What is Life?

"Life is marked by the ability to nourish oneself, grow over time, and eventually decline."

According to the Greek philosopher Aristotle, this is the definition of life. In other words, life is the capacity for self-sustenance, growth, and decay. His definition captures the cyclical nature of being alive on planet Earth.

But when we try to put this definition to the test, we quickly come across problems.

What about fire? — we might ask. Doesn’t fire sustain itself with oxygen, grow, and die out?

Back to the drawing board.

Part of the problem with definitions of life is that the organisms on Earth are almost impossibly diverse.

Life ranges from single-celled organisms like bacteria to complex multicellular entities like humans and trees. These life forms exhibit a vast array of metabolic activities, reproductive strategies, and survival mechanisms.

Any definition of life must be broad enough to include both bacteria and humans while being specific enough to exclude non-living matter.

We need to ask questions like ‘What do humans and bacteria have in common?’

— But our answer cannot simply be something like ‘they are both on earth’ because that would also include rocks and refrigerators.

The diversity of life is not the only problem.

Another challenge arises from edge cases in biology, such as viruses. Viruses display some characteristics of life, such as the ability to evolve, but lack others, like cellular structure and independent metabolic processes.

Viruses cannot reproduce on their own, relying instead on the machinery of a host cell. This has led to ongoing debates about whether viruses should be considered living.

Yet another difficulty is that the concept of life is also deeply dynamic. Living organisms evolve, adapt, and change over generations.

This evolutionary perspective suggests that any definition of life must encompass the very concept of change across time, accommodating both past and future forms of life.

Life as it existed a billion years ago differs vastly from life today, and it will likely continue to evolve in ways we can't yet predict. Therefore, a comprehensive definition of life must be flexible enough to account for new life forms that may emerge in the future.

So how do we overcome these problems?

Firstly, we can be more specific than Aristotle, because we now have a greater understanding of biological processes.

For example, instead of ‘self-sustenance and growth’, we can talk specifically about metabolism: the chemical processes that enable organisms to convert energy from their surroundings into forms they can use.

As well as ‘growth, and decay’ we can talk about evolutionary adaptation: an organism's ability to respond to changes in its environment, a key factor in the survival of species over time.

We now have a precise understanding of the different characteristics that are displayed by living organisms, including cellular structure, reproduction, growth and development, metabolism, and adaptive evolution.

This tile will track through each of these key characteristics, asking to what extent they help us to define life.

Our first orb drew attention to some of the problems with defining "life".

This orb will examine some of these definitions a little more closely. To what extent do they succeed at defining life (and if not: why not?)

Let's start with one essential characteristic of life: energy intake.

Erwin Schrödinger described life as an "open system" that stays organized by taking in energy and materials from its environment.

This might sound a bit abstract, but essentially, he means that life resists breaking down over time, a process called entropy. Without a constant supply of energy, living things would fall apart and die. To avoid this, organisms take in energy from their surroundings to maintain their structure and function.

For example, plants use sunlight through photosynthesis to produce energy, grow, and stay organized. When a plant dies, it stops taking in energy, and entropy takes over, causing it to break down and decay.

Yet a definition based on energy intake has its limits.

A hurricane also takes in energy, grows, and maintains structure. Yet, a hurricane is not alive. Why?

Well, hurricanes lack the intricate biochemical processes that consume energy in living organisms.

When we talk about life needing energy, we are really referring to metabolism—the chemical reactions that power everything an organism does. To truly define life, we may want to specify certain metabolic processes — not just the intake of energy.

Another pretty essential characteristic of life is reproduction.

Reproduction is a fundamental characteristic of all living things. It is the process by which living organisms produce new individuals, ensuring the continuation of their species.

Some have attempted to define life by this ability to reproduce.

The biophysicist Edward Trifonov, for example, proposed that “life is self-reproduction with variations.” This idea suggests that life is defined by its ability to replicate itself while introducing small differences.

Let's take bacteria: bacteria reproduce by dividing, often resulting in mutations that may provide advantages in changing environments.

But does this capacity really 'define' life?

Non-living entities, like crystals, can grow and exhibit variations during their formation. Crystals can even propagate by "seeding" new growth.

Yet, despite these similarities, crystals aren’t alive. Why?

Crystals are not considered alive, despite their ability to grow and exhibit variation, because they lack the fundamental processes of genetic inheritance and evolution.

So, to be more specific in defining life, we could turn to the concept of Darwinian evolution.

NASA defines life as “a self-sustaining chemical system capable of Darwinian evolution.” Here, “self-sustaining” means an organism can maintain balance and respond to its environment, while “chemical system” refers to metabolism.

As we’ll discuss more later, Darwinian evolution is different from Trifonov’s "reproduction with variation." It shows how species change over time through natural selection. Traits such as size or behavior can help some individuals survive and reproduce better than others, allowing these traits to be inherited. Over time, this changes the species.

This is not the case with crystals.

A broad definition of life should consider the ability of organisms to adapt and evolve across generations.

To conclude: definitions of life frequently come across problems and gaps.

As we've seen, hurricanes, fires, and crystals can mimic some life aspects. A hurricane extracts energy, grows, and eventually "dies." A fire consumes fuel, spreads, and reacts to its environment. Crystals exhibit 'variation' during formation.

Each characteristic—reproduction, metabolism, and response to stimuli—provides insight into life’s nature but is insufficient alone.

Even specifying a capacity for 'Darwinian evolution' comes across problems. Under this definition, if NASA astronauts encountered a life-like entity on a distant planet without knowing its evolutionary history, could we say it was alive?

In the end, it's the combination of these characteristics that captures what it means to be alive.

We've touched on some of these already, but the next two tiles will examine these characteristics in greater detail:
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• Cellular structure
• Growth and Development
• Response to Stimuli
• Metabolism
• Homeostasis
• Adaptation through Generations