The hypothetical ancestor common to all species is called LUCA (Last Universal Common Ancestor). It is believed to have lived 3,500 to 4,000 million years ago and have been composed of only one cell.

_{Source: The physiology and habitat of the last universal common ancestor (2016) / Physiology, phylogeny, and LUCA (2016) / The nature of the last universal common ancestor and its impact on the early Earth system (2024)}