What makes something alive? It’s a simple question, but it gets deep, fast. Fire moves, grows, and consumes things. So does a car engine. But we don’t call them alive. Why?

In biology, “life” isn’t about the ability to do things, instead it’s about meeting specific, scientific criteria. Let’s explore what those are.

The Seven Signs of Life

Biologists generally agree that something is alive if it exhibits all seven of the following characteristics:

1. Cells

Everything alive is made of one or more cells. Cells are the basic building blocks of life, and if it’s not made of cells, it’s abiotic.

2. Homeostasis

Living things must keep their internal environment stable. Think: sweating when you’re hot or shivering when you’re cold. This balance-keeping is called homeostasis.

3. Metabolism

Living organisms need energy. They eat, process, and absorb nutrients and then break them down to fuel their activities. This chemical processing is metabolism.

4. Growth and Development

All living things grow and change over time. A seed becomes a tree, and a baby becomes an adult. This development is usually guided by DNA.

5. Reproduction

Life must make more life. Whether it’s a flower pollinating or a bacteria splitting in two, reproduction is a sign of life.

6. Response to Stimuli

Living things react to their environment. Plants bend toward light, animals flee danger, single-celled organisms can detect and move away from toxins, and we wear clothes based on the seasons! These are all things that prove our livelyness

7. Adaptation through Evolution

Over many generations, all living organisms evolve in some way, shape, or form. They adapt to survive better in their environments, oftentimes being driven by natural selection.

What’s not alive?

Some things might seem alive but don’t check all the boxes:

• Fire consumes fuel and spreads, but it doesn’t have cells or DNA.
• Viruses contain genetic material and evolve, but they can’t reproduce on their own. They hijack a host’s cells to do it. Many scientists consider viruses to be in a gray area: not quite alive, but not purely non-living either.
• Robots can move, respond, and even “think,” but they don’t have cells or metabolism.

Is life a spectrum?

Some researchers argue that life may be more of a spectrum than a strict checklist. Especially since we’ve exploring alien organisms or synthetic biology, a broader definition is in order.

For now, though, if you want to know if something is alive, ask:

“Does it have all seven characteristics?”

If the answer’s yes, it’s alive!

Understanding life helps us not only appreciate its beauty, but also helps us notice its fragility. Whether you’re watching a bird fly, a tree sway, or cells divide under a microscope, you’re witnessing miracles.