The hypothesis that all living things share a common ancestry was already part of Darwin's thinking. It crystallized in the 20th century and became solidly established with the recognition that basic molecular strategies are universal, including transcription, translation and especially the genetic code. LUCA, the Last Universal Common Ancestor, is represented on the universal tree by its deepest node, that which marks the divergence of Bacteria from Archaea/Eukarya. The nature of that entity has been much debated and remains controversial. It now appears that LUCA was a much more advanced organism than originally expected, endowed with genes, membranes, enzymes, metabolism and the basic mechanisms of gene expression, replication and energy transduction. LUCA was a cell of sorts, but probably represents a stage before discrete lineages, whose members swapped genes and evolved communally. The chapter concludes with a brief presentation of dissenting opinions, chiefly those of Thomas Cavalier-Smith, Radhey Gupta and James Lake.