The six characteristics of life are cellular organization, reproduction, growth and development, energy use (metabolism), homeostasis, and response to the environment. Every living organism on Earth, from single-celled bacteria to blue whales, shares all six of these traits. Some frameworks add a seventh, evolution/adaptation, but most biology courses teach the core six. If something meets all six criteria, it’s alive. If it’s missing even one, biologists generally classify it as non-living.
1. Cellular Organization
All life is built from cells. Some organisms are a single cell, like bacteria or yeast. Others, like humans, are made of trillions of cells organized into tissues, organs, and systems. But no matter the scale, the cell is the basic unit. Every living thing has at least one.
Cells themselves are highly ordered structures. They contain genetic material, a boundary membrane, and internal machinery for carrying out the work of staying alive. Beyond the cellular level, many organisms also show larger-scale structural patterns: bilateral symmetry in humans (your left side roughly mirrors your right), radial symmetry in starfish (identical sections radiating from a center point). This layered organization, from molecules up to whole-body architecture, is something no non-living object achieves on its own.
2. Reproduction
Every living thing produces new copies of itself. This happens in two broad ways: sexual reproduction, where two parents combine genetic material to create offspring, and asexual reproduction, where a single organism copies itself. Bacteria reproduce asexually by splitting in half. Yeast buds off smaller copies of itself. Animals and most plants reproduce sexually, shuffling genes from two parents into each new individual.
At the molecular level, reproduction comes down to DNA. The two strands of a DNA molecule separate, and each strand serves as a template for building a new matching strand. Because the chemical bases always pair the same way (adenine with thymine, guanine with cytosine), the copy is nearly identical to the original. This template replication is one of the oldest features of life, and it’s the mechanism that passes traits from one generation to the next.
3. Growth and Development
Living things don’t just get bigger. They change in organized, predictable ways over time. A caterpillar transforms into a butterfly. A single fertilized human egg develops into a body with hundreds of specialized cell types. Growth means an irreversible increase in size, while development refers to the process of maturing and gaining new capabilities.
This is different from, say, a crystal growing in a solution. Crystal growth is simple accumulation. Biological growth involves cells dividing, differentiating into specialized types, and assembling into structures that didn’t exist before. A liver cell and a nerve cell contain the same DNA, but they develop into radically different things based on which genes are active. That guided, programmed progression from simple to complex is unique to life.
4. Energy Use (Metabolism)
Every living organism takes in energy and uses it to power the chemical reactions inside its cells. This collection of reactions is called metabolism, and it’s happening constantly in every cell of every living thing. Without a continuous supply of energy, cells stop working and the organism dies.
The ways organisms obtain energy are wildly diverse. Animals eat food and break it down through digestion. Plants capture sunlight and convert it into sugars. Some deep-sea bacteria harvest energy from the radioactive decay of rocks. But the end goal is the same: converting raw energy into a usable chemical form that cells can spend on growth, repair, movement, and everything else they do. Your body processes food in stages, first breaking complex molecules into simple ones, then running those through a series of chemical reactions that ultimately produce the energy currency your cells run on. Every bite of food you eat passes through this cascade.
5. Homeostasis
Homeostasis is the ability to keep internal conditions stable even when the outside environment changes. Your body temperature stays around 98.6°F whether you’re hiking in the desert or playing in the snow. Your blood maintains a narrow range of acidity. Your cells regulate their water and salt content within tight limits. All of this is homeostasis at work.
The mechanism typically works through negative feedback loops. When your body temperature rises too high, you sweat to cool down. When it drops too low, you shiver to generate heat. The brain’s temperature-monitoring center detects even slight variations and triggers the appropriate response. Similar feedback systems regulate blood pressure, blood sugar, oxygen levels, and dozens of other variables. When these systems fail, the consequences show up quickly: a high core temperature, too much salt in the blood, or too little oxygen can trigger urgent signals like overheating, thirst, or breathlessness. These sensations are your body’s way of pushing you to take action and restore balance.
6. Response to the Environment
All living things detect changes in their surroundings and react to them. This is true for organisms with complex nervous systems and for organisms with no nervous system at all. The reactions can be behavioral, chemical, or physical, but the pattern is always the same: sense a change, then respond.
Examples span every branch of life. Cuttlefish sense a predator nearby and instantly change their skin color to match the background. Plants grow toward sunlight, a response called phototropism. The sensitive plant (touch-me-not) folds its leaves shut when something brushes against them. Frogs emerge after rain. In humans, the smell of food triggers saliva production, while the smell of ammonia drives you to leave the area. Even at the cellular level, organisms respond to stimuli: bacteria swim toward nutrients and away from toxins. Many plants shed their leaves in winter as a response to shorter days and colder temperatures. This constant sensing and reacting is what allows organisms to survive in changing conditions.
Where Evolution Fits In
You’ll sometimes see lists that include seven characteristics of life instead of six, with the seventh being adaptation or evolution. Populations of living things change over generations in response to environmental pressures. Organisms with traits better suited to their environment survive and reproduce more, gradually shifting the characteristics of the whole population. NASA’s astrobiology program, for instance, lists adaptation as a seventh trait alongside the core six.
The reason some courses stick with six is that evolution operates on populations over long timescales rather than on individual organisms during their lifetimes. The other six characteristics are things every individual organism does right now: it has cells, it metabolizes energy, it maintains internal balance, it responds to stimuli, it grows, and it can reproduce. Evolution is the cumulative result of those processes playing out across generations. Whether your textbook lists six or seven, the core six remain the same.
Why Viruses Don’t Make the Cut
Viruses are the classic edge case. They contain genetic material (DNA or RNA) and they evolve over time, but they lack several key characteristics of life. A virus has no cells. It cannot reproduce on its own; it must hijack a living cell’s machinery to make copies of itself. It has no metabolism, meaning it doesn’t take in or use energy independently. During the phase when a virus is inside a host cell, it essentially disappears as a distinct entity, reduced to nothing but its genetic instructions. Outside a host, it’s an inert particle.
This is why most biologists classify viruses as non-living, though the debate continues. Some researchers have argued for expanding the definition of life to include viruses, particularly after the discovery of giant viruses that carry surprisingly complex genetic toolkits. But under the standard six-characteristic framework, viruses fail on multiple counts.