What Is “Life” Anyway? An Evolutionary Take

Back in 2012, I wrote a Psychology Today post entitled “What Is 'Behavior' Anyway?” In the end, I argued we might define behavior as “the coordinated responses of whole living organisms to internal and/or external stimuli.” Still, even posing the question of how to define behavior assumes we know how to define life. Inspired by evolutionary thinking, I want to suggest a way to do it.

Oftentimes, discussions of what constitutes life begin by analogy. Suppose you have never seen fire before, but while you are out in the forest today, you do. What you see is that fire grows, and it appears to move. It engulfs living material in its path, and smoke and ash appear to be waste products produced by fire. Fire looks like it “reproduces,” splitting off and forming new, smaller fires. Living beings you have encountered do similar things and so you might think that the fire is alive. But then someone explains to you that fire isn’t alive; it just appears to be.

The problem here is that there is something unsettling, and downright unscientific, about saying fire is not alive because, well, we just know it isn’t. Instead, we need an operational way to address what constitutes life.

Following arguments that Carl Bergstrom and I have made in our textbook Evolution, rather than trying to construct a hard and fast definition of life, perhaps the best we can thing do is to identify a set of properties that are usually, if not always, associated with living things, including the ability to: 1) adjust the internal environment to maintain a stable equilibrium (homeostasis); 2) maintain distinct parts and the connections between them (structural organization); 3) control chemical reactions (metabolism); 4) grow and reproduce; and 5) respond to environmental conditions or stimuli.

Here’s the evolutionary kicker that needs to be added to that list: Life is subject to the process of natural selection. Indeed, the origin of life on Earth was more than just the origin of self-replicating entities: The origin of life is the origin of those entities subject to natural selection—we need replication, but we also need heritable variation in traits that cause fitness differences.