thumb|upright=1.75|[[Phylogenetic tree linking all major groups of living organisms, namely the bacteria, archaea, and eukaryota, with the last universal common ancestor (LUCA) shown at the root]]
The last universal common ancestor (LUCA) is the hypothesized latest common ancestral cell population from which all subsequent life forms descend under the three-domain system of<!-- An early tree of life was sketched by Jean-Baptiste Lamarck in his Philosophie zoologique in 1809. Charles Darwin more famously proposed the theory of universal common descent through an evolutionary process in his book On the Origin of Species in 1859: "Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed." The last sentence of the book begins with a restatement of the hypothesis:
The DNA was kept double-stranded by an enzyme, DNA polymerase, which recognises the structure and directionality of DNA. The integrity of the DNA was maintained by a group of repair enzymes including DNA topoisomerase. If the genetic code was based on dual-stranded DNA, it was expressed by copying the information to single-stranded RNA. The RNA was produced by a DNA-dependent RNA polymerase using nucleotides similar to those of DNA<!--, with the exception that the DNA nucleotide thymidine was replaced by uridine in RNA-->. The genetic code was expressed into proteins. These were assembled from 20 free amino acids by translation of a messenger RNA via a mechanism of ribosomes, transfer RNAs, and a group of related proteins.
LUCA's functionality, and evidence for the early evolution of membrane-dependent biological systems, together suggest that LUCA was a cell with membranes. It contained a water-based cytoplasm enclosed by a lipid bilayer membrane; it reproduced by cell division.
By phylogenetic bracketing, analysis of its offspring groups, LUCA appears to have been a small, single-celled organism. It likely had a ring-shaped coil of DNA floating freely within the cell. Morphologically, it would likely not have stood out within a mixed population of small modern-day bacteria. The originator of the three-domain system, Carl Woese, stated that in its genetic machinery, the LUCA would have been a "simpler, more rudimentary entity than the individual ancestors that spawned the three [domains] (and their descendants)".
Because bacteria and archaea differ in their structure of phospholipids and cell wall, ion pumping, most proteins involved in DNA replication, and glycolysis, it is inferred that LUCA had a permeable membrane without an ion pump. The emergence of Na<sup>+</sup>/H<sup>+</sup> antiporters likely led to the later evolution of impermeable membranes in eukaryotes, archaea, and bacteria. This would accord with LUCA's having made use of the natural geochemical proton gradient in its environment across a leaky membrane to provide it with energy. Cell walls, too, would have evolved later. Although LUCA likely had DNA, it is unknown if it could replicate DNA: as Weiss et al. write, it "might just have been a chemically stable repository for RNA-based replication".
LUCA's genome was likely similar in size to that of modern prokaryotes, encoding around 2,600 proteins, based on statistical inference using the probabilistic gene- and species-tree reconciliation algorithm ALE. It may have been an acetogen, respiring anaerobically, and may have had an early CAS-based anti-viral immune system. The inferred metabolic features are consistent with the early Earth hydrothermal systems with high concentrations of CO<sub>2</sub> and H<sub>2</sub>.