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. On the basis of a formal statistical test, this theory of a universal common ancestry (UCA) is supported in preference to competing multiple-ancestry hypotheses. The first universal common ancestor (FUCA) is a hypothetical non-cellular ancestor to LUCA and other now-extinct sister lineages.
Whether the genesis of viruses falls before or after the LUCA—as well as the diversity of extant viruses and their hosts—remains a subject of investigation.
With no discovered fossil evidence of LUCA, it may precede the earliest known life forms. However, the detailed biochemical similarity of all current life makes its existence widely accepted by biologists. Its characteristics can be inferred from shared features of modern genomes. These genes describe a complex life form with a lipid bilayer, and many co-adapted features, including the genetic code and ribosomes for transcription and translation mechanisms, to convert information from DNA to mRNA to proteins.