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The last universal common ancestor (LUCA) is a hypothetical ancient microbe from which all present-day life descends. About 60,000 years ago, there lived a human in Africa from which all living humans descend. The LUCA is an idea based on a similar principle, but being the common ancestor of all life rather than just humans.
The LUCA is believed to have lived between 3.6 and 4.1 billion years ago. Life may have existed for 100-500 million years before the LUCA appeared. The LUCA is not the first living thing ever or the most primitive possible living organism, just the universal common ancestor of all extant organisms.
Although fossils from the period are scant and highly degraded, we can extrapolate characteristics of the LUCA by seeing what features all of life has in common today. This includes a genetic code based on double-stranded DNA, including four nucleotides, making up 64 possible three-nucleotide codons. This selection of nucleotides is arbitrary but universal to all earthly life.
Another shared feature is the way DNA instructions are expressed via single-stranded RNA intermediates. These RNA intermediates lead to the construction of proteins by ribosomes, tRNA and a group of related proteins. These proteins are built from 20 amino acids, and the synthesis pathways are arbitrary but universal. All forms of life use glucose (simple sugar) as a source of energy and carbon. ATP is always used as the energy currency of the cell. The LUCA would have had a simple locomotion system based on microtubules.
It is uncertain whether the LUCA more closely resembles the domain Bacteria or Archaea. Both have extremely primitive variants. Until 2002, a bacterium, Mycoplasma genitalium, was thought to have the shortest genome of all living things, consisting of 582,970 base pairs. Then that title was stolen by the Archaea Nanoarchaeum equitans, with 490,885 base pairs. In 2006, Candidatus ruddii, a bacterium, took the title again, with a genome only 159,662 base pairs long. The LUCA probably had a genomic complexity in this general range.
There is another theory about LUCA and an anatomic model about it. See the model at The Universal Matrix of Natural Systems and Life's Cycles.