Recent studies on the physiology of LUCA (Last Universal Common Ancestor) associated to molecular dating methods suggest that the ancestor cell appeared very early in the history of the Earth, in a hostile environment, consequence of an intense cosmic bombardment occurred at that time. In the present investigation, the population size of LUCA was estimated by assuming that the ancestor cell had a metabolism similar to the methanogen M. jannaschii fed essentially by H2 and CO2, one of the oldest known metabolism. The cell (wet) mass was evaluated to be approximately 530 fg and the energy per cell required to drive its metabolism and growth was estimated to be around 2:7 10?13 W=cell. If LUCA was fed by the chemical energy available from submarine hydrothermal vents, then its cell population in the Hadean oceans was expected to be around 1:8 1024. Such a value corresponds to approximately ve orders of magnitude less than the present estimated number of prokaryote cells in the Earth's oceans. The study of LUCA's physiology can reveal essential biochemical processes that might be common in extraterrestrial life, guiding the search for biosignatures on other planets.