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Universal common ancestry, LUCA, and the Tree of Life: three distinct hypotheses about the evolution of life

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Abstract

Common ancestry is a central feature of the theory of evolution, yet it is not clear what “common ancestry” actually means; nor is it clear how it is related to other terms such as “the Tree of Life” and “the last universal common ancestor”. I argue these terms describe three distinct hypotheses ordered in a logical way: that there is a Tree of Life is a claim about the pattern of evolutionary history, that there is a last universal common ancestor is an ontological claim about the existence of an entity of a specific kind, and that there is universal common ancestry is a claim about a causal pattern in the history of life. With these generalizations in mind, I argue that the existence of a Tree of Life entails a last universal common ancestor, which would entail universal common ancestry, but neither of the converse entailments hold. This allows us 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 uncontroversial truth of universal common ancestry.

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Notes

  1. When biologists claim that we know that “all life is related,” this does not require a precise definition of what counts as a living thing. However, it does require several clarifications. First, while viruses may or may not be alive, they are not being considering here. The origins of viruses are not at all clear. While it is possible that they all emerged out of cellular life, it is also possible that some viruses existed before cellular life and may or may not have originated independently of it (Forterre 2016; Nasir et al. 2017). Second, the claim is not necessarily about all life in the universe, but just the life on Earth that we have actually discovered and identified.

  2. Of course lateral gene transfer is not the only process that causes gene histories to differ. For example, in sexual species, hybridization and incomplete lineage sorting also lead to gene tree incongruence (Maddison 1997).

  3. Clarke (2010) lists thirteen different candidate definitions of individual or organism that have been defended in the literature.

  4. In response to my claim that the Tree of Life must have a root, a number of audience members have pointed out at various talks that there are rooted and unrooted phylogenetic trees. But the Tree of Life is supposed to be a representation of evolutionary history and as such it must have a root. To see this, it is perhaps enough to note that on an unrooted phylogenetic tree, there is no distinction between a “clade” and its complement (all the tips not in that clade). To ensure that collections such as the “non-mammals” (which includes everything from lizards, the mosses, and E. coli) are not real groups requires a root.

  5. A different story might be told utilizing the model of Doolittle and Booth’s (2017) “it’s the song not the singer” idea. Here, we could imagine a community where a song (a pattern of interactions such as metabolic, structural, or developmental interactions) gets preserved, replicated, and altered over time without the underlying physical lineages necessarily being the ancestors of the future lineages singing the song. Different singers (different taxa) can be recruited horizontally at different points during evolutionary history. This might also be called common ancestry without a LUCA.

  6. A reader here may worry about the definition of “life” and how that is being used in the argument. If we want to answer the question “How many times did life begin?” we obviously need to know what qualifies as life. That problem is extremely tricky and may not even have a correct answer at all. But we do not have to answer the “what is life” question in order to answer the question of whether a given collection of entities has a common origin. For example, whether the LUCA in Weiss et al.’s (2016) story counts as alive or not is irrelevant since it is a causal bottleneck in the sense that living things can causally trace their properties back to the properties of LUCA.

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Acknowledgements

I would like to thank Ford Doolittle, Maureen O’Malley, Jeremy Schwartz, Elliott Sober, Quayshawn Spencer, several anonymous referees, and audiences at the University of Pennsylvania and the 2016 Philosophy of Science Association meeting for helpful comments and discussion.

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  1. Texas Tech University, Lubbock, TX, USA

    Joel Velasco

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Velasco, J. Universal common ancestry, LUCA, and the Tree of Life: three distinct hypotheses about the evolution of life. Biol Philos 33, 31 (2018). https://doi.org/10.1007/s10539-018-9641-3

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