In recent years, it has became clear that most proteins involved in cellular DNA precursor synthesis or DNA replication have been 'invented' more than once, indicating that the transition from RNA to DNA genomes was more complex than previously thought. Several authors have suggested that DNA viruses, which often encode their own version of these proteins, played an important role in this process. The nature of the genome of the last universal cellular ancestor (LUCA) -- that is, RNA or DNA, prokaryotic-like or eukaryotic-like -- remains in dispute. A hyperthermophilic LUCA would have suggested a circular, double-stranded DNA genome; however, recent data favor a mesophilic or moderately thermophilic LUCA.