The nature of the last universal common ancestor.

Cytologically, prokaryotes appear simpler and thus evolutionarily 'older' than eukaryotes. In terms of RNA processing, however, prokaryotes are sophisticated and eukaryotes, which retain many features of an RNA-world, appear primitive. The last universal common ancestor may have been mesophilic and could have had many features of the eukaryote genome, but its cytology is unknown.