One thing we hope to accomplish here at SC is to provide readers with a better understanding of interesting ideas in biology. Perhaps, then, we should come up with a working definition of “biology.” You probably learned long ago that biology is the study of life (from the Greek bios, life; not from “bi,” as in “two,” as in “plants and animals,” which is an inappropriate dichotomy for all that biology entails). But what is life? That’s a question I can’t entirely answer in a single post, nor probably in a single lifetime. However, I’ll try to convey a sense of what distinguishes life from non-life in a biological sense.
For a long time, people thought that living things possessed some kind of metaphysical essence that non-living things didn’t have. Many people still believe this. It turns out, though, that you can explain the inner workings of living things using the same laws of physics and chemistry that apply to non-living things. Since it’s extremely hard to prove that something does not exist, scientists can’t really say that you do not have an immaterial soul. But if you can basically explain everything that happens inside living things without invoking something supernatural, why should you? I personally see no reason to.
Notice I said “immaterial soul.” I do believe in souls. Physical souls, made up of atoms and energy like everything else. You don’t have a soul, you are one. Dust with breath, as it says in Genesis 2:7. A soul is the emergent complexity that arises from all the physical and chemical reactions constituting a being. Like a story. A story isn’t anything supernatural, but it’s more than the sum of the molecules of ink and cellulose making up a book. A “scientific,” reductionist analysis of a novel, that weighed the pages and tallied the data on when and where each letter of the alphabet was used, would never find the story. A skeptic might even conclude that stories don’t exist.
But I still haven’t explained what life is. Let me explain what fire is, first. Fire is a lot like life. A fire uses oxygen in the air to turn organic molecules into carbon dioxide and water, releasing energy. So do living things. All the living things you’re most familiar with, dogs and houseflies and trees, do it exactly that way. Some microbes use different molecules, but its still a process of chemical metabolism. Fire can also grow, reproduce, and die. Why isn’t fire considered alive?
For one thing, fires don’t have cells. The reason we even bother to distinguish between living and nonliving is because living things catch our attention with their intricately detailed functional structures and abilities. A cell is organized above and beyond anything non-living. Every cell, even the simplest bacterium, has an amazingly complex system of enzymes that copy DNA, ribosomes that read the copied transcripts and molecules called tRNAs that build protein from the instructions. Then there are the parts that keep the process supplied with a constant supply of energy, protected from the assaults of world, and structurally sound. A cell is a super-organized, awe-inspiring machine unlike anything else anywhere, except maybe systems built from aggregations of cells. It’s this complexity that what we notice about living things.
The main reason complexity is important is because it allows for homeostasis. This means a living thing is organized enough to monitor its surroundings and adjust accordingly. Temperature, salinity, and other chemical balances are kept within a reasonable range. Fire, on the other hand, is subject to the whims of the world. It goes where the wind blows it, grows bigger when there’s lots of fuel, withers when fuel is scarce, and changes temperature depending on what its burning. It has no way of maintaining conditions so that they’re optimal for it. That’s why fires, and their offspring, eventually go out.
Fires go out, but life doesn’t. Individuals die, but life as a whole keeps on going. And that means it’s had time to find better ways to survive, which keeps it going longer, in a self-reinforcing process. No fire is as organized as a living cell, because no fire lineage lasts long enough to get there.
Burning is everywhere. Fires are a widespread feature of the universe. But, once, a long time ago, there was a fire that didn’t go out. It was able to control its environment enough to keep itself going and producing offspring. Some of the offspring were able to control their environment a little more, which helped them survive better, and eventually this led to the first living cell. I speak of fire in the broad sense; this may have happened underwater, after all, where flames don’t do so well. But it was, truly, the process of burning: a chemical reaction that releases energy. And all living things are simply this same chemical reaction, still sustaining itself billions of years later. Fire is not a type of life, but life is a fire. A single, individual fire, with a particular origin billions of years ago, that has become complex enough to maintain homeostasis and continue indefinitely. That’s what we are. And that’s what biologists study.