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Three comic-style circles in a black background with various shapes and styles of living unicellular microorganisms, some with flagellae, and a microscope in the center top. Circle on the right shows rectangular plant cells lined up as if magnified under a microscope.

3.5. How can we tell if something is alive or not?

A core learning question from the Astrobiology Learning Progressions

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4.1. Where do life's building blocks come from?

Grades K-2 or Adult Naive Learner

As we look around a classroom, our homes, in a forest, or anywhere we go, we can tell things that are alive from things that are not alive. You are alive, so is a bug, a fish, and your teacher. But things like a desk, a rock, a pencil, or a building are not alive. Things that are alive do certain things, like move and grow and sometimes change. If you sit and watch a pencil for a long time without using it, it won’t change or grow at all. It will just keep on being a pencil.

Portrait photo of Three comic-style circles in a black background with various shapes and styles of living unicellular microorganisms, some with flagellae, and a microscope in the center top. Circle on the right shows rectangular plant cells lined up as if magnified under a microscope.
Key traits shared by all life include growth, reproduction, metabolism, and response to the environment. Life is defined by cellular activity structures like mitochondria, which act as powerhouses.
NASA/Aaron Gronstal

Grades 3-5 or Adult Emerging Learner

Everywhere you look there are things that are alive and things that are not alive. Things that are alive have common characteristics even though they are very different from each other. For example, living things grow and move and nonliving things often do not grow or move on their own. Living things can be hurt and can repair themselves, like when you get a scratch. Living things at some point will die. Living things also reproduce and can have babies or produce seeds to make new life. Non-living things cannot heal themselves or reproduce. There is a huge variety of both living and nonliving things on our planet. As we explore the rest of the solar system and beyond, it may be that we cannot always easily tell the difference between living and nonliving things in places beyond Earth.

Portrait photo of Three comic-style circles in a black background with various shapes and styles of living unicellular microorganisms, some with flagellae, and a microscope in the center top. Circle on the right shows rectangular plant cells lined up as if magnified under a microscope.
Key traits shared by all life include growth, reproduction, metabolism, and response to the environment. Life is defined by cellular activity structures like mitochondria, which act as powerhouses.
NASA/Aaron Gronstal

Grades 6-8 or Adult Building Learner

What defines something as living or nonliving? It turns out that it’s not as easy to define as you might think, but we can look at the characteristics living things have in common to better understand what life is. For instance, living things are made of cells, maintain homeostasis (a stable internal environment), grow and develop, reproduce, metabolize, respond to the environment, and over time, evolve. Although many nonliving things could have a few of these characteristics they do not have all of them. As we look out beyond Earth in order to find life we must consider what being alive means. On Earth, we do not have much trouble figuring out what is living and what is nonliving, however, when exploring beyond Earth we’ll need to design our spacecraft so that we can detect living from nonliving things.

Portrait photo of Three comic-style circles in a black background with various shapes and styles of living unicellular microorganisms, some with flagellae, and a microscope in the center top. Circle on the right shows rectangular plant cells lined up as if magnified under a microscope.
Key traits shared by all life include growth, reproduction, metabolism, and response to the environment. Life is defined by cellular activity structures like mitochondria, which act as powerhouses.
NASA/Aaron Gronstal

Grades 9-12 or Adult Sophisticated Learner

As we search for possible life beyond Earth, it’s important for us to consider what exactly life is. It turns out that it’s not as easy to define life as you might think, but we can at least look at the characteristics that living things have in common to better understand life. For instance, life is made of cells, maintains homeostasis, grows and develops, has the ability to reproduce, metabolizes, responds to the environment, and, over time, evolves. However, just having some of these characteristics doesn’t necessarily make something alive. There are examples of nonliving phenomena on Earth such as fire, crystals, computer algorithms, and even artificial intelligence that have many of the characteristics of life. Additionally, there are living things that also only hit most of the characteristics. For instance, mules are animals that come from a mix of a horse and a donkey and actually cannot reproduce.

Defining life has proven to be a difficult task, even though many of us can look at most things and tell if they’re alive or not. The most widely accepted scientific definition for life right now is this: “life is a self-sustaining chemical system capable of Darwinian evolution”. However, there are still problems with this definition. For instance, viruses further blur the distinction between life and non-life since they cannot live without a living host and yet they have their own genetic material, they act as biological machines, and they evolve. As we continue to consider how life works on Earth and how we might best be able to find extraterrestrial life if it exists, we’ll continue to improve our understanding of what life really is.

Portrait photo of Three comic-style circles in a black background with various shapes and styles of living unicellular microorganisms, some with flagellae, and a microscope in the center top. Circle on the right shows rectangular plant cells lined up as if magnified under a microscope.
Key traits shared by all life include growth, reproduction, metabolism, and response to the environment. Life is defined by cellular activity structures like mitochondria, which act as powerhouses.
NASA/Aaron Gronstal