Defining Life: What Are We Looking For?

Scientists searching for life beyond Earth face a challenging task: how do we know what to look for when we have only one example—life as we know it on our own planet? The definition of life shapes the search for organisms on other worlds. Understanding what makes something alive is a foundation for astrobiology, the scientific study of life in the universe.

What Makes Something Alive?

Biologists generally agree that life has six main characteristics: organization (made of cells), metabolism (using energy), growth, reproduction, response to the environment, and the ability to evolve. For example, all known living things are made of cells. Cells organize and use energy through metabolism, like plants using sunlight in photosynthesis or humans digesting food. Growth and reproduction ensure that living things can make more of their kind. Response to the environment means organisms can react to changes around them, such as bacteria moving toward nutrients. Over generations, life evolves—populations change to adapt better to their environments. These processes interact: for instance, metabolism provides energy for growth, and evolution results from reproduction and variation.

Life As We Know It—And Beyond

So far, our definition of life is based on Earth. All known life requires liquid water as a solvent, carbon for building complex molecules, and an energy source. But could life elsewhere be different? Some scientists propose that alien life could use silicon instead of carbon, or ammonia instead of water. To keep an open mind, researchers look for signs of life’s functions—using energy, reproducing, and adapting—rather than just familiar forms. Studying extremophiles on Earth supports this idea. Extremophiles are organisms that thrive in extreme environments, like boiling hot springs, deep ocean vents, acidic pools, and frozen Antarctic ice. Some, like tardigrades, can survive in the vacuum of space, extreme radiation, or temperatures near absolute zero. The existence of extremophiles expands our view of where and how life could exist elsewhere.

The Limits and Challenges of Defining Life

Not everything that seems alive fits neatly into our definitions. Viruses, for example, can reproduce and evolve but lack cells and don’t use energy on their own. Are they alive? Fire grows, reproduces (spreads), and metabolizes fuel, but it is not alive because it lacks cells and does not evolve in the biological sense. These examples show that life’s boundaries are complex. Astrobiologists use these challenges to refine their search. They ask questions, conduct experiments, and compare findings from biology, chemistry, and astronomy to look for patterns and exceptions. By studying Earth’s most extreme and unusual life forms, scientists hope to recognize unfamiliar life if we find it.

In summary, defining life requires looking at systems and interactions, not just single features. Our Earth-based definition is a starting point, but discoveries of extremophiles and debates about viruses remind us that life is adaptable and diverse. As technology improves, the search for life elsewhere may force us to rethink what it really means to be alive.

Interesting Fact: 
Tardigrades, also known as water bears, are less than 1 mm long but can survive being frozen, boiled, dried out, and even exposed to the vacuum of space!