7 Fundamental Characteristics That Define Life: A Scientific View of Living Things

Understanding life and studying it in depth have fascinated humanity throughout history. Nobel Prize-winning biologist Paul Nurseoffered an important framework by identifying five fundamental characteristics of life. We have expanded this list slightly by adding two new items: balance and the interdependence of living things. We will explore the seven fundamental characteristics of life in an accessible popular science style. With current examples and data under each heading, we will take a fresh look together at the question "what is life?"

1. Cells (The Basic Unit of Life)

The fundamental building block of all living things is the cell. Cells can be considered the smallest and simplest units of life. A single cell can carry out all of life's fundamental processes on its own. Bacteria, for example, are single-celled organisms, and these tiny life forms can eat, produce energy, and reproduce. In multicellular organisms (including us humans), the body consists of trillions of cells with different roles. The human body contains approximately 30 trillion human cells, most specialized for particular functions (such as muscle, nerve, and blood cells). Cells grow and divide to make copies of themselves, allowing life to continue. In the 1800s, the scientists considered the fathers of cell theory defined life's foundation by stating: "all living things consist of one or more cells, and every new cell arises through the division of another cell" as their fundamental principle.

Cells vary in size and shape according to their functions. Human red blood cells, for example, are disc-shaped and specialized to carry oxygen, while nerve cells transmit signals throughout the body using long extensions. The largest cell in the world is an ostrich egg — a huge single cell visible to the naked eye! The microscopic world also holds surprises: in 2022, scientists announced that a bacterium discovered in a mangrove swamp, Thiomargarita magnifica , could reach approximately 1 centimeter in length. This single-celled bacterium is 50 times larger than an average bacterium and is the first bacterium visible to the naked eye. The discovery surprised scientists by pushing the assumed limits of cell size. Cells, then, are the building blocks of life and contain all its characteristics, whether microscopic or visible to the naked eye.

2. Genes and Heredity (The Code of Life)

The code that allows an organism's traits to pass from generation to generation is made possible by genes . Genes are specific sections of a molecule called DNA and each gene encodes a particular trait in an organism (such as eye color or blood group). DNA's famous double helix is rather like a book of life : it contains instructions written in a four-letter chemical alphabet (the nucleotides A, T, C, G). These instructions tell cells which proteins to make, and proteins act as an organism's building blocks and machine parts. Genetic information passes from parents to offspring through reproduction, which is why children resemble their parents. This phenomenon of heredity was first discovered through pea experiments by Gregor Mendel . Mendel's discoveries were later explained at the molecular level when DNA's structure was deciphered.

Modern science has made incredible advances in genetics. The human genome (the complete set of human genetic information) contains around 25 thousand genes, and our DNA comprises a sequence of 3 billion letters. In 2003, the Human Genome Project marked a milestone by largely deciphering 92% of our genome sequence; however, some difficult regions remained missing. Finally, in 2022, an international consortium of scientists completed the remaining missing 8% and published, for the first time, a complete and gapless human genome sequence . This means all the pages in our genetic information book can be read. Fully decoding our genetic information opens the door to groundbreaking advances in understanding and treating hereditary diseases, and even designing new organisms (synthetic biology). Gene-editing technologies (e.g., CRISPR) also allow us to deliberately modify particular genes. Genes and heredityare the foundation of life's continuity and the raw material for evolutionary change, giving them a key place in the list of life's characteristics.

3. Evolution and Natural Selection (The Ability to Adapt)

Living things undergo evolution over time to adapt to changing environmental conditions and survive. Evolution is based on the mechanism discovered by Charles Darwinknown as natural selection . In its simplest form, natural selection means organisms better adapted to their environment survive and reproduce, while those unable to adapt are eliminated . A population contains inherited variation (genetic differences); individuals with traits better suited to the environment leave more offspring, and those traits become more common over time. Thus, across generations, species change to become better adapted to their environments.

There are striking everyday examples of evolution. Bacteria developing antibiotic resistance, for example, is literally evolution happening before our eyes. An antibiotic used to eliminate an infection kills susceptible bacteria while leaving a rare few with mutations conferring resistance alive. Resistant bacteria reproduce and quickly become dominant, creating a resistant bacterial population that no longer responds to that antibiotic. This process of natural selection can be observed even in a laboratory. Another historical example occurred during England's Industrial Revolution: a change in moth color. Normally light-winged peppered moths began to become darker as tree trunks were coated in soot; dark mutant moths survived because they were camouflaged against blackened trees and hidden from birds, and soon became the majority of the population. Later, as air pollution decreased, light moths regained an advantage. This shows how natural selection pressures can reverse as environmental conditions change.

The finches Darwin studied in the Galápagos Islands are also symbols of evolution. Their beaks differ according to the food sources on their islands — some have thick beaks suited to cracking hard seeds, others pointed beaks suited to catching insects on slender branches. Darwin used these examples to explain that species share a common ancestor and diverged to adapt to different conditions . Today, genetic analyses, fossil findings, and observational studies strongly support evolutionary theory. Evolution and natural selectionrepresent life's changeable, dynamic, environmentally responsive nature. They have allowed life to persist for 3.5 billion years by adapting to countless different environments on our planet.

4. Chemical Processes (Life's Metabolism)

Living things are like chemistry laboratories! Chemical processesencompass all biochemical reactions essential for life to continue. Thousands of reactions occur at every moment in our bodies and all other living things. For example, as we breathe and produce energy, our cells' mitochondria use oxygen to break down a sugar molecule called glucose and produce the universal energy molecule ATP (adenosine triphosphate). ATP supplies the energy cells need to work (muscle contraction, nerve transmission, molecular synthesis, etc.), which is why biology calls it an "energy currency." Every day, a human body produces and consumes an amount of ATP close to its own weight — approximately 50 kilograms — although the same ATP molecules are recycled and reused many times. This astonishing figure shows how intense the chemical traffic in our metabolism is.

The chemical processes of living things essentially revolve around obtaining nutrients, transforming energy, and removing waste products. Photosynthetic organisms such as plants and algae use sunlight to turn atmospheric carbon dioxide into food (photosynthesis), storing energy for themselves while producing oxygen and making the atmosphere habitable. When animals eat, their digestive systems break large molecules into small building blocks that fuel their cells. All these reactions are accelerated by biocatalysts called enzymes and controlled through a delicate balance.

Life's chemistry is so finely tuned that even a small change in body temperature or pH can disrupt reaction rates. At high fever, for example, enzymes lose their structure, chemical processes falter, and life-threatening situations may arise. Organisms therefore try to maintain stable internal conditions so chemical reactions work properly (which brings us to the next characteristic: balance). In essence, chemical processes underpin life: everything from respiration to digestion, growth to repair occurs through chemical interactions in cells. Life is, in fact, nothing more than a dance of elements and molecules.

5. Information Flow

One of the most fascinating aspects of life is how it uses information. Living systems operate on information as well as energy. This information is mostly stored in DNA and determines when, where, and what organisms do. But information is not limited to the genetic code. As Paul Nurse emphasizes, life is a complex network system that stores, processes, and transmits information.

The first layer contains the classical flow of information: DNA → RNA → Protein . This process — transcription and translation — governs the basic functions of living cells. Through RNA, DNA controls the cell's protein production, and proteins serve as the cell's building blocks and molecular machines. This central dogma is life's universal language.

But information flow is not limited to this. Organisms also manage information through complex networks within cells. For example, which genes are active in a liver cell depends on protein levels, energy status, and even environmental stress signals. This works like biological software: the cell's internal sensors decide which information will be read and which will be silenced.

Cells also exchange information with one another. Chemical signals released by injured tissue direct immune cells to the site. During embryonic development, signaling molecules produced by one group of cells tell neighboring cells to "become a neuron" or "become a muscle cell." This information flow organizes life and enables different cell types to form.

Plants, too, act on information when growing toward light or sensing water in the soil. Although genetically based, this information is carried by signals sent between cells. Information therefore exists not only in DNA sequences, but also in intracellular networks, communication between cells, and developmental pattern formation.

In recent years, revolutionary technologies have been developed to understand and manage this information system. DNA sequencing allows us to read organisms' genetic books; gene-editing tools such as CRISPR allow lines in these books to be deleted or rewritten. Synthetic biology allows genetic circuits not found in nature to be designed. All these technologies show that life is an information-based system as well as a chemical one.

In summary: information flow is the fundamental mechanism sustaining both life's existence and its diversity. Beginning with the genetic code, information is processed, transmitted, and interpreted through cellular regulation, communication, and development. Life is an exquisite organization of this multilayered information system.

6. Balance (Homeostasis)

Living things can maintain a balance both in their internal conditions and in their relationships with the environment where they live. In biology, this internal balance is called homeostasis . Homeostasis means an organism continually adjusts to keep its conditions at a stable point (or within a narrow range). Maintaining body temperature around 36-37 °C, blood glucose at a certain level, and stable cellular pH are all results of homeostasis. When this balance is successfully maintained, life continues; when it is disrupted, diseases arise or the organism faces life-threatening risks.

As an everyday example of homeostasis, consider our body temperature . A healthy person's internal body temperature is approximately 37 °C. When the surroundings become colder, we begin to feel cold; we try to prevent heat loss through responses such as shivering (producing heat through muscle movement), constricting blood vessels in the skin (reducing heat loss), and putting on more clothes. Conversely, when body temperature rises on a hot day, we begin to sweat; as sweat evaporates, it cools the body and removes excess heat. These processes occur because a center in the brain called the hypothalamus acts like a thermostat. Similarly, when blood glucose rises, the pancreas releases insulin, which tells body cells to "clear the blood, take in sugar." When blood glucose falls, glucagon takes over, telling the liver and muscles to "break down reserves, release sugar into the blood" and raising the level. This keeps blood glucose in an ideal range, neither too high nor too low.

Homeostasis is not limited to temperature or glucose; it applies to almost every biological parameter. Blood pressure, the body's water and salt balance, tissue oxygen levels, and even the amounts of proteins produced inside cells are regulated by homeostatic mechanisms. Through feedback systems, cells reduce production when a substance becomes too abundant and increase it when levels fall. For example, when blood oxygen drops, the brain increases breathing to take in more oxygen; when there is too much oxygen, it slows breathing. Without these fine-tuning systems, our bodies could not function steadily despite environmental changes.

Homeostasis makes living things flexible and resilient rather than fragile in the face of their environment. A desert mouse adjusts its kidneys to survive on minimal water when water is unavailable; a polar bear prevents heat loss in the cold through a thick fat layer and a body with low surface area, without lowering its body temperature. The human body also sends signals through sensations such as sweating, feeling cold, thirst, and hunger to keep us in balance. Thus, balance (homeostasis) is essential for life to continue. Without internal balance, remaining alive despite the outside world's changing conditions would be impossible.

7. Interdependence (Ecosystems and Symbiosis)

No organism can exist alone, completely isolated. Interdependenceis the fabric of life's web. In nature, all organisms interact directly or indirectly with other organisms. These interactions can take many forms, including feeding, protection, reproduction, and providing habitats. Forest trees, for example, share nutrients and information through fine fungal threads wrapped around their roots (mycorrhizae); a tree needing water may "borrow" from a neighbor, or a tree attacked by harmful insects may send chemical signals to warn nearby trees. These reciprocal interactions are like a network of solidarity extending throughout the ecosystem.

One of the clearest examples of dependence is the relationship involved in pollination (the transfer of pollen). Approximately two-thirds of the world's flowering plants depend on animal pollinators; approximately 35% of global food crops (one in every three mouthfuls!) also depend on pollination by animals such as bees, butterflies, birds, and bats. As bees collect nectar, they unknowingly carry flowers' reproductive cells, enabling plants to produce fruit and seeds. When we eat the fruit, we benefit from this cycle. If pollinators disappeared, many plants could not reproduce and agricultural production would be badly affected; humanity would therefore be directly affected too. This is a striking example of living things' chain of dependence on one another.

Moreover, even every organism's body is an ecosystem. For example, the human gut contains trillions of bacteria, fungi, and other microorganisms forming the microbiome. It helps digestion, trains the immune system, and may even influence behavior. There are as many bacterial cells as cells in our bodies — so numerically, we could say we are half human, half bacterial! Without these microbes, we could not digest many foods or produce certain vitamins, and we would be more vulnerable to disease. They depend on us (we provide nutrients), and we depend on them (they support digestion and health) — a mutually beneficial relationship.

Close forms of living together called symbiosis are also common in nature. Lichens, for example, consist of a fungus and an alga; the fungus supplies water and minerals, the alga photosynthesizes to make food, and together they can live even on hard rocks. Similarly, underwater coral reefs exist through a partnership between coral polyps and single-celled algae; the algae feed the coral, and the coral provides shelter. Predator–prey relationships, parasite–host relationships and competitive relationships also shape organisms' populations and evolution. Other species limit excessive growth of a species, maintaining balance in the ecosystem (for example, wolves control rabbit numbers, while limits on available prey control wolf numbers).

The image aboveshows how microorganisms (bacteria, viruses, fungi) circulate among humans, animals, plants, soil, water, and air. Humans interact reciprocally with this ecosystem; microbes can move from the environment to humans and from humans to the environment. This cycle highlights the spread of infections, environmental transmission routes, and how closely ecological health is intertwined with human health.

Ultimately, interdependencemeans that every organism in life's great web is directly or indirectly connected to another. Understanding this web of connections is also critical for protecting ecosystems. Like links in a chain, if one species disappears, many others interacting with it are affected. If human activities reduce bees in an area, not only honey production but fruit and vegetable yields fall; if large predators disappear from a forest, herbivores may grow out of control and destroy vegetation. These examples show that organisms exist in an interdependent balance and that no organism is truly "an island unto itself."

These seven characteristics help us understand life's complexity and remarkable balance. From cells to genes, evolution to chemistry, information flow to internal balance and ecological relationships, they remind us once again how valuable and intricate life is. Lifeis too rich a phenomenon for a single definition; yet these basic principles reveal the shared "signature of life" found in all organisms. As science advances, new dimensions may be added to life's definition: borderline cases such as viruses may be better understood, or artificial life forms developed. At their core, however, the seven fundamental characteristics listed here will remain the touchstones distinguishing life from nonliving processes. Understanding these principles does more than satisfy our curiosity about biology; it also guides us in protecting ourselves, nature, and all its living things, and living in harmony with them.