What did Charles Darwin conclude about the existence of a common ancestor for all life

The last universal ancestor (LUA), also called the last universal common ancestor (LUCA), or the cenancestor, is the most recent organism from which all organisms now living on Earth descend.The LUA is estimated to have lived some 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era).

It is not that there is a specific ancestor, but Darwin's theory of evolution states that every being on Earth has a common descent, all life forms deriving from the same beginning. It would start with the simplest of micro-organisms, and continue until this very day!