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.

The last universal ancestor (LUA), also called the last universal common ancestor (LUCA), or the cenancestor, is the most recent organism from which all organisms now living on Earth descend.^[1] Thus it is the most recent common ancestor (MRCA) of all current life on Earth. The LUA is estimated to have lived some 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era).^[2][3]

"There is strong quantitative support, by a formal test"^[1] for the theory that all living organisms on Earth are descended from a common ancestor.^[4] A statistical comparison of various alternative hypotheses has shown that universal common ancestry is significantly more probable than models involving multiple origins.^[1] "A universal common ancestor is at least 10²⁸⁶⁰ times more probable than having multiple ancestors'"^[5] "A model with a single common ancestor but allowing for some gene swapping among species was'10³⁴⁸⁹ times more probable than the best multi-ancestor model'"^[5]

[edit] Features

Based on the properties currently shared by all independently living organisms on Earth,^[6][7][8][9] it is possible to deduce the defining features of the LUCA. These features were present in the LUCA:

• The genetic code was based on DNA.
  • The DNA was composed of four nucleotides (deoxyadenosine, deoxycytidine, deoxythymidine and deoxyguanosine), to the exclusion of other possible deoxynucleotides.
  • The genetic code was composed of three-nucleotide codons, thus producing 64 different codons. Since only 20 amino acids were used, multiple codons code for the same amino acids. This coding structure was arbitrary.
  • The DNA was kept double-stranded by a template-dependent DNA polymerase.
  • The integrity of the DNA was maintained by a group of maintenance enzymes, including DNA topoisomerase, DNA ligase and other DNA repair enzymes. The DNA was also protected by DNA-binding proteins such as histones.
• The genetic code was expressed via RNA intermediates, which were single-stranded.
  • RNA was produced by a DNA-dependent RNA polymerase using nucleotides similar to those of DNA with the exception of thymidine in DNA was replaced by uridine in RNA.
• The genetic code was expressed into proteins.
• All other properties of the organism were the result of protein functions.
• Proteins were assembled from free amino acids by translation of an mRNA by ribosomes, tRNA and a group of related proteins.
  • Ribosomes were composed of two subunits, one big and one small.
  • Each ribosomal subunit was composed of a core of ribosomal RNA surrounded by ribosomal proteins.
  • The RNA molecules (rRNA and tRNA) played an important role in the catalytic activity of the ribosomes
  • Only 20 amino acids were used, to the exclusion of countless non-standard amino acids
  • Only the L-isomers of the amino acids were used.
• Glucose could be used as a source of energy and carbon; only the D-isomer was used.
• ATP was used as an energy intermediate.
• There were several hundred protein enzymes which catalyze chemical reactions that extract energy from fats, sugars, and amino acids, and that synthesize fats, sugars, amino acids, and nucleic acid bases using arbitrary chemical pathways.
• The cell contained a water-based, cellular protoplasm that was surrounded and effectively enclosed by a lipid bilayer membrane.
• Inside the cell, the concentration of sodium was lower, and potassium was higher, than outside. This gradient was maintained by specific ion pumps.
• The cell multiplied by duplicating all its contents followed by cellular division.

[edit] Hypotheses

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.^{[citation needed]}

It is possible that all of LUA's contemporaries became extinct and only LUA's genetic heritage lived to this day. There are some indicators that LUA was no single agent but part of a swarming community (LUCAs).^[10] 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.^[11] These hypotheses are ruled out by Douglas L. Theobald^[1] who "finds strong support for the unity of life compared with even two independent origins."^[12] "The question of whether or not all life on Earth has an ultimate common origin is a subtle one, complicated by the phenomenon of lateral gene transfer. It has now been tackled with a formal statistical analysis [by Theobald]."^[4]

[edit] See also

• Timeline of evolution
• Common descent
• Origin of life
• Cell (biology)#Origin of the first cell
• Most recent common ancestor

[edit] References