To be or not to be: The search for a definition of life
Sara George
Sara George
Sara George is majoring in Journalism and Spanish at the University of Missouri and is a student in the Liminality and the Sciences seminar of the Honors College.
Since the beginning of human existence, the question of life has dominated scientific, religious, and political conversations. As living beings, our understanding of life seems simple at initial glance. We know that a rock is not alive, for example, but the bugs crawling around on it are alive. Yet, trying to explain why a rock is different from a bug may be a much more difficult question. Such is the question of defining life.
Many early philosophers and scientists defined life based on observation, constrained by limits in technology. Plato was one of the first to create a classification system for life, in which life could be categorized as a vegetable, animal, or rational being (Mariscal, 2021). Aristotle came up with one of the first definitions of life as something that “grows, is self-sustaining and reproduces” (Love, 2018). In the mid 19th-century, advances in scientific knowledge and technology led to cell theory (Love, 2018). This concept speculates that all life is made up of cells, the smallest unit of life. This theory presents an interesting question, though. We do not consider parts of the cell, such as the mitochondria or ribosomes, to be life forms. So, what gives the cell life? And what are the other characteristics that life shares?
Biology, the study of life, continues to debate the definition of life. There are more than 120 definitions of life that continue to be scrutinized and debated (Trifonov, 2012). In order to be able to conduct field studies and reach consensus among scientists, many of these definitions are called working (or operational) definitions. They allow scientists to set constraints to reach a consensus, but don’t necessarily set a philosophical, all-encompassing definition for life.
One common working definition highlights simple characteristics that “life” often shares as part of its working definition. These include organization, homeostasis, metabolism, growth, response to stimuli, reproduction, and adaptation (Khan Academy, n.d). Most of what we consider life meets some variation of these characteristics; a mule, for example, cannot reproduce, but is very much alive. Others are more complex. And as the definition of life continues to be debated among scientists, the answer to whether or not these parts of matter meet the characteristics of life largely depends on who you ask.
For example, viruses meet many of the characteristics for life. Historically, viruses have not been thought of as a life form. They are not made up of cells. And they are not able to meet many characteristics, such as reproduction, without the presence of a host. Although, as many experts have noted, while viruses may be considered non-living for failing to be self-sustaining, many organisms classified as living cannot sustain themselves. Parasitic organisms need a host to survive. And heterotrophs depend on the existence of other living organisms in order to obtain energy through consumption.
Viruses are thought to have first been studied in the late 19th century. A 1937 New York Times article states that viruses occupy the “shadowy borderland where matter is ready to leap into life” (Zimmer, 2021). This concept is further expanded on by virocell theory (Forterre, 2011), which suggests that viruses are alive. Under this theory, virions, the viral particle often confused with the virus, have the potential for life that is met when infecting a host cell. When in the cell, the virus (or viral factory) acts as if it was alive. Under this theory, viral matter can appear both dead and alive, depending on the constraints acting upon it.
Our knowledge around viruses is still expanding. For example, the newly discovered mimivirus contains a much larger genome than other viruses. Its genes code for proteins responsible for metabolism and translation: both abilities associated with living beings (PBS Eons, 2022). The mimivirus is also unique in its ability to be a host to other viruses (Love, 2018). Previously, viruses were thought to be incapable of becoming a host. The unique nature of viruses leads to some questions around the nature of life itself. Is life something that can be conditional? And if viruses are not considered alive, do we consider them dead? Or do we recognize a new state in the “shadowy borderland” between life and non-life?
The consideration of viruses as “non-life” or “on the verge of life” have left viruses out of parts of studies reserved for life. One such example is evolution. Due to the exchange of genetic information with their “host” and their ability to rapidly mutate, viruses undoubtedly have some impact on the evolution of other lifeforms. Research by Patrick Forterre, a scientist at the University of Paris-Sud, concluded that the enzymes responsible for DNA replication in eukaryotes likely originated from a viral source (Villarreal, 2008). Viral proteins are also found in the reproductive system and immune system in humans (Scudellari, 2021). And the algae in Earth’s oceans may have gained their photosynthetic abilities from viruses (Zimmer, 2021).
Some definitions of life seek to find a simpler and broader approach to life. One such definition is the NASA definition, which states that “life is a self-sustained chemical system capable of undergoing Darwinian evolution” (Zimmer, 2021). This definition is intended as a working definition when attempting to define what life on other planets may look like.
We have dedicated science fiction novels, cinema, and even sensationalized journalistic articles to appeal to this interest in understanding whether or not we are alone in the universe. Any suggestion that there may be water on mars or livable conditions on another planet has always generated human interest. Yet, when reading these articles, I’ve always questioned whether or not we would know extraterrestrial life if we found it. This led me to question whether extraterrestrial life would be subject to the same constraints we place on life on Earth.
When looking for extraterrestrial life, scientists focus on two key areas (Mariscal, 2021). Certain chemical compounds are believed to be dependent on life to exist; these are called biosignatures. The other area of study focuses on signs of technology that may indicate intelligent life. This area has yielded no results, despite attempts to send information into “promising” parts of the universe.
However, since life is largely defined by our observations of Earth, it is fair to question whether we are limiting ourselves when seeking out extraterrestrial life. Life may have evolved under different constraints, forcing us to widen our definition of life. Some theories suggest that we ditch the definition all together. Instead of using definitions, one study suggests using tentative criteria when looking at possible life-forms on other planets. This would prevent us from dismissing the phenomena we don’t understand as abiological, and encourage further study of them as possible signs of life (Cleland, 2019). In an interview with Astrobiology Magazine, Dr. Carol Cleland references the ability of microbes to survive “interplanetary journeys” often encased in meteors (Astrobiology Magazine staff, n.d.). Other theories use the example of cosmic matter to suggest the possibility that life is a universal and connected concept.
Similarly to concepts that arise in the studies of consciousness and astrophysics, discussions of life often fall in the intersection of science and philosophy. While science can seek to understand what life is, I think a widely-accepted definition of life is nearly impossible. Not only are we constrained by the observable world, but ideas around life are often subjective. We have spoken in class before about our desire in western culture to classify and define; perhaps, trying to find an answer to the question of life is an unsolvable idea that we are attempting to find an answer to.
Stepping outside of the binary can send us into a liminal space, even if it’s only for a span of a few seconds. From a young age, we assume that we are alive and conscious. When we stop and consider what these ideas really mean, we can be plummeted into anti-structure. Suddenly, very simple ideas become incredibly complex and lead to further questions. How did life originate on Earth? Why are we here? And would we be able to distinguish whether or not we were alive?
I have expressed these moments of anti-structure as beautiful and necessary in past reflections. Yet, one question remains: if these ideas remain unknown, how do we avoid existing in a perpetual anti-structure? Here, I propose a possible theory. Perhaps, acceptance of the unknown marks the end of a liminal experience. Our structure crumbles when we stop and consider a once-simple concept as an unknown variable; we are pulled over the threshold, either willingly or unwillingly. This occurs for people at different times and in different contexts. Scientists can encounter liminality when new discoveries are made that challenge existing scientific literature. Often, people leaving deeply-held religious or familial beliefs encounter liminality when confronted with new concepts. And everyone may encounter liminality when evaluating their place in life.
While experiences with liminality are highly individualized, the experience is marked with the common quality of anti-structure. The previous constraints and worries that determine an individual’s outlook on life fade into the background as they enter a
near-existential experience. A liminal experience may last seconds to years. Over time, people will undoubtedly return to this liminal space, but acceptance of the unknown may (at the very least) challenge us to step outside of the binary and remain open to new ideas and non-definitions. We can only hope to meet that space of thought by embracing anti-structure as part of the process.
References
Astrobiology Magazine staff. (n.d.). Carol Cleland on Life’s Working Definition: Does It Work? NASA. https://www.nasa.gov/vision/universe/starsgalaxies/life%27s_working_definition.html
Cleland C. E. (2019). Moving Beyond Definitions in the Search for Extraterrestrial Life. Astrobiology, 19(6), 722–729. https://doi.org/10.1089/ast.2018.1980
Forterre, P. (April, 2011). Manipulation of cellular synthesis and the nature of viruses: The virocell concept. Comptes Rendus Chimie. 14(4). 392-399. https://doi.org/10.1016/j.crci.2010.06.007.
Khan Academy. (n.d.). What is life? Khan Academy. https://www.khanacademy.org/science/biology/intro-to-biology/what-is-biology/a/what-is-life
Kurzgesagt – In a Nutshell. (July 26, 2017). Optimistic Nihilism [Video]. YouTube. https://www.youtube.com/watch?v=MBRqu0YOH14
Love, L. G. (Sept. 19, 2018). Does the discovery of the mimivirus call into question attempts to define life? Bioscience Horizons: The International Journal of Student Research, 11(). https://doi.org/10.1093/biohorizons/hzy006
Mariscal, C. (Nov. 30, 2021). Life. Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/cgi-bin/encyclopedia/archinfo.cgi?entry=life
Nurse, P. (Oct. 25, 2019). What is Life? – with Paul Nurse. [Video]. The Royal Institution. https://www.youtube.com/watch?v=_z-SUo2wP4I
Nuwer, R. (June 17, 2020). Why the world needs viruses to function. BBC. https://www.bbc.com/future/article/20200617-what-if-all-viruses-disappeared
PBS Eons. (June 29, 2022). Giant Viruses Blur The Line Between Alive and Not [Video]. YouTube. https://www.youtube.com/watch?v=EJvxOO2WVzg
Scudellari, M. (Nov. 1, 2021). Are viruses alive, not alive or something in between? And why does it matter? Science News. https://www.sciencenews.org/article/viruses-alive-coronavirus-definition
Trifonov, E. N. (2011). Vocabulary of Definitions of Life Suggests a Definition. Journal of Biomolecular Structure and Dynamics. 29(2). 259-266. DOI: 10.1080/073911011010524992
Villarreal, L. P. (Aug. 8, 2008). Are Viruses alive? Scientific American. https://www.scientificamerican.com/article/are-viruses-alive-2004/
Zimmer, C. (Aug. 26, 2021). Are Viruses Alive – with Carl Zimmer. [Video]. The Royal Institution. https://www.youtube.com/watch?v=Tryg5UCp6fI
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