Review of The Quest for a Universal Theory of Life: Searching for Life As We Don't Know It by Carol E. Cleland

W hat is life ? This question—simple to pose, famously difficult to answer—has challenged philosophers and scientists since Aristotle addressed it more than two millennia ago. In the modern era, answers have frequently been proposed in the form of a definition—a set of properties that would allow one to classify something as life or not-life. An example from an elementary biology textbook (Simon et al., 2019) lists eight defining properties: order; cells; growth and development; energy processing; regulation; response to the environment; reproduction; and evolution. A central problem with this and many definitions is that of obvious exceptions. An individual organism does not evolve, yet we know it is alive. A mule cannot reproduce, neither can one rabbit, yet we know they are alive. Some things like a forest fire or a growing crystal may satisfy many of the properties, yet we know they are not alive.

Various methods have been used in attempts to escape this dilemma. A common approach has been to subsume individual properties into more overarching criteria. The so-called “NASA definition of life,” credited to Gerald Joyce (1994), is probably the best-known example of this: Life is a self-sustained chemical system capable of undergoing Darwinian evolution. But here again, those familiar with theories of the origin of life will note an obvious exception: living systems preceding the last universal common ancestor (LUCA) of all known life on Earth in which rampant (horizontal) exchange of genetic material likely dominated over the parent-to-progeny (vertical) genetic transfer of Darwinian evolution.

In her intriguing analysis of The Quest for a Universal Theory of Life, philosopher Carol Cleland expands on work she published almost two decades ago with scientist Chris Chyba (Cleland and Chyba, 2002; see also Cleland and Chyba, 2007) addressing why this dilemma has been so intractable. They argued that attempting to define life in the current epoch is analogous to attempting to define “water” before the advent of molecular theory. Under that circumstance (say, in the 16^th century), water might be defined by reference to its properties as being wet, transparent, odorless, tasteless, thirst quenching, a good solvent, and so on. Such a definition would face the same problems as many of our definitions of life. There are exceptions (e.g., ocean water, swamp water) and other substances that share many of the properties but obviously aren't water (e.g., hydrochloric and nitric acids). But with an understanding of the molecular nature of matter we can now say that water is H₂O. Even though this is not strictly accurate (liquid water naturally includes dimers, trimers, OH^-, H⁺, H₃O⁺, HDO, etc.), we understand that this definition provides the most scientifically informative and useful answer to the question, “What is water?” It then follows that the reason we cannot define life is that we lack an overarching theory of biology comparable to atomic and molecular theory.

In The Quest for a Universal Theory of Life, Cleland builds provocatively on this earlier work, arguing not only that we can't define life but that we should give up trying. She argues that the quest for a definition (in contrast to the titular quest for a universal theory) is futile and counterproductive. Her argument for futility involves both the nature of definitions and the concept of natural kinds. Natural kinds are things that would exist in nature in the absence of human interests, concerns, and conventions. Water and gold are examples. A classic example of a non-natural kind is a bachelor, generally defined as an unmarried adult human male. Definitions lend themselves to non-natural kinds, since standard definitions are concerned with language and human concepts—with the analysis of the meanings of terms. Since the concept of a bachelor is entirely a human construct, it can be satisfactorily defined in terms of other human concepts such as adult and male.

Natural kinds like water and gold can be defined through what philosophers call stipulative definitions, in which a technical meaning is provided on the basis of an underlying scientific theory. But in these cases, as Cleland points out, it is the theory that does most of the work. So in the absence of a theory of biology, a stipulative definition of life, which is most likely a natural kind, is not possible. (If life is not a natural kind, then it can be defined in terms of human concepts; but in that case there is no natural unity to life, the quest for a universal theory is doomed, and any definition is of limited scientific value.) Given all of these limitations, Cleland argues that a definition of life will likely only serve to entrench our misconceptions, blinding us to truly novel forms of life should they be encountered.

In contrast to the quest for a definition, Cleland argues that the quest for a universal theory of life should continue. She rejects arguments to the contrary as premature, noting that we know only the single example of life on Earth descended from LUCA (the so-called N = 1 problem) and hence face a fundamental impediment to generalizing about life, for example, to discriminating accidental from essential characteristics. Furthermore, what we do know about that single example is biased toward concepts associated with readily observed complex multicellular eukaryotes, particularly plants and animals. Rather, she argues, we should think of life from the perspective of the more characteristic and diverse microbial world. Until we adjust our thinking in this way, and perhaps most importantly, discover additional examples of life—life as we don't know it—we will not know whether or not a universal theory of life is possible.

Cleland supports this argument by citing historical examples of flawed ontologies—fundamental sets of theoretical concepts and principles—that hindered the development of theoretical understanding. A prominent example is Aristotle's concepts of motion. Aristotle believed that the natural state of inanimate terrestrial objects is rest and that the challenge was therefore to explain how an object begins to move and stays in motion. The latter aspect of the challenge led to the development of the concept of “impetus,” an internal, motive cause for keeping an object, such as a projectile after launch, in motion. It was not until Newton embraced the then-radical idea that matter is kinetically “inert”—replacing impetus with inertia and reframing the question as “what causes a moving body to stop?”—that the stage was set for his universal theory of motion. By analogy, Cleland argues that it may be a flawed ontology—based on an Aristotelian, plant-and-animal-biased perspective of a single example—that hinders our quest for a universal theory of life.

So what are we to do? Cleland devotes the last three chapters of her book to three strategies all aimed at overcoming the N = 1 problem: developing artificial life (ALife); searching for life beyond Earth; and searching for a microbial “shadow biosphere,” that is, microbial life on Earth descended from a separate origin of life than that leading to LUCA. She dismisses ALife as too much based on our current concepts of life and too much removed from natural life to tell us much about the potential for truly novel forms of life. For the latter two strategies, she argues we should focus on searching for anomalies that are biologically promising using tentative (not defining) criteria for life. As an example of a problematic use of defining criteria, she cites the 1976 Viking biology experiments in which the criteria for life detection were determined in advance, leading to debate continuing to this day about how to completely account for the results obtained on Mars. In contrast, tentative criteria would focus on whether an anomalous result was potentially of biological interest. This seems to this reviewer akin to the astrobiology community's focus on “agnostic biosignatures,” indicators not tied to characteristics of life as we know it but which may manifest as, for example, unexpected (anomalous) complexity in a planetary environment or in preserved molecules (National Academies of Sciences, Engineering, and Medicine, 2019).

Finally, regarding the odds of success of the two strategies—searching for life elsewhere and searching for a shadow biosphere on Earth—Cleland firmly places herself on the side of searching for N = 2 at home. She contests arguments against the existence of a shadow biosphere, noting three examples of anomalous phenomena which she argues should be considered with more openness to the possibility that they represent novel forms of life. She does not argue that a shadow biosphere exists but rather that we openly question the widely accepted assumption that all life on Earth (known and potentially unknown) is descended from a common ancestor.

This relatively slim book (220 pages of text) covers a lot of ground. Cleland helps the reader by adhering to the old dictum—tell them what you're going to tell them, tell them, then tell them what you've told them—beginning with the introduction, in which she summarizes the book's major arguments, continuing in the structure of each chapter (and sometimes even within chapter sections), and concluding, appropriately, in the conclusion. Her aim was to make the content accessible to both philosophers and scientists. I think it likely she has been successful. So if you're a scientist wanting to understand the philosophical underpinnings of attempts to define life spanning more than two thousand years, or a philosopher wanting to learn about exploration of Earth's biosphere and efforts to find a second example of life, then The Quest for a Universal Theory of Life is a great place to start.