Last universal ancestor

Definition
noun
The hypothetical primordial organism from which all other species of organisms on Earth descended
Supplement
The last universal ancestor is a hypothetical primordial organism that served as the ancestor to all organisms on Earth. It is estimated to have existed some 3.5 to 3.8 billion years ago, i.e. in Paleoarchean era.^{1, 2}
The hypothesis may be traced back from the proposal of Charles Darwin in his book On the Origin of Species in 1859. Accordingly, there could be one progenitor from where all life forms on Earth descended.³ A formal quantitative testing was done on the last universal ancestor hypothesis, and was published in 2010. By applying the model selection theory to molecular phylogenies on a set of ubiquitously conserved proteins that are orthologous, the results provided evidence of monophyly of life, i.e. the existence of a lone universal common ancestor.⁴ Furthermore, it was hypothesized that this last universal might not be alone but coexisted with other microbial forms. Nevertheless, only its descendants might have survived to have given rise to various life forms beyond the Paleoarchean era.
Abbreviation / Acronym: LUA
Synonym(s):

• last universal common ancestor (LUCA)
• cenancestor
• progenote

Reference(s):

¹ Doolittle, W. F. (February 2000). “Uprooting the tree of life”. Scientific American 282 (2): 90–5.

² Glansdorff, N., Xu, Y., & Labedan, B. (2008). “The last universal common ancestor: emergence, constitution and genetic legacy of an elusive forerunner”. Biology Direct 3: 29.

³ Darwin, C. (1859). On the Origin of Species. London: John Murray, Albermarle Street. Pg. 484 and 490.

⁴ Theobald, D. L. (May 2010). “A formal test of the theory of universal common ancestry”. Nature 465 (7295): 219–22.