In evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor.
Charles Darwin proposed the theory of universal common descent through an evolutionary process in On the Origin of Species, twice stating the hypothesis that there was only one progenitor for all life forms and ending with "There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one". The theory asserts that all currently living organisms on Earth share a common genetic heritage with each being the descendant from a single original species, though the suggestion of substantial horizontal gene transfer during early evolution has led to questions about monophyly of life.
The last universal ancestor (LUA) (also called the last universal common ancestor, LUCA), that is, the most recent common ancestor of all currently living organisms, is believed to have appeared about 3.9 billion years ago