First universal common ancestor

The first universal common ancestor (FUCA) is a proposed non-cellular entity that is the earliest ancestor of the last universal common ancestor (LUCA) and its descendants, including every modern cell. FUCA would also be the ancestor of ancient sister lineages of LUCA, none of which have modern descendants.

FUCA is thought to have been composed of progenotes, proposed ancient biological systems that would have used RNA for their genome and self-replication, in place of DNA. By comparison, LUCA would have had a complex metabolism and genome with hundreds of genes and gene families.

The now-extinct sister lineages descended from FUCA are thought to have horizontally transferred some of their genes into the genome of early descendants of LUCA.