaryotes" in their evolution from LUCA towards complexity. == See also ==