The last universal common ancestor (LUCA), also called the last universal ancestor (LUA), cenancestor, or (incorrectly[R 1]) progenote, is the most recent population of organisms from which all organisms now living on Earth have a common descent.[1] LUCA is the most recent common ancestor of all current life on Earth. LUCA is not thought to be the first living organism on Earth, but only one of many early organisms, whereas the others became extinct.
While there is no specific fossil evidence of LUCA, it can be studied by comparing the genomes of its descendants, all organisms known to be living today. By this means, a 2016 study identified a set of 355 genes inferred to have been present in LUCA. This would imply it was already a complex life form with many co-adapted features, including transcription and translation mechanisms to convert information between DNA, RNA, and proteins.[2][3] However, some of those genes could have developed later and spread universally by horizontal gene transfer between archaea and bacteria.[4]
LUCA is estimated to have lived some 3.5 to 3.8 billion years ago in the Paleoarchean era,[5][6] a few hundred million years after the earliest evidence of life on Earth, for which there are several candidates. Microbial mat fossils have been found in 3.48 billion-year-old sandstone from Western Australia,[7][8] while biogenic graphite has been found in 3.7 billion-year-old metamorphized sedimentary rocks from Western Greenland.[9][10] Recent studies have tentatively proposed evidence of life as early as 4.28 billion years ago.[11][12][13]
Charles Darwin proposed the theory of universal common descent through an evolutionary process in his book On the Origin of Species in 1859, saying, "Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed."[14] Later biologists have separated the problem of the origin of life from that of the LUCA.
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