Last universal common ancestor

The last universal common ancestor (LUCA), also called the last universal ancestor (LUA), cenancestor, or (incorrectly) progenote, is the most recent population of organisms from which all organisms now living on Earth have a common descent. LUCA is the most recent common ancestor of all current life on Earth. LUCA should not be assumed to be the first living organism on Earth. The LUCA is estimated to have lived some 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era). The composition of the LUCA is not directly accessible as a fossil, but can be studied by comparing the genomes of its descendents, organisms living today. By this means, a 2016 study identified a set of 355 genes inferred to have been present in the LUCA.

The earliest evidence for life on Earth is biogenic graphite found in 3.7 billion-year-old metamorphized sedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia. A 2015 study found potentially biogenic carbon from 4.1 billion years ago in ancient rocks in Western Australia, but such findings would indicate the existence of different conditions on Earth during that period than are generally assumed today, and point to an earlier origin of life.

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."

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