What’s the difference between life and nonlife? To answer this question, we need to think like a biologist. A key idea in biology (the study of living things) is that every living organism engages in two basic activities: self-generation (making more life) and self-maintenance (staying alive).

Self-generation

Reproduction: the process by which living organisms produce young or copy themselves

Organism: a living thing that has (or can develop) the ability to act or function independently

Protein: A complex molecule made of amino acids. What cells are made of

Molecule: the smallest distinguishing unit of a compound, formed when atoms of different elements are held together by chemical forces

Cell: a system considered analogous in structure or function to a living body

Photosynthesis: the conversion of light energy to chemical energy, which is stored in sugars or other organic compounds, and is performed by plants, algae, and a few other organisms

Gene: a segment of DNA that is involved in producing a polypeptide chain

Genome: a complete, single set of chromosomes

"The genes that copy DNA are very accurate… most of the time."

Mutation: the act or process of changing, especially genetically

One of the most basic things that all living things do is make new life. Animals, plants, and even tiny, single-celled organisms can reproduce, whether by making babies, seeds, or dividing their cells. This is how life continues from one generation to the next. Nonliving things, like rocks or refrigerators, can’t reproduce.

All organisms on our planet have proteins, the keys to self-generation. Proteins are a type of complex molecule that make up most parts of a cell. Proteins develop in many different shapes and combine to generate chemical reactions in cells that control how the cells act.

Self-maintenance

All living organisms do things to keep themselves alive. They eat food, drink water, breathe air, and do other things that allow them to get energy. They use this energy to grow, repair themselves, and reproduce. Animals—like you—eat food to get this energy. Plants, on the other hand, get energy from sunlight, through a process called photosynthesis. But both you and the tree down the street need water to survive, and you also eat a whole lot of plant molecules, which were made by photosynthesis. Refrigerators, on the other hand? They might have food inside them, but they can’t heal a dent or make a litter of tiny fridge babies.

Proteins, so important for self-generation, are also critical for self-maintenance. But here, rather than forming chemical bonds that allow self-generation, proteins help break chemical bonds to create the energy needed to keep a cell going. Self-maintenance also involves things like avoiding dangers in the environment—like predators and disease.

DNA: The blueprints of life

Proteins control the functions of self-generation and self-maintenance that are typical of all living organisms. But what controls the proteins? The answer is what makes you, you—your DNA, the blueprint for everything about you.

The proteins that run your cells are programmed in genes inside DNA molecules. Genes are instructions for cells, and each gene specifies what special shape proteins fold up into.

The instruction book of all the genes that an organism needs to grow, stay alive, and reproduce, is called a genome. It’s the genome of an organism that makes one living thing different from another. For example, a cell in a banana tree and a cell that’s part of your body both share many of the same genes, especially the ones that help with staying alive. But obviously, humans also have many genes that make them different from bananas.

When living things reproduce, they pass copies of their genomes to their offspring. This genetic inheritance is why children usually look like their parents, and why you look like you, rather than a banana.

When it’s time for your body to complete certain processes, such as making blood, healing a cut, or fighting a cold, the genes in charge of that job are “switched on,” and when it’s done, they’re “switched off.” This is the power of the genome. It does much more than just controlling what you look like. It’s constantly working to make sure your self-maintenance and self-generation functions are working.

The evolution of life

The genes that copy DNA do a very accurate job…most of the time. Sometimes, mistakes happen. When there’s an error in the copying of DNA, it’s called a mutation. Genes with mutations in their DNA encode proteins that are different shapes than what was coded in the genome.

The mutation may have no effect. But sometimes, mutations can have big consequences that can disrupt an organism’s ability to self-generate and self-maintain, and it will not survive.

Some mutations, however, create changes that make the organism different from its parent in ways that make it better at surviving and reproducing. For example, a parent duck may have delicate foot webbing while the webbing of its mutant offspring may be extra-thick. What happens next depends on the environment the ducks live in. If the ducks hang out on mudflats, the mutant feet may provide for better footing, allowing the mutant duck to be better at finding food and fleeing predators. This success makes the thick-footed duck more likely to survive and reproduce, enabling it to pass on its mutation to future generations. If, however, the ducks live in grasslands, the mutant feet may slow the duck down, and the trait will be less likely to be passed on. This process is called natural selection. When you combine inherited mutations with natural selection, you get something called evolution.

The evolution of life

So, is there a single characteristic that life has that makes it different from nonlife? Is there one clear difference between a rock and a whale? A refrigerator and you? After all, they’re all made of molecules. They all change through time.

Perhaps the most interesting quality that living organisms share is that they have purpose while nonlife does not. That means living things have goals. The short-term goal of every living thing is to self-generate and self-maintain in their environment. The long-term goal is to pass genome copies to offspring, a goal that succeeds only if self-generation and self-maintenance succeed. Lots of rocks are cool, but they don’t have goals. You might like cold soda from your refrigerator, but your fridge doesn’t care what you like.

Taking this perspective, you could say that when life showed up on Earth, something completely new showed up: the emergence of purpose. Whether life—and purpose—exists anywhere else in the Universe is unknown and may remain a mystery. Meanwhile, we can enjoy the amazing variety of life that surrounds us here on Earth.