Pluralism is popular among philosophers of biology. This essay argues that negative judgments about universal biology, while understandable, are very premature. Familiar life on Earth represents a single example of life and, most importantly, there are empirical as well as theoretical reasons for suspecting that it may be unrepresentative. Scientifically compelling generalizations about the unity of life (or lack thereof) must await the discovery of forms of life descended from an alternative origin, the most promising candidate being the discovery of extraterrestrial life. Nonetheless, in the absence of additional examples of life, we are best off exploring the microbial world for promising explanatory concepts, principles, and mechanisms rather than prematurely giving up on universal biology. Unicellular microbes (especially prokaryotes) are by far the oldest, metabolically most diverse, and environmentally tolerant form of life on our planet. Yet somewhat ironically, much of our theorizing about life still implicitly privileges complex multicellular eukaryotes, which are now understood to be highly specialized, fragile latecomers to Earth. The problem with pursuing a pluralist approach to understanding life is that it is likely to blind us to the significance of just those entities and causal processes most likely to shed light on the underlying nature of life.
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RNA viruses, which store hereditary information as RNA, are the only known exceptions.
The term ‘microbe’ is used loosely even among microbiologists. All microbiologists include prokaryotes (Archaea and Bacteria) and unicellular eukaryotes (e.g., protozoa). Many uses of the label also include acellular viruses, and some include microscopic multicellular eukaryotes (e.g., rotifers) as “microbes.” My focus in this paper is on prokaryotes.
Not everyone agrees, however (Kurland et al. 2006).
And viruses in turn outnumber them (Edwards and Rohwer 2005; Rohwer and Barott this issue). Viruses are found in large numbers in every microbial community and play a central role in microbial evolution as a source of genetic variation.
The concept of species is highly problematic for prokaryotes. I will have more to say about this in the next section.
And of course their acellular viral companions, who are critical to the structure and dynamics of the microbial world (Suttle 2007) and are discussed elsewhere in this special issue (Rohwer and Barott this issue).
It is important to keep in mind that a scientifically compelling universal theory of life is unlikely to explain all biological phenomena on our particular planet any more than knowledge of basic physics and chemistry can explain all geological phenomena occurring on our planet. Knowledge of the basic chemistry of water, for instance, cannot tell us the source of Earth’s supply of water (volcanism), or what causes monsoons, or droughts. Similarly, knowledge of the chemistry of water cannot tell us whether Mars was ever wet for an extended period of time or whether it merely experienced sudden, high volume flows for brief periods of time; this is a hotly debated question among planetary scientists. The point is it is unrealistic to expect a universal theory of life to explain all the details of life on a particular planet, such as our Earth, since many of them will be contingent upon extraneous conditions. Critics of the prospects for universal biology sometimes ignore this important point.
Horizontal gene transfer also goes under the label “lateral gene transfer” (aka LGT).
It is important to keep in mind that explaining water in terms of its molecular composition is not the same as identifying water with it (Cleland 2012, pp. 138–139). A single molecule of H2O lacks temperature and pressure. Moreover many of the distinctive phenomenal properties of water (mentioned above) result from secondary structures (dimers, trimers, and hydrogen bonded networks) produced by “weak” hydrogen bonds among H2O molecules, and the purest samples of liquid water are not composed of just H2O molecules because they invariable dissociate into ions (H+, OH−, and H3O+) when combined together; for more on the chemistry of water, see (Snoeyink and Jenkins 1980). Nonetheless modern chemistry explains the behavior of water by reference to its molecular composition in the context of molecular theory.
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I am very grateful to Maureen O’Malley for comments on the manuscript during its development. I would also like to thank two anonymous reviewers for comments on the original version.
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Cleland, C.E. Pluralism or unity in biology: could microbes hold the secret to life?. Biol Philos 28, 189–204 (2013). https://doi.org/10.1007/s10539-013-9361-7
- Universal theory of life