Defining Life

What is Life? Why should we care about defining it? As biologists, our research tends to focus on the details, not the bigger picture. Today, Josi Buerger steps back and looks at how the basic concept of “Life” affects topics from conservation to astrobiology.


The Life Sciences, though young in comparison to other disciplines, have made incredible progress in explaining the tangible world around us. Yet even at the most fundamental level, biological processes remain a mystery. Basic questions remain unanswered: Why did living systems evolve on this planet? Could it have taken radically different forms? Why do organisms display such complexity and diversity?

The fact that we can ask and discuss these questions surely indicates that defining life is unnecessary – we all know what it means and know it when we see it. Our efforts should be aimed at understanding biological processes, not quibbling over a definition…

There are two main concerns why scientists should care about definitions: Firstly, even if we do not fully understand biological processes, there are scenarios where we need to be able to identify and label them. Space exploration comes to mind, or the deep ocean. Secondly, arguing about definitions may not be traditional examples of the “Scientific Method”, but there are concrete examples where definitions affects experimental design: in early evolutionary biology, dentifying life forms from interacting organic material is vital. Or consider the construction of “minimal genomes”: does a self-replicating plasmid suffice as a minimal organism, or does it require a more complex cellular set-up?


To better understand the debate, we’ll look at two common definitions. The first one is from NASA’s guidelines for space exploration:

“Life is a self-sustaining chemical system capable of Darwinian evolution.”

Any chemical traces found in deep space are analysed with this definition in mind. It allows researchers to conclude whether a pattern stems from non-biological phenomena or potential alien life forms. This definition is very broad and perhaps too-encompassing, But with good reason: this definition must be able to identify alien life forms entirely different to what we know. And while it may sound like pure conjecture, this working definition actively influences the experiments carried out by the Mars Rovers Expeditions.


Though the search for extraterrestrial life is fascinating in its own right, there are reasons much closer to home why defining “life” matters: the notion of life is at the very heart of the biological sciences. The most fundamental theories in biology require a notion of what “alive” means.This concept is the difference between a single-celled organism and a microscopic piece of dust, which in turn  distinguishes ecosystems from other physical processes. Questions of preservation and conservation also depend on this distinction. Things that might otherwise be seen as mere resources may be subject to conservation efforts when recognised as living beings.


“A system displaying compartmentalisation, metabolism, homeostasis, growth, stimulus response, reproduction, and adaptive evolution.”

Here is an alternative, conditional definition of “life” proposed by Carl Sagan in 1970 (and indeed other illustrious scientists, such as Schrödinger of feline fame). When objects fulfill all the aforementioned conditions, they are classified as living. Organisms whose living status is disputed, such as viruses, may fail to fulfill one or more of these conditions (viruses fail to reproduce in an independent manner). The conditional approach is useful in clarifying these disputes, by drawing attention to those factors we see as most important for living things to have. It helps explain why we think of things like fire as non-living. Although fire exhibits certain attributes characteristic of living things (growth in response to fuel, homeostasis, response to stimuli such as wind), it fails to meet other important criteria (e.g. adaptive evolution).

Clearly the conditional approach is a useful tool. However, it also brings drawbacks. Most notably, it is a descriptive definition that was developed by scientists based on those particular living things they had encountered. And this encounter encompasses a shockingly unconvincing sample size of one biosphere: the Earth. The conditional definition of life is tailored well to our understanding of living systems, indeed it is tailored too much towards living systems on Earth and thus not suitable for identifying life forms outside of this environment.

In a paper from 2002, Cleland and Chyba make an interesting point in regard to exactly this problem. They explain that before we understood how molecules make up the world around us, any definition of “water” was bound to be too broad (since it would likely include the impurities commonly found within water) or too narrow (not considering ice as water). And this is true also for Life: “prior to the elaboration of such a theory [of living systems], it is not possible to be certain that a definition will, in fact, ever be formulated” by only using descriptive or conditional facts.

So what do we take from all of this? If we think the current definition of “water” as H2O is correct, then we take it to be correct in the context of molecular theory, i.e. part of an expansive scientific field. Before this field was thoroughly developed, we had to make do with less refined definitions of “water”, which nonetheless remained useful for particular purposes.  Any refined definition of “life” will similarly be correct in the context of an advanced biological theory of living systems. While biology may provide such a theory for the concept of life as chemistry has done for H2O, a complete understanding of life still eludes even the most brilliant of scientists. The aforementioned definitions must then be seen as rough-and-ready generalisations suited to a variety of differing present day purposes, to be supplanted by increasingly more refined definitions as the biological theory matures. As much as a suitable definition of Life will be groundbreaking both for the debate on Origin as well as Extraterrestria, it will remain elusive until a concrete theory of Life has been established.


Cleland, C.E. and Chyba, C.F., 2002. Defining ‘life’. Origins of Life and Evolution of the Biosphere, 32(4), pp.387-393.

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