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Universal Common Ancestry, LUCA, and the Tree of Life: Three Distinct Hypotheses about the Evolution of LifeIt is widely agreed that common ancestry is a central feature of the modern theory of evolution, yet there does not appear to be a shared understanding of what “common ancestry” actually means. This becomes especially clear when we examine the (rare) attempts to directly test its truth. There are many more discussions of “the Tree of Life” and of “the last universal common ancestor”, but as I will argue, while each of these also has an unclear meaning, it is best to think of these three phrases as referring to distinct hypotheses ordered in a logical way: the existence of a Tree of Life entails a last universal common ancestor which would entail universal common ancestry, but none of the converse entailments hold. When understood this way, it is easiest to make sense of the debates surrounding the Tree, as well as our lack of knowledge about the last universal common ancestor, while still maintaining the shared agreement of the truth of universal common ancestry.
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