LUCA is a hypothetical ancient microbe that is believed to be the universal common ancestor of all life on Earth. It lived between 3.6 and 4.1 billion years ago and had a simple microtubule-based locomotion system. Its characteristics can be extrapolated from what all life today has in common, including a genetic code based on double-stranded DNA and the use of glucose and ATP as sources of energy. It is unclear whether LUCA more closely resembles the domain Bacteria or Archaea, but it probably had genomic complexity in the range of the shortest genomes of living organisms.
The last universal common ancestor (LUCA) is a hypothetical ancient microbe from which all life today is descended. About 60,000 years ago, there lived a human being in Africa from whom all living humans are descended. LUCA is an idea based on a similar principle, but being the common ancestor of all life rather than just humans.
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 LUCA appeared. LUCA is not the first living thing ever or the most primitive living organism possible, just the universal common ancestor of all existing organisms.
Although fossils from the period are scarce and highly degraded, we can extrapolate the characteristics of LUCA by seeing what characteristics all life today has in common. 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 for all life on earth.
Another shared feature is the way DNA instructions are expressed via single-stranded RNA intermediates. These RNA intermediates lead to the construction of proteins from ribosomes, tRNAs, and a group of related proteins. These proteins are made up of 20 amino acids and the synthesis routes are arbitrary but universal. All life forms use glucose (simple sugar) as a source of energy and carbon. ATP is always used as the energy currency of the cell. LUCA would have had a simple microtubule-based locomotion system.
It is unclear whether 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 Archaea Nanoarchaeum equitans, with 490,885 base pairs. In 2006, Candidatus ruddii, a bacterium, retook the title, with a genome just 159,662 base pairs long. LUCA probably had genomic complexity in this general range.
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