Last Universal Common Ancestor

The last universal common ancestor (LUCA) is the most recent population of organisms from which all living cells today are descended. It was not the first life, but the last point at which the lineages of bacteria and archaea joined.

Estimates from molecular clocks place LUCA roughly 3.5 to 4.2 billion years ago, only a few hundred million years after Earth formed. A 2016 study of gene families shared by bacteria and archaea reconstructed a set of about 355 genes, suggesting an anaerobic organism that used hydrogen and carbon dioxide for energy, and possibly lived near a hydrothermal vent. These conclusions rest on assumptions that are still debated.

All known life uses DNA to store information, RNA and ribosomes to make proteins, the same basic genetic code, and ATP as an energy carrier. Carl Woese's work on ribosomal RNA in the 1970s helped establish the three domains and the idea of a deep common root, shown in the tree of life.

LUCA was already complex, so it must have been preceded by simpler systems, a subject of research on abiogenesis and the RNA world hypothesis. Gene exchange between early lineages makes the root of the tree blurry, and the later eukaryogenesis produced the third domain from within the archaea.

• Tree of Life — where LUCA sits at the root
• Abiogenesis — how life may have begun
• Genetic Code — a feature shared by all life