A group of organisms is said to have common descent if they have a common ancestor. In modern biology, it is generally accepted that all living organisms on Earth are descended from a common ancestor or ancestral gene pool.
A theory of universal common descent via an evolutionary process was proposed by Charles Darwin in his book On the Origin of Species (1859), and later in The Descent of Man (1871). This theory is now generally accepted by biologists, and the last universal common ancestor (LUCA or LUA), that is, the most recent common ancestor of all currently living organisms, is believed to have appeared about 3.9 billion years ago. The theory of a common ancestor between all organisms is one of the principles of evolution, although for single cell organisms and viruses, single phylogeny is disputed (see: origin of life).
In his book The Ancestor's Tale, Richard Dawkins uses the word concestor as a substitute for "common ancestor." This new word is very gradually entering scientific parlance.
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