What is Life?

How do we define what is and isn't life? A checklist of course! There are certain agreed upon criteria that define what we consider living and non-living. Clearly, something like a rock is non-living. But what about a virus? Let's take a look at the list of what we consider makes something alive:

1. Organized - Life is complex and organized, with different molecules that are highly structured performing different functions. This includes single-celled and multicellular organisms.

2. Metabolism - Living organisms channel and use energy, through chemical reactions. Some of the more common metabolic processes include photosynthesis (found in plants) and respiration (found in animals and many other organisms).

3. Homeostasis - Organisms maintain a stable internal environment to allow metabolic processes to function. In humans, we maintain a very well defined temperature, while other organisms, like reptiles, make use of outside environments to maintain their temperatures, like lying in the sun.

4. Growth - Life grows! Whether its multicellular organisms producing more cells, or single-celled organisms building larger membranes, life grows in stages, commonly referred to ontogeny.

5. Reproduction - Organisms create copies of themselves or mix their genetic code to create new versions of themselves. Some organisms simply produce clones while others sexually reproduce.

6. Stimulus-Response - Organisms respond to their environments, often referred to as stimuli. If someone pokes you, you respond!

7. Evolution - Life changes over generations in response to the environment and other organisms through natural selection. As conditions change, only those that develop ways to survive better (adaptation) in their respective environments are able to continue to live.

For something to be considered alive, it must meet the criteria listed above (thanks Khan Academy). However, there may be edge cases where we see some of the list fulfilled, but not entirely, like viruses, which do not have their own metabolisms. Interestingly, there is an innate human recognition of living organisms vs. non-living objects that shows up as our brains respond to the recognition of these objects (Ahmadi-Pajouh et al., 2018). Whether we realize it or not, our brains recognize at least several features of living beings.