The Smallest Living Thing: An Exploration On Life

A sea of tumultuous clouds crowds the sky like an army readied for battle. Grey and thunderous, they rush to cover any space left blank, fighting to ensure no light leaks through. Despite it all, a singular ray of sun struggles and shoves its way to an opening, bursting open a spotlight onto something precious. Its target? A speck of meaningless grey so miniscule, a dust particle seems like a skyscraper in comparison. If you put a magnifying glass to it, you might even think there was nothing there. Though, you might not want to count it out just yet. See, that grey dot is very precious indeed. Why? Its name is the Nanoarchaeum Equitans, also known as the Smallest Living Thing we’ve discovered thus far. But how do we know this?

What is life?

We need to have a solid understanding of what we think Life is. As your primary teacher has undoubtedly drilled into your head, there are seven characteristics we use to identify a living thing, which are conveniently packaged into the well-loved acronym “MRS. GREN”: movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition. As you’ve developed and matured, MRS. GREN has evolved into the more thorough, albeit less catchy, characteristics of homeostasis, cell organisation, metabolism, growth, adaptation, reproduction, and a response to stimuli. These characteristics do a great job of laying the basic groundwork down, and are all foundational attributes, but there is a lot more nuance involved when classifying matter as being life or not. 

Millions of scientists around the globe constantly contest what a living being is, evident by the fact they all have their own attributes they assign to life. There are roughly 123 lists of characteristics to what life is, all of which physicist Trifonov (2011) went through and analysed as a method of determining a common link. His definition, that "Life is self-reproduction with variations”, is compact, yes, but not entirely accurate. Mules, which are hybrids between horses and donkeys, don’t have the ability to reproduce. A mule has 63 chromosomes, which aren’t in pairs, and as meiosis takes place, their sex cells are not complete most of the time. By Trifonov’s definition, a mule is considered a non-living thing and so does NASA’s definition of “a self-sustaining chemical system capable of Darwinian evolution” (Joyce, 2019). 

The most debated topic in this area though, is undoubtedly the question of whether or not a virus is a living thing. They don’t partake in metabolism (converting food into energy) like other living organisms do, yet they are able to reproduce and evolve given a host and sufficient amount of time. Even more perplexing, by some definitions, computer viruses would be classified as living things (Trifonov, 2012). It seems impossible for something that’s digital by nature to be considered a living thing, and yet, sometimes it is. The main issue is that viruses seem more like chemistry sets than organisms, due to them consisting of nucleic acids in a protein coat (Villareal, 2008). Yet when it enters a host cell, it starts to perform metabolism and replication, though some argue that without other organisms, viruses would not be able to, therefore still classifying it as non-living (Çengel, 2023). A laundry list of rebuttals goes on and on, but at the crux of it all, viruses remain between the border of non-living and living. 

In any case, for our purposes today, we will stick to living things having to have all seven characteristics mentioned above, meaning viruses are not in this discussion. Homeostasis, which is self-regulation performed by a biological system to maintain equilibrium in order for survival. Organisation, defined as being composed of a hierarchical structure composed of the basic building block of life, cells. Metabolism, the earlier mentioned conversion of food into energy. Growth which is self-explanatory. Adaptation, a process an organism undertakes to have better chances of survival in its habitat. Response to stimuli, which is exemplified by the leaves of a plant turning towards the sun. Finally, reproduction, which is producing new organisms either asexually or sexually. 

What is the smallest living thing?

In the world of microorganisms, one type of species stands out as one of immense population size and variation: bacteria. Microscopic and seemingly indetectable, infinite species of bacteria populate every surface we see in our daily lives. Just how small do they go? Normally, scientists measure bacteria in the micrometer (µm) unit, which is around 1 millionth of a meter. However, certain types of bacteria, like the Mycoplasma genitalium, reach sizes so small they’re measured in nanometers (nm), which are a billionth of a meter. M. genitalium, as it’s often shortened to, is considered the smallest organism discovered thus far at around 200 to 300 nm that can grow and reproduce independently, albeit in lab environments. (Conrad, 2001). 

Even smaller than that is the Nanoarchaeum equitans, a microbe discovered in 2011 sitting in a hydrothermal vent in Iceland. With a measurement of 400 nanometers across its cells, it was officially crowned by the Guinness Book of World Records as the smallest living organism in the world. Though, if we break the boundaries of our current definitions, there are theories that the true smallest form of life could be nanobes. Sitting at 20 nm in diameter, these structures were originally found in rocks (theorised to be present everywhere) and are a tenth the size of mycoplasma. Though, the ever-present problem and running theme is that they’re widely debated as being non-living. 

Final thoughts

As we’ve explored throughout this piece, life escapes definition no matter which way you put it. Through the restraints we’ve set, the Nanoarchaeum equitans stays the smallest organism scientists know. But I can’t help but wonder if, through a different set of restraints, we would find ourselves a new world record. As I’ve said before, I find it fitting that something as incomprehensible and mystical as life cannot be defined even by scientific terms. In an earlier article of mine on Panpsychism, I explored all the different ways life could be categorised philosophically. Even then, I reached the conclusion of leaving it up to the reader. Going into this article, I believed my findings would differ; after all, this is science - the realm of rigid rules and categorisation. But it seems Life, in any manner of speaking, is eternally undefined. Mystical and confusing it is, just the way it should be. 

References

Çengel, Y. A. (2023). Eighteen distinctive characteristics of life. Heliyon, 9(3), e13603. https://doi.org/10.1016/j.heliyon.2023.e13603 

Life (Stanford Encyclopedia of Philosophy). (2021, November 30). https://plato.stanford.edu/entries/life/#Conc 

Nanobes and nanobacteria. (n.d.). Nanobes and Nanobacteria. https://serc.carleton.edu/microbelife/topics/nanobes/index.html 

Re: What is the smallest living thing? (2001, September 25). https://www.madsci.org/posts/archives/sep2001/1001464156.Gb.r.html 

Trifonov, E. N. (2012). Definition of Life: Navigation through Uncertainties. Journal of Biomolecular Structure and Dynamics, 29(4), 647–650. https://doi.org/10.1080/073911012010525017 

Trifonov, E. N. (2011). Vocabulary of definitions of life suggests a definition. Journal of Biomolecular Structure and Dynamics, 29(2), 259–266. https://doi.org/10.1080/073911011010524992 

Villarreal, L. P. (2025, March 31). Are viruses alive? Scientific American. https://www.scientificamerican.com/article/are-viruses-alive-2004/ 

Why can mules not reproduce? (no date) Extension Horses. Available at: https://horses.extension.org/why-can-mules-not-reproduce  (Accessed: 8 June 2025).