Last universal common ancestor

Every living thing on Earth, from bacteria to blue whales, descends from a single ancestral population of cells known as LUCA, the last universal common ancestor. LUCA was not the first life; it was the most recent population whose descendants include all life alive today, while other early lineages died out. A 2024 study led by researchers at the University of Bristol used genetic 'molecular clocks', which estimate time from the rate at which genes accumulate changes, and dated LUCA to about 4.2 billion years ago. Other estimates range from about 3.5 to over 4 billion years, and the date remains debated.

The evidence for a common ancestor lies inside every cell. All known life uses DNA to store genes, RNA to carry instructions, and ribosomes to build proteins, and nearly all organisms use the same genetic code, in which three-letter DNA 'words' stand for the same amino acids. Cells everywhere also use ATP as their energy currency. Such detailed shared machinery is very unlikely to have evolved separately many times. Charles Darwin suggested in 1859 that all life might come from one original form; modern genetics confirmed it.

By comparing genes shared by the two oldest branches of life, bacteria and archaea, scientists reconstruct what LUCA was like. The 2024 study suggests it was already a fairly complex single cell with about 2,600 proteins, living without oxygen, possibly feeding on hydrogen and carbon dioxide, and part of an ecosystem with other microbes. Earlier work pointed to hot environments such as hydrothermal vents. Genes swapped between ancient microbes make the reconstruction difficult.