The last common ancestor

The experimental data that prove the existence of the three primary kingdoms do not tell us much about the last common ancestor (LCA), but we can still say that such a progenitor must have existed, because all cells of the three kingdoms had the same genetic code, the same metabolic currency based on ATP, and roughly 50% of bacterial genes have homologues in eukaryotes.
As for the first living cells (the first common ancestor or FCA) we know even less, but again we are not completely in the dark. The evidence that we do have tells us that they came from the RNP world, and their genomes were made therefore almost completely of RNAs. This means that during the transition from first to last common ancestor, the cells substituted RNA with DNA in their genes, probably by using enzymes that were very similar to inverse transcriptases. Traces of this substitution, in fact, seem to have survived, because many modern enzymes that produce DNA (the DNA polymerases) are still capable of functioning as inverse transcriptases (Poole, Jeffares and Penny, 1998).
The reason for adopting DNA genomes was probably the fact that DNA is more stable than RNA, and was therefore a more suitable material for heredity, but there could also have been another reason. RNA’s linear molecules can be used both as genes and messengers, and in the RNP world these two roles were performed by the same molecules, which could create some confusion. The necessity to distinguish between genes and messengers could well have been a good reason for subtituting the RNA genes, and in this case it is not surprising that the choice fell on DNA, because this molecule is easily obtained from RNA and transports exactly the same information.
In addition to changing the genome’s nucleic acids, it is possible that other modifications took place during the evolution from first to last common ancestor, but for the moment we know virtually nothing about these developments. The characteristics of the last common ancestor are therefore highly hypothetical, and yet many have already decided that they were bacterial features.