The P-Value Podcast

Defining Life

September 22, 2023 Rachael Brown Season 2 Episode 4
The P-Value Podcast
Defining Life
Show Notes Transcript

Can we know the answer to the question "what is life?"? Could our definitions of life blinker us to new life forms we might find in the future? 

In 2010 scientists found some bacteria in remote Lake Mono in Yosemite National Park in Californian which threatened to redefine life as we know it. The Lake Mono bacteria appeared to be able to substitute arsenic for phosphorous in their DNA and proteins, overturning the long held view that just six biological elements—carbon, hydrogen, nitrogen, oxygen and sulfur—were necessary for life. Unfortunately, later the discovery was shown to be in error—no arsenic substitution was actually taking place. Nonetheless the discovery revived debates about the nature of life, and our ability to recognise new or novel ways of living that we might come across. Well we might ask, what was it about arsenic bacteria that would have warranted extending our account of life? Would we have been so happy to make such a move had arsenic life looked less familiar to us? Surprisingly perhaps, the answers to these questions are not as clear as they might be. 

The question of what life is might seem obvious. We seem to be pretty good at discerning when we are looking at living or non-living things. Rock - non living, fish - living, plant - living, dirt - non-living, roads - non-living etc… but this sort of folk knowledge doesn’t work well when we look at the more strange parts of our world. 

Viruses, for example, might seem to be living but aren’t capable of reproducing for themselves, have no capacity for respiration, or metabolism or anything really unless inside the cell of a living organism. They are essentially little protein capsules containing RnA or DNA and very little else which has led some to say they are not living at all. 

Scientifically we lack a clear definition of life which makes clear what it is it about all the apparently living things, the bacteria,, fungi, birds, penguins, whales and humans which distinguishes them from other non-living things. And what any new entity we come across must have in order to be classed as life. 


This week in the p-value we talk about life, what it is, and whether we would be able to recognise alien life if we saw it. 


NASA, the US space agency defines life as any “self-sustaining chemical system capable of Darwinian evolution”. This, they argue, captures all the cases of life we know of and thus is a good working definition for life which can be updated should new evidence about life, such as finding alien life, arise. 

Whilst this may seem reasonable, one might worry it is too parochial - too heavily indexed to our own planet and life here. Philosopher of Biology Emily Parke, from the University of Auckland expresses this worry well in a 2020 paper where she says “it cannot be assumed that life elsewhere would resemble life as we know it in even the most basic ways. So we need some way to recognise it that abstracts away from life as we know it.” Is such a thing possible? How do we define life in such a way that it leaves open the possibility that there are very different living things elsewhere we just haven’t found yet?

Some philosophers of biology dismiss the project altogether. Such definitonal skeptics argue that we are asking too much to expect a definition of life that is universal, and may not even need one anyway. 


Definitional skeptics, Carol Cleland and Christopher Chyba argue that we aren’t in an epistemic position to define life as we only have a single sample –life on Earth— from which to derive any general principles. They say ““No purported definition of ‘life’ can provide a scientifically satisfying answer to the question ‘‘What is life?’’ because no mere analysis using human concepts can reveal the nature of a world that lies beyond them.”

Cleland and Chyba are pointing here to the limited inductive base we have to work with when it comes to any defintion life. Life on Earth is widely accepted to have arisen from just one source, one origination. This, they argue, gives us insufficient grounds for making claims about life in general. Any concept of “life” we have can only reflect life on Earth and thus the peculiarities of life here. Not life more broadly. Moreover, says Cleland, trying to define life from such a limited base risks hindering or obscuring our ability to find life on other planets because it may mean we overlook life that is too different to that on Earth.

If this sort of definitional pessimism is true, then perhaps the best we can do is to construct and empirically test scientific theories about the general nature of living systems and try to use what we find to settle our classificatory dilemmas by explaining puzzling cases – why things that are alive sometimes lack features that we associate with life and why things that are non-living sometimes have features that we associate with life.

Edouard Machery is also a definitional skeptic but for slightly different reasons arguing that attempts to find a definition of life are either pointless or impossible and should be abandoned. He makes his case in two parts, arguing firstly that any definition based on folk concepts is unscientific off the bat. Any project based on these sorts of folk definitions of life is, he says, simply pointless as it is unable to yield useful predictions and explanations. Definitions of life based in science might seem a better place to go, but he also dismisses them on the grounds that there is no unified scientific consensus on the matter, and moreover, such a consensus is unlikely. This is because the concept of life and attempts to define it are spread across many scientific disciplines from astrobiology, to evolutioary biology, artificial life, synthetic biology and ethics. These different sciences have very different research agendas around “life” and their definitions reflect this. An evolutionary definition of life (i.e. one like the NASA definition), for example, is appealing to synthetic biologists as they can observe whether the artificial “life” they build can evolve. Astrobiologists would not find such definitions as satisfactory, as frequently they are looking for life in situations that preclude such observations (e.g. when looking at dead or fossilized structures for which we can’t make claims about evolution). “We are now” says Machery, “likely to end up with several definitions of life, which naturally raises the following question: Which of these definitions would tell us what life is?”. For these reasons Machery thinks the project of defining life is either impossible or pointless and should be discarded. Machery goes on to say that the whole idea that we need a definition to proceed with a science of life is mistaken anyway—research into the nature of life, its origins and diversity, can continue without a definition just fine. 

Whilst the definitional skeptics appear to have a point, not everyone agrees. We will look at three such views after the break. 

Leonardo Bich, Sara Green and others, including Emily Parke, concede that the skeptics might be right about the prospects of actually getting an agreed upon definition of life, but conclude more optimistically that definitions of life play a useful role in science even if no definitional consensus is possible. Weaker, localised, operational definitions, they point out, can be used to help narrow down our research agenda and interests, for example, helping us decide which biomarkers to test for in samples from Mars. They help guide our search and resources to the sorts of places most likely to harbour life. Such working or operational definitions need not be universal— different disciplines can adopt what suits their purposes, and they are typically understood to be revisable given new evidence. NASA see their definition to fall under this sort of category—a guide but not a definitive or irrefutable one. This sort of response will not, of course, convince the skeptic like Cleland who sees the NASA concept as hindering our ability to discover, to use her words “life as we don’t know it”.

For Cleland, a working definition, if based in biology on Earth risks us stumbling over life very differnt to our own and missing it entirely. 

Kelly Smith offers a more fatalistic sort of optimism in arguing that there is simply no viable alternative to the pursuit of definitions of life (at least given our current epistemic and theoretical situation) and thus it is the very best we can do. To quote Smith ““The bottom line is that there is simply no viable alternative, either pragmatically or theoretically, to the pursuit of definitions. If nothing else, the empirical data the pessimists demand will be a very long time coming and scientists will of necessity continue to employ definitions of life in the interim. Chastising them for this will only drive their ideas underground where they can escape critical analysis, making the problems caused by problematic conceptions of life worse…

… Therefore, however messy and confused things may be, science really has no choice but to follow Churchill’s advice and just ‘keep plodding on.’ In fact, we would be well advised to produce more definitions of life and wholeheartedly embrace the debate that ensues.”

Smith and Parke also points out that we may be being too narrow with how we look at the definitional projec in the first place. Definitions, they say, can take many forms (not just a single set of necessary and sufficient conditions) and hence failure to achieve such a definition for life is no reason to give up on the project of defining life (Smith 2016, Parke 2020). Biological systems, are, afterall typically messy. Attempts to define all sorts of biological phenomena from species, to human nature, disease, the gene, even biolgoical individuality have all resisted simple categorisations defined by necessary and sufficient conditions. Why, then think life will be any different? If this is the case, perhaps we need to think of life dimensionally? Or as having vague boundaries? Rather than being all or nothing?

Such a reconceptualisation of life would be radical, but perhaps, as Cleland points out, radical is what is needed to fully capture the true richness of living things in the universe?