Look around you. Do you see anything that’s alive? You’re alive, of course, as are any friends of family members you can see. Do you see an animal? How about a plant? These things are alive, too. But what about the soil that plant lives in? The food in your lunch box? Are these things alive?
At first glance, it may seem pretty easy to determine what’s alive and what’s not. However, when we look a bit closer, it can become more difficult. Consider the tiniest things in our world. Are electrons alive? How about viruses and bacteria?
Even scientists disagree about what it means for something to be alive. NASA defines life as “a self-sustaining chemical system capable of Darwinian evolution.” Some experts get a little more specific—they lay out seven properties of life.
What are these properties? The first is organization. That means that living things are made of cells. To be alive, something must have specialized parts that all do their own job. Second, living things must have metabolism. This includes all the biochemical reactions that take place in something that is alive.
The third property is homeostasis. That means that living things regulate the conditions inside their bodies. Living things also experience growth. They reproduce in some way, although this can take many forms. Living things must give response to changes in their environment. Finally, the last property of life is evolution. Living things must be able to change over time.
Not all experts agree with these properties of life. After all, it’s pretty easy to find examples that don’t fit. Consider the mule. Like most animal hybrids, it can’t reproduce. However, most people would consider the mule to be alive. Still, the properties of life provide a starting point for understanding what makes something alive.
There are still many questions to answer. For example, what about viruses? Experts disagree over whether they are living things. Some claim the answer is no because viruses can’t reproduce without help from a host cell. They also aren’t made of cells. Others disagree, as viruses do have many of the other properties of life.
How about life on other planets? Will it look like life on Earth? Most likely not. That means those searching for alien life have to keep open minds about what makes something alive. Our understanding of this will probably continue to develop as humans explore deeper into the universe.
What do you think? Would you classify viruses as living things? Do the properties of life seem right to you? What do you think alien life could look like? Human beings are still searching for solid answers to these questions and many more.