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A formal test of the theory of universal common ancestry


Universal common ancestry (UCA) is a central pillar of modern evolutionary theory1. As first suggested by Darwin2, the theory of UCA posits that all extant terrestrial organisms share a common genetic heritage, each being the genealogical descendant of a single species from the distant past3,4,5,6. The classic evidence for UCA, although massive, is largely restricted to ‘local’ common ancestry—for example, of specific phyla rather than the entirety of life—and has yet to fully integrate the recent advances from modern phylogenetics and probability theory. Although UCA is widely assumed, it has rarely been subjected to formal quantitative testing7,8,9,10, and this has led to critical commentary emphasizing the intrinsic technical difficulties in empirically evaluating a theory of such broad scope1,5,8,9,11,12,13,14,15. Furthermore, several researchers have proposed that early life was characterized by rampant horizontal gene transfer, leading some to question the monophyly of life11,14,15. Here I provide the first, to my knowledge, formal, fundamental test of UCA, without assuming that sequence similarity implies genetic kinship. I test UCA by applying model selection theory5,16,17 to molecular phylogenies, focusing on a set of ubiquitously conserved proteins that are proposed to be orthologous. Among a wide range of biological models involving the independent ancestry of major taxonomic groups, the model selection tests are found to overwhelmingly support UCA irrespective of the presence of horizontal gene transfer and symbiotic fusion events. These results provide powerful statistical evidence corroborating the monophyly of all known life.

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Figure 1: Selected class I evolutionary hypotheses, excluding HGT.
Figure 2: Selected class II evolutionary hypotheses, including HGT.


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I thank J. Felsenstein, P. Garrity, N. Matzke, C. Miller, C. Theobald and J. Wilkins for critical commentary.

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Correspondence to Douglas L. Theobald.

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The author declares no competing financial interests.

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This file contains Supplementary Tables S1-S12, Supplementary Equations and Discussion 2.1-2.3, Supplementary Figures S1-S2 with legends, Supplementary Methods and Results 3.1-3.4, Supplementary Notes 4.1-4.3 and References. (PDF 352 kb)

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Theobald, D. A formal test of the theory of universal common ancestry. Nature 465, 219–222 (2010).

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