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Comic-style illustration with two hands and colorful molecules in front of each, both seemingly mirror images of each other.

3.1. What are the characteristics of life?

A core learning question from the Astrobiology Learning Progressions

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2.3. Where could life have gotten started on Earth?

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3.2. What does life need for survival?

Grades K-2 or Adult Naive Learner

The things that live here on Earth all have a lot in common. The tiny parts inside you that are responsible for helping you do everything you do, like walking, breathing, and digesting your food – those tiny parts are almost exactly the same in all types of life, from flies and fish to trees and cows. All types of life have other things in common, too. All living things have a body of some kind. Some things have body parts that allow them to walk or fly or swim. These are things like legs and wings and fins. All of the living creatures we know have to eat food in order to have the energy to move and to grow. All living things can respond to their surroundings, just like you can taste something awful then spit it out and shout “YUCK!” And all life comes from other life – just like how you came from your mother and father. So even though we look so different from other living things, we are much more the same than different.

Portrait photo of Comic-style illustration with two hands and colorful molecules in front of each, both seemingly mirror images of each other.
Chirality, described sometimes as 'handedness' is the way molecules do not mirror each other. It is a fundamental characteristic of life on Earth. Most biomolecules are chiral.
NASA/Aaron Gronstal

Grades 3-5 or Adult Emerging Learner

All living things on Earth have a lot in common. Both inside and outside. On the inside, all lifeforms have tiny structures inside their bodies that are almost exactly the same and help them do all the things that they need to do so they can live – things like walking, flying, or swimming, and even basic functions like breathing and digesting food. All lifeforms have similar characteristics or behaviors, too. There is order and structure to living things. All lifeforms exist in a body of some kind that is highly organized. Think of a tree – it has roots to absorb water, a trunk and branches, and leaves to absorb sunlight.

All living things have the ability to reproduce, or procreate. Mammals, like whales and giraffes, do so by giving birth to young. All living things consume energy (they eat food), and use that energy for work, play, and growth. All living things respond to changes in the world around them. When the environment changes, life responds to those changes. For example, when the weather turns cold, bears return to their dens and hibernate for the winter. When spring comes, the bears can come out again. All across different types of life, some individuals will have characteristics that make them better at a particular task, which makes them more likely to survive. For example, in a population of giraffes, there may be a few who have longer necks than the rest. When the only leaves remaining to be eaten are at the very tops of the trees, only the giraffes with longest necks can reach them, and this makes them more likely to survive. Even though living things look very different, they are more the same than different.

Portrait photo of Comic-style illustration with two hands and colorful molecules in front of each, both seemingly mirror images of each other.
Chirality, described sometimes as 'handedness' is the way molecules do not mirror each other. It is a fundamental characteristic of life on Earth. Most biomolecules are chiral.
NASA/Aaron Gronstal

Grades 6-8 or Adult Building Learner

Have you ever thought about how we know something is alive or not alive? For instance, what makes a rock not alive while a whale is alive? All living things share many different traits. One way to consider something to be alive is if it has all of the traits shared by living organisms. For instance, the bodies of all living things are structured into cells, and use the same types of molecules to carry out functions inside of their cells. These cellular functions translate into everyday activities such as breathing, walking, flying, digesting food, etc.

All living things have the ability to reproduce, or procreate. Mammals, like humans, do so by giving birth to live young while birds lay eggs that have to hatch and plants make seeds from which new plants can grow. All living things consume energy (they eat food), and use that energy for work, play, and growth. All living things exhibit “homeostasis,” which is the ability to maintain a steady internal environment regardless of their external environment. For example, most humans maintain a body temperature of 98.6 degrees Fahrenheit regardless of whether they are out playing in the snow or hiking in the hot desert. Homeostasis is achieved because of strict biochemical regulations in cells and organs.

Living things also respond to stimuli (or changes in the environment). For example, when autumn comes, the leaves in many trees stop doing photosynthesis. The leaves then turn from green to brown and fall off, and the tree becomes dormant throughout winter. And all living things have the ability to adapt to their environment as it changes. For example, if there is a prolonged drought (or a period with little water), certain plants in a population will have the ability to survive it and others won’t. The ones that survive it pass on that ability to their offspring. Over time in a prolonged drought, the population of plants will be more adapted to its environment. We call this process natural selection, wherein the environmental pressures “select” for certain traits in a population.

All life shares these aspects and abilities, from worms to birds to fish to tigers. Things that we don’t consider to be alive may have some of these traits, but not all of them. For example, a fire exhibits some of these traits. It consumes energy (wood and oxygen) and gives off by-products such as carbon dioxide and heat, it grows in size as it consumes more and more fuel, and may even appear to reproduce as it spreads. But because it doesn’t exhibit all of the traits of life, we don’t consider fire to be alive. Defining life from the viewpoint of examining its characteristics reveals how much life on Earth has in common, and helps distinguish between living and non-living things, which is important for learning if life could be possible beyond Earth.

Portrait photo of Comic-style illustration with two hands and colorful molecules in front of each, both seemingly mirror images of each other.
Chirality, described sometimes as 'handedness' is the way molecules do not mirror each other. It is a fundamental characteristic of life on Earth. Most biomolecules are chiral.
NASA/Aaron Gronstal

Grades 9-12 or Adult Sophisticated Learner

How do we know that something is alive? The answer seems obvious and intuitive, but when you ask that question in the context of searching for life on other planets, it becomes more difficult to pin down. In the unfamiliar environments of other planets, we need to find a way to distinguish between a phenomenon that isn’t alive as compared to one that is. One way to address this is by considering the characteristics of living things, which reveals how much all living things have in common. All life on Earth adheres to a common biochemistry. In all living things, cells are the main unit of organization, cellular membranes are made up of molecules called phospholipids, genetic information is made up of molecules called nucleic acids, and functions within and between cells are mostly carried out by molecules called proteins. This means that a fly has the same basic biochemistry as an elephant!

Beyond this shared biochemistry, all life has certain general traits in common, too. Here are some of the key traits of life as we know it:
1) All life is highly ordered and structured. Not only do all living things that we know of have cells and cellular structures, but many living things also have larger-scale structure such as bilateral symmetry (in humans) or radial symmetry (in starfish).
2) All life reproduces itself, either sexually (as animals do) or asexually (such as budding in yeast or one cell splitting into two identical daughter cells via binary fission as bacteria do).
3) All life grows and develops to reach maturity, such as from a caterpillar to a butterfly.
4) All life takes in and utilizes energy to carry out the functions of its cells, which results in growth and development. Mechanisms for energy intake are vastly different across all species, and can range from eating food like humans do, to converting sunlight into sugars like plants do, to the harnessing of the energy produced when rocks radioactively decay like some bacteria do.
5) All living things exhibit homeostasis, which is the ability to maintain a steady internal environment regardless of their external environment. For example, most humans maintain a body temperature of 98.6 degrees Fahrenheit regardless of whether they are out playing in the snow or hiking in the hot desert. Homeostasis is achieved because of strict biochemical regulations in cells and organs.
6) All living things respond to their environment by sensing external stimuli and changing their biochemistry and/or behavior. For example, when cuttlefish sense danger, they can instantaneously change their colors to match whatever background they are against to avoid being seen by a predator.
7) Finally, all living things adapt to external pressures, and evolve because of them. Adapting is much like responding to a stimulus in the environment, but takes it to the next level. In evolutionary adaptation, one cuttlefish will have the ability to change colors more quickly and effectively than another (because of its genetic makeup), and it will inherently be more likely to survive than another one that doesn’t do it as well or as quickly. The first one is more likely to pass on its genes to its offspring, and that offspring will pass it on to their offspring, and so on. Over time, the population of cuttlefish descended from that one who changed colors more quickly and effectively is more highly adapted to its environment. They have undergone the process of natural selection and are more likely to survive. Their genes were “selected for” by the external pressures of the environment.

Something that is alive will exhibit all of these traits, while phenomena that we do not consider to be alive can exhibit some, but not all of them. For example, a fire exhibits some of these traits – it consumes energy (wood and oxygen) and gives off by-products such as CO~2~ and heat, it grows in size as it consumes more and more fuel, and it may appear to reproduce as it spreads. But because it doesn’t exhibit all of these traits, we don’t consider fire to be alive. Defining life from the viewpoint of examining its characteristics reveals how much life on Earth has in common, and helps distinguish between living and non-living things. If there is other life out there in the cosmos and it’s like the life that we know, then we would expect it to also show these traits of living things.

Portrait photo of Comic-style illustration with two hands and colorful molecules in front of each, both seemingly mirror images of each other.
Chirality, described sometimes as 'handedness' is the way molecules do not mirror each other. It is a fundamental characteristic of life on Earth. Most biomolecules are chiral.
NASA/Aaron Gronstal