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Last universal ancestor (LUA), the hypothetical latest living organism from which all currently living organisms descend. As such, it is the most recent common ancestor of the set of all currently living organisms. Also LCA (last common ancestor) or LUCA (last universal common ancestor). It is estimated to have lived some 3.5 billion years ago.
It is hypothesized that the last universal ancestor already had all of the properties that are shared by all currently living organisms, such as a (prokaryotic) cell structure, DNA, the modern genetic code and mRNA, tRNA and ribosome mediated translation.
Notes on possible misconceptions:
- The LUA was not necessarily the first living organism ever,
- Likewise, it likely was not the most primitive possible living organism.
- Finally, it almost certainly was not alone.
Construction of cladograms, based upon genetic distance between all living cells, shows that there was a very early split between the Archaeobacteria that are highly resistant to environmental extremes of great salinity, high temperature or high acidity, and the remainder of life. This has led some to suggest that LUCA evolved in areas like the deep ocean vents, where such extremes prevail today.
All its contemporaries have since become extinct; only the LUCA's genetic heritage lives on to this day. Carl Woese proposed  that our pre-LUCA genetic heritage derives from a community of organisms, rather than an individual.
- ^ Note that Viruses are not usually considered organisms, a prerequisite for the assertion that all organisms are DNA based.
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