For lowest common ancestors in graph theory and computer science, see lowest common ancestor.
A cladogram linking all major groups of living organisms to the LUA (the black trunk at the bottom). This graph is derived from ribosomal RNA sequence data.
A cladogram linking all major groups of living organisms to the LUA (short trunk at the center). This graph is derived from complete genome sequencing data.
Based on the properties shared by all independently living organisms on Earth,[2][3] it is possible to derive the features that the LUA must have had before branched out:
The genetic code is composed of three-nucleotide codons, thus producing 64 different codons. Since only 20 amino acids are used, multiple codons code for the same amino acids. These choices are arbitrary and are shared by all eukaryotes and prokaryotes. Archaea and mitochondria use a similar code with minor differences.
The DNA is kept double-stranded by a template-dependent DNA polymerase
The integrity of the DNA is maintained by a group of maintenance enzymes, including DNA topoisomerase, DNA ligase and other DNA repair enzymes. The DNA is also protected by DNA-binding proteins like histones
The genetic code is expressed via RNA intermediates, which are single-stranded.
RNA is produced by a DNA-dependent RNA polymerase using nucleotides similar to DNA with the exception of Thymidine, replaced by Uridine.
The genetic code is expressed into proteins. All other properties of the organism (e.g. synthesis of lipids or carbohydrates) are the result of protein enzymes.
Inside the cell, the concentration of sodium is lower, and potassium higher than outside. This choice is arbitrary. This gradient is maintained by specific ion pumps.
The cell multiplies by duplicating all its contents, followed by cellular division.
When LUA was hypothesized, cladograms based on genetic distance between living cells indicated that Archaea split early from the rest of life. This was inferred from the fact that all known archaeans were highly resistant to environmental extremes such as high salinity, temperature or acidity, and led some scientists to suggest that LUA evolved in areas like the deep ocean vents, where such extremes prevail today. But archaeans were discovered in less hostile environments and are now believed by many taxonomists to be more closely related to eukaryotes than bacteria, though this is still somewhat contentious.
It is possible that all of LUA's contemporaries became extinct and only LUA's genetic heritage lived to this day. Or, as proposed by Carl Woese, perhaps no individual organism can be considered a LUA, but the genetic heritage of all modern organisms derived through horizontal gene transfer among an ancient community of organisms.[4] Another hypothesis to explain the paucity of alternative life forms is panspermia, the inoculation of Earth by life carried on meteorites.
