Last universal common ancestor

The Last Universal Common Ancestor, or LUCA, is the name given to last common ancestor of Earth's current life. It is believed to be the first living organism from which both Prokaryotes and Eukaryotes decended. In the Universal SSU rRNA Tree of Life, the phylogenetic tree shows that the LUCA organism occurs just before the basal split in the tree into the two main domains, Bacteria and Archaea.

Multiple hypothesises try to answer the question as to how LUCA came to life. The current most accepted suggestion to this is the RNA world hypothesis.