Last universal ancestor (LUA), also LUCA (last universal common ancestor), is 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. It is estimated to have lived some 3.6 to 4.1 billion years ago .
The LUA is not hypothesised as:
- the first living organism ever.
- the most primitive possible living organism.
At the time when LUA was first hypothesized, construction of cladograms based upon genetic distance between all living cells indicated that there was an early split between the archaeans and the remainder of life. This was inferred from the fact that, at that time, all known archaeans were highly resistant to environmental extremes such as high salinity, temperature or acidity, and led some to suggest that LUA evolved in areas like the deep ocean vents, where such extremes prevail today. However, since that time, archaeans have been 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 a matter of some contention.
It is possible that all of LUA's contemporaries would have since become extinct with only the LUA's genetic heritage living on to this day. However, this suggestion ignores the possibility of horizontal gene transfer. Carl Woese has proposed that there was no individual organism which could be considered a LUA, but rather that the genetic heritage of all modern organisms derives from an ancient community of organisms.
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