Last universal common ancestor

The last universal common ancestor (LUCA) is the hypothesized latest common ancestral population of single-celled organisms from which all subsequent life forms descend under the three-domain system of Bacteria, Archaea, and Eukarya. Most studies suggest that the LUCA existed by 3.5 billion years ago, and possibly as early as 4.3 billion years ago or earlier. The nature of this point or stage of divergence remains a topic of research.

All earlier forms of life preceding this divergence and all extant organisms are generally thought to share common ancestry.