3.1: Definition of Life

Biology is the branch of science concerned with the study of living things, or organisms. That definition is pretty straightforward. However, it opens the door to more difficult—and more interesting—questions: What is life? What does it mean to be alive?

Learning Objectives

• Discuss some examples of living things and how we know they’re alive
• Describe the attributes that living things have that distinguish them from nonliving things
• Discuss some of the various definitions of life, and why scientists argue over whether they are or are not satisfactory

You are alive, and so am I. The dog I can hear barking is alive, and so is the tree outside my window. However, snow falling from the clouds is not alive. The device you’re using to read this article is not alive, and neither is a chair nor a table. The parts of a chair that are made of wood were once alive, but they aren’t any longer. If you were to burn the wood in a fire, the fire would not be alive either.

What is it that defines life? How can we tell that one thing is alive and another is not? Most people have an intuitive understanding of what it means for something to be alive. However, it is surprisingly hard to come up with a precise definition of life. Perhaps because of this difficulty, since ancient times there have been “vitalistic” hypotheses—the idea that there must be some quality that life has which makes it different from non-life. This quality has alternatively been called “elan vital” or “life force” or “divine spark.” In spite of many proposals that this is the key distinguishing feature of life, it has consistently eluded measurement. The proponents of these vitalistic hypotheses have claimed to be able to directly observe the thing that distinguishes life from non-life, and have consistently been shown to be incorrect.

A famous example was the case of Franz Mesmer who in the eighteenth century claimed to be able to capture and use what he termed the “magnetism” of living things to cure diseases. Investigations by scientists at the time showed that his claims were based on wishful thinking; when they secretly switched his potions with decidedly non-living vials of water, Franz Mesmer could not tell the difference.

In the nineteenth century, a school of “vitalist chemistry” proposed that the chemistry of living things would always be separate from non-living things. They predicted that it would be impossible to distill chemical compounds produced by living things through artificial or non-living processes. This idea of vitalist chemistry was shown to be incorrect when urea, an organic compound, was synthesized from two inorganic compounds thereby proving that life was not needed to produce an organic chemical.

Even with the debunking of vitalism, proponents continued to argue that there is something special and ineffable about life that science has not yet discovered. The scientific community has largely moved on from this argument and vitalism has been relegated as pseudoscience in most circles.

The struggle to clearly define life led to many attempts that are operational descriptions of life—such definitions allow us to separate living things from nonliving ones, but they don’t actually pin down what life is. To make this separation, we might try to come up with a list of properties that are, as a group, uniquely characteristic of living organisms. This is the most common solution employed today.

Test Your Intuition

Classify the following as either living or non-living.

How did you decide whether a virus is living or non-living? Note that there is not a generally accepted “yes” or “no” answer to the question of whether a virus is alive.

Properties of life

We can determine properties of life by identifying various traits common to all the living organisms that we know of. Although nonliving things may show some of these characteristic traits, living things consistently show all of them.

Concept Check: What properties do living things all share?

One strategy scientists use to define life is to list the things all life has in common. Make a list of the properties that you think all living things share.

Show Answer

You may have listed reproduction or evolution . These things are very important for ensuring the survival of a species . A living thing must be able to reproduce or its species won’t live on after it dies. A living thing must also be able to evolve to the changing conditions of its environment, or it won’t be able to survive to reproduce.

There are many things that nearly all living things share in common, but there are often exceptions which make defining life difficult. For example, your intuition may have been to label the mule in question one as living; however, mules are sterile, and can neither reproduce nor evolve. A mule is one of those exceptions that makes defining life difficult.

Some non-living things have things in common with life. For example, 3D printers can print all sorts of objects, from models of the human brain to life-sized boats. What if a 3D printer printed a copy of itself? Technically the printer would be reproducing.

1. Organization

Living things are highly organized, meaning they contain specialized, coordinated parts. All living organisms are made up of one or more cells, which are considered the fundamental units of life.

Even unicellular organisms are complex! Inside each cell, molecules are organized into cell organelles and coherent structures. Multicellular organisms—such as humans—are made up of many cells. The cells in multicellular organisms may be specialized to do different jobs. Similar cell types can be organized into tissues, such as connective tissue, epithelial tissue, muscle, and nervous tissue. Tissues, in turn, coordinate to create organs, such as the heart or lungs, which carry out specific functions needed by the organism as a whole.

Figure 1 – Left: unicellular bacterium, with the exterior of the cell cut away to show the multiple layers of the cell and the DNA in its interior. Center: multicellular tissues in humans. Small drawings of connective tissue, epithelial tissue, muscle tissue, and nervous tissue. Right: diagram of a human upper body, showing one location where epithelial tissue like that shown in the center panel could be found—the lining of the mouth.

2. Metabolism

All living things require energy to grow, respond to the environment, and reproduce. The series of chemical reactions inside the body that create energy from nutrients are known as metabolism. Life depends on an enormous number of interlocking chemical reactions. These reactions make it possible for organisms to do work, such as moving around to catch prey, growing, reproducing, and maintaining the structure of their bodies. Living things must use energy and consume nutrients to carry out the chemical reactions that sustain life.

3. Homeostasis

Homeostasis alludes to the maintenance of a stable, internal environment, even in the face of a changing external environment. Living organisms regulate their internal environment to maintain the relatively narrow range of conditions needed for cell function. For instance, your body temperature needs to be kept relatively close to 98.6°F (37°C). This stability can be achieved either by sweating to cool off or shivering to warm up.

Concept Check: Homeostasis

Can you think of an example of something non-living that maintains an internal temperature in spite of the temperature of its surroundings?

Show Answer

Stars produce energy in their interiors and maintain a steady temperature for billions of years in spite of being situated in cold, frigid space. Hot springs are also warm regardless of the temperature of the environment above them. They get their temperature from geothermal heat below.

Check out this 7 second video of a hot spring in Lassen Volcanic National Park in California:

A video element has been excluded from this version of the text. You can watch it online here: https://pb.libretexts.org/astrobio/?p=84

4. Growth and Change

Living organisms undergo regulated growth. Individual cells become larger in size, and multicellular organisms accumulate more and more cells through cell division. You yourself started out as a single cell and now have tens of trillions of cells in your body!

Concept Check: Growth and Change

All living things have the ability to grow and change. A seed grows into a tree, a baby grows into an adult, and even a tiny bacterium grows in size until it splits into two. Can you think of some non-living things that can grow and change?

Show Answer

Crystals, glaciers, rivers, and mountains grow in size and change over thousands of years. However, the causes of growth and change to these things are based on physics rather than biology, e.g., temperature-pressure conditions, climate, and plate tectonics.

Check out this 30 second time lapse of a crystal growing:

A YouTube element has been excluded from this version of the text. You can view it online here: https://pb.libretexts.org/astrobio/?p=84

5. Reproduction

Living organisms are capable of reproduction, which creates new organisms. Reproduction can be either asexual, involving a single parent organism, or sexual, requiring two parents. Single-celled organisms, like the dividing bacterium shown in the left panel of the image at right, can reproduce themselves simply by splitting in two!

Figure 2 – (left) Salmonella bacterium dividing into two bacteria.(right) image of sperm and egg meeting during fertilization.

In sexual reproduction, two parent organisms produce sperm and egg cells containing half of their genetic information, and these cells fuse to form a new individual with a full genetic set. This process, called fertilization, is illustrated in the image above on the right.

6. Response

Living organisms show “irritability,” meaning that they respond to stimuli or changes in their environment. For instance, people pull their hand away—fast!—from a flame; many plants turn toward the sun; and unicellular organisms may migrate toward a source of nutrients or away from a noxious chemical.
See a plant (mimosa) respond to touch:

Concept Check: Response to changes in environment

All living things are capable of responding to some changes in their environment. Changes in temperature, humidity, sound, light, and physical contact with another living thing can cause a response, such as growth, movement, dormancy, or aggression. Can you think of a few non-living things that respond to changes in the environment?

Show Answer

A pond changes with the weather. It freezes over in the winter, dries up during a drought, and becomes full during periods of rain. Fire also responds to changes in the environment. A forest fire is intensified if it’s windy, but if you put water on the fire, it will extinguish the flames.

7. Evolution

Populations of living organisms undergo evolution, meaning that the genetic makeup of a population changes over time. Living organisms must therefore have a system for storing genetic material, such as in DNA and/or RNA, and a pathway through which this genetic material can change.

In some cases, evolution involves natural selection, in which a heritable trait, such as darker fur color or narrower beak shape, lets organisms survive to reproduce better in a particular environment. Over generations, a heritable trait that provides a reproductive advantage may become more and more common in a population, making the population better suited to its environment. This process is called adaptation.

Separating living and non-living things

How well do the properties above allow us to determine whether or not something is alive? Let’s revisit the living and nonliving things we saw in the introduction as a test.

The living things we listed in the introduction—humans, dogs, and trees—easily fulfill all seven criteria of life. We, along with our canine friends and the plants in our yards, are made of cells, metabolize, maintain homeostasis, grow, and respond. Humans, dogs, and trees are also capable of reproducing, and their populations undergo biological evolution.

Living things may keep some properties of life when they become nonliving, but lose others. For instance, if you looked at the wood in a chair under a microscope, you might see traces of the cells that used to make up the living tree. However, the wood, having been made into a chair, can no longer grow, metabolize, maintain homeostasis, respond, or reproduce. A wooden chair is therefore not alive.

Nonliving things may exhibit some of the criteria for life. For instance, crystals of snow are organized—though they don’t have cells—and can grow, but they don’t meet the other criteria of life. Similarly, a fire can grow, reproduce by creating new fires, respond to stimuli, and can arguably even be said to carry out chemical reactions that constitute a metabolism. However, fire is not organized, does not maintain homeostasis, and lacks the genetic information required for evolution.

Indeed, it seems biochemistry is a key component of living things that allows us to make a clear distinction between living and nonliving things. The four main types of chemicals living things utilize for this are proteins, nucleic acids, carbohydrates, and lipids. We will spend considerable time investigating the details of these chemicals to understand the contexts where they arise and speculate on whether and how they might exist in environments well outside the Earth.

Concept check: Can a robot be considered alive?

Consider the properties listed above and assess whether a robot fulfills each criteria or not.

Show Answer

Let’s test out our list of properties on a trickier case. Imagine a very sophisticated robot, such as R2D2 or C3PO from the Star Wars movies. Such a robot would show organization—without cells—respond to stimuli, and even have a metabolism of sorts, using energy to power the circuitry of its “nervous system”. It might even maintain homeostasis, with an internal fan or heater that turned on if the temperature changed.

A typical robot would not grow, reproduce, or be part of an evolving population, and thus wouldn’t be considered alive. However, what if a robot were programmed to add pieces onto itself? To build more robots? To build more robots with variations in the “DNA” of their programs? As these ideas show, a sophisticated enough computer or robot—far beyond what we have today!—could start to bend the definition of life.

What counts as life is still being defined.

Even with the biochemical definition of life in hand, the question of what it means to be alive remains unresolved. Living organisms have many different properties related to being alive, and it can be hard to distinguish the exact set that best constitutes an operational definition of life. Different thinkers have developed different lists of the pertinent properties of life. For instance, some lists might include movement as a defining characteristic, while others might specify that living things carry their genetic information in the form of DNA. Still others might require that life is carbon-based, meaning the molecules that make up a living organism must incorporate carbon.

Figure 3 – Image of a mule on a farm. The mule looks similar to a donkey and is clearly a living animal, despite the fact that it cannot reproduce.

It’s also true that the list above is not foolproof. For instance, a mule, pictured below, is the offspring of a female horse and a male donkey. Mules look and act similar to donkeys and are easy to categorize as “living.” However, mules are unable to reproduce, thereby failing to fulfill one of our seven criteria of life listed above. Does this make a mule non-living?

Viruses are another debated category. Viruses are tiny structures made up of proteins and nucleic acid genetic material that can only reproduce inside host cells. They have many of the properties of unicellular life—they are organized structures, respond to stimuli, and evolve. However, they cannot reproduce without a host. They also aren’t capable of carrying out their own metabolism, and it is not clear that they maintain homeostasis.

For these reasons, biologists do not generally consider viruses to be alive, but not everyone agrees with this conclusion. Whether viruses count as life remains a topic of debate. Some even simpler molecules, such as self-replicating proteins—like the “prions” that cause mad cow disease—and self-replicating RNA enzymes, also have some of the properties of life, like viruses, but not all. Should these simpler molecules be considered living or non-living?

Moreover, we are basing our definitions of life on the characteristics of life we see on earth. If extraterrestrial life exists, it may or may not share these same characteristics. Indeed, NASA’s working definition of life states that:

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

Let’s break down this definition into two parts: 1) Self-sustaining chemical system, and 2) capable of Darwinian evolution.

Self-sustaining chemical system: A system can be made up of many parts. As individual humans, we cannot live forever, but as a species (i.e., Homo sapiens), we can live indefinitely. Our method of self-sustaining our species is known as sexual reproduction.

Concept check: Reproduction

Let’s return to the selections you looked at earlier in assessing what is alive.

The original version of this chapter contained H5P content. You may want to remove or replace this element.

For each selection, ask if it can reproduce copies of itself without a continued source of energy. If they can reproduce, identify how. For example, mules cannot sustain themselves as a species as they cannot reproduce.

Show Answer

Dogs are self-sustaining. Similar to humans, dogs can sustain themselves indefinitely via sexual reproduction.
Trees are self-sustaining. Trees reproduce both sexually and asexually, and cam therefore sustain their species indefinitely.
A 3D printer is self-sustaining. If a 3D printer was programmed to take raw chemicals from the environment and print a copy of itself, it could sustain itself until the chemicals run out.
A virus is self-sustaining. There are many species of viruses. Each species could potentially be considered self-sustaining as they can reproduce and live indefinitely as long as living cells are present in their environment.
Fire is not self-sustaining. A fire dies out as soon as it runs out of fuel (e.g., wood). Even big forest fires only last for about five weeks. There is no methos a fire has to sustain itself in the face of its rapidly depleting fuel source.
A crystal is not self-sustaining. Individual crystals are very strong because the atoms they are made out of are arranged in a compact, organized manner. Their atomic structure helps them exist in the environment for billions of years. However, they cannot copy or reproduce themselves as a chemical system.

The other half of the NASA definition is:

Capable of Darwinian evolution: Darwin published his theory of evolution in a book titled, On the Origin of Species in 1859. The general concept is that when living things reproduce, they do so imperfectly (e.g. we are not exact clones of our parents). This allows variation amongst a group of similar living things (i.e. a species). Variations can lead to competitive advantages amongst individual living things, which are passed on to their children, evolving the species to be better suited to the environment.

At its core, being capable of Darwinian evolution means being capable of copying yourself imperfectly.

What do you think?

The list given here provides a reasonable, though not foolproof, set of properties to help distinguish between things that are alive and those that are not. How would you define life? Would you add something to the list of properties above, subtract something, or use an entirely different definition? Can you think of exceptions or special cases that aren’t covered by the list?

Key Concepts and Summary

The definition of Life is central to the search for life. We want to know the attributes of our quest, but an all-encompassing definition of life currently eludes us. Life on Earth is organized as one or more cells that carry out chemical reactions (biochemistry) for metabolism, growth, and reproduction. The hallmark of life is that populations of organisms undergo evolution through natural selection, with genetic modifications to adapt to the environment. However, evolution will be difficult to monitor or detect on distant worlds.

This article is remixed from https://www.khanacademy.org/science/biology/intro-to-biology/what-is-biology/a/what-is-life under a CC BY-NC-SA 4.0 license.