Walk into any high school biology class and you’ll see it. The list. You know the one—it usually says something about growth, reproduction, and metabolism. It looks clean. It looks settled. But honestly? The definition of biological life is a total mess.
The more we look at the edges of our world, the more that neat little list falls apart. NASA scientists, synthetic biologists, and philosophers have been arguing about this for decades. Some say we have over 100 different definitions. Others think the whole quest is a waste of time because "life" isn't a fundamental thing like an atom, but just a word we made up to describe stuff that acts a certain way.
It’s weird. We can tell a cat is alive and a rock isn't. That’s easy. But what about a virus? What about a self-replicating computer program? What about a crystal that grows and organizes itself? That’s where things get blurry.
The "NASA Definition" and Why It’s Not Perfect
If you ask the folks looking for aliens, they’ll give you a very specific answer. Back in the 90s, a committee chaired by Gerald Joyce came up with what we now call the "working definition." It says life is a self-sustaining chemical system capable of Darwinian evolution.
It's a clever definition. It covers the basics: you need chemistry (sorry, AI), you need to be able to keep yourself going (metabolism), and you need to be able to change over generations. But there’s a massive problem.
Think about a mule.
A mule is clearly alive. It breathes, it eats, it kicks. But a mule is sterile. It can’t evolve because it can’t pass on its genes to a next generation. Under a strict interpretation of the "Darwinian evolution" rule, an individual mule—or an elderly person past reproductive age—doesn't technically fit the definition. That feels wrong, doesn't it?
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Then you have the virus problem. Viruses are basically just some genetic code wrapped in a protein shell. They don’t have a metabolism. They don’t eat. They can't reproduce on their own; they have to hijack a cell to do the work for them. Most biologists say they aren't alive. But they evolve. They have DNA or RNA. They are more "alive" than a pebble, but less "alive" than a bacterium.
The Seven Pillars of Life (Koshland’s Perspective)
Daniel Koshland, a heavy hitter in the world of biochemistry, tried to fix this by proposing the "Seven Pillars of Life." He used the acronym PICERAS: Program, Improvisation, Compartmentalization, Energy, Regeneration, Adaptability, and Seclusion.
- Program: This is your organized plan. In our case, DNA. It’s the blueprint.
- Improvisation: This is basically evolution. The ability to change the program when the environment gets tough.
- Compartmentalization: You need a container. A cell membrane. You have to keep the "inside" separate from the "outside."
- Energy: Life is a hungry process. You need a way to take in fuel and turn it into movement or repair.
- Regeneration: You wear out. You need to fix yourself.
- Adaptability: This is different from evolution. It’s a short-term response. You get hot, you sweat.
- Seclusion: Your chemical reactions need to stay private. You can't have every enzyme in your body bumping into every other molecule at once.
It’s a robust list. But even Koshland admitted it’s not a "definition" so much as a set of properties. And even then, some things we consider "alive" might fail a pillar, while some non-living things might pass.
Why Thermodynamics Might Hold the Key
Some researchers, like Jeremy England at MIT, think we’re looking at this all wrong. Instead of looking at biology, we should look at physics. Specifically, entropy.
The universe loves chaos. Everything tends to break down and spread out. That’s the Second Law of Thermodynamics. But life? Life is weirdly good at staying organized. We take energy from the sun or food and use it to build complex, orderly structures.
Schrödinger—the "cat in the box" guy—actually wrote a book called What is Life? in 1944. He argued that life is something that feeds on "negative entropy." We essentially suck order out of our environment to keep ourselves from decaying into a pile of dust. It’s a beautiful way to think about it. We are islands of order in a sea of cosmic chaos.
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The Synthetic Life Dilemma
We’re getting to a point where the definition of biological life isn't just a philosophical debate; it's a lab reality. In 2010, Craig Venter’s team created "Synthia," the first self-replicating bacterial cell with a completely synthetic genome.
If we can build it from scratch, is it "biological"? If we eventually build a robot that can repair itself, gather its own energy, and print copies of itself with slight "mutations" in its code, is it alive?
The Swedish philosopher Nick Bostrom and others have pointed out that our definitions are incredibly "carbon-centric." We assume life has to be wet and squishy because that’s all we’ve seen. But that’s like a fish assuming all "travelers" must have scales.
The Gray Zone: Prions and Mimiviruses
Let’s talk about the weird stuff.
Prions are just misfolded proteins. They cause Mad Cow Disease. They don't have DNA. They don't have a cell. But when they touch a healthy protein, they "convert" it into another prion. They replicate. They spread. They are almost certainly not alive, but they act with a terrifying, life-like purpose.
Then you have Mimiviruses. These things are huge—bigger than some bacteria. They have genes for metabolism. They have genes that look like they belong in a "real" cell. They sit right in the middle of the gap between "chemistry" and "biology." They remind us that nature doesn't use buckets; it uses gradients.
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Moving Beyond the Checklist
Maybe the reason we can't find a perfect definition is that there isn't one.
Think about the word "game." Ludwig Wittgenstein, a famous philosopher, pointed out that there’s no single feature that all games share. Some have winners, some don't. Some are played with balls, some with cards. Some are for fun, some are professional. Instead of a definition, they have a "family resemblance."
Life might be the same.
Instead of looking for a "yes/no" switch, we should probably look at "lifeness" as a spectrum. A rock is at 0. A human is at 100. A virus might be a 30. A synthetic cell might be a 85.
Practical Next Steps for Understanding Life
If you want to dig deeper into how we define our own existence, don't just stick to biology textbooks. The real action is happening at the intersections of different fields.
- Read "What is Life?" by Erwin Schrödinger. It's short, and even though it's old, it changed how we think about the physics of biology.
- Explore the "RNA World" hypothesis. Look into the work of researchers like Walter Gilbert. It explains how life might have started as simple molecules that eventually "became" alive.
- Follow the James Webb Space Telescope (JWST) findings. We are currently looking for "biosignatures" in the atmospheres of distant planets. How we define life determines what gases we look for. If we’re too narrow, we might miss an entire alien civilization just because they don't breathe oxygen.
- Look into Synthetic Biology. Keep an eye on the Jennifer Doudna and CRISPR space. As we gain the power to edit and "write" life, the boundary between "made" and "born" is going to disappear.
The definition of biological life is a moving target. As our tech gets better and our telescopes see further, we’re going to have to keep rewriting the rules. And honestly? That’s the most exciting part. We are the only part of the universe that is actively trying to define itself. That alone makes us pretty special.