How Do We Define ‘Life’?

How did an inanimate universe become animate? That question has been the cause of wars and the inspiration for artists, painters, writers, and of course, scientists.

More than any other physical observable, life appears akin to a miracle. For many people, perhaps most people alive in the world today, it still is. Yet, at the heart of science is the presumption that natural effects have natural causes. If we accept that life is subject to physical laws, we are obliged to then move from the sacred to the profane. Or at least to the natural.

The conversation, when attempting to address the known unknowns of the universe, would be colossally incomplete if it did not raise what—for most people—are the two most awe-inspiring mysteries of nature: life and consciousness.

While both of these domains are typically in the purview of the biological sciences, nature doesn’t divide itself along the lines of 19th century academic disciplines. The laws of biology are determined by the laws of chemistry, which in turn are determined by the laws of physics. Any fundamental understanding of life will ultimately reflect the workings of these laws as well.

This fact has inspired some outstanding physics minds in the 20th century to ponder the problem of the nature and origin of life, as well as its possible robustness in the universe. Erwin Schrödinger, one of the fathers of quantum mechanics, wrote an influential book in 1946 entitled What is Life?, which inspired a young student, planning to be an ornithologist, to turn instead to genetics. That student, James Watson, later discovered the double helix nature of DNA as the basis for the genetic code of life. (His scientific collaborator, Francis Crick, was trained as a physicist.)

As it turns out, Schrödinger was influenced by Max Delbrück, a physicist who had done seminal work on fundamental physics interactions but later turned his attention to genetics. His work in 1935 on molecular genetics strongly impacted Schrödinger. That work on bacteria and viruses, which won Delbrück the Nobel Prize in Physiology or Medicine, was done while he still had a teaching position in physics. He didn’t become a professor of biology until 1947.

In the 21st century, the tools necessary to address the fundamental mysteries associated with the nature and origin of life are likely once again to be found in physics laboratories, and perhaps even astrophysics departments. The questions are too important and too fundamental to be relegated to merely one area of science.

Let’s first consider Schrödinger’s question:

What is life?

While it seems obvious and easy to tell if something is alive, upon more thought, the definition of life becomes particularly slippery. Ultimately, as Justice Steward said when referring to pornography, one seems driven to simply say, “I know it when I see it.”

For example, one might posit the following definition: living systems reproduce faithfully and have an internal metabolism that draws energy from the environment and stores and expends that energy to grow and reproduce.

OK then—is fire alive?

It ticks all these boxes. A forest fire draws energy from the environment. It reproduces, and even somewhat faithfully, depending on the nature of the fire and the availability of combustibles. It certainly has a metabolism, expending energy to grow and reproduce.

But I don’t think anyone would argue a fire is alive, so we need to do better. Here is a definition from Wikipedia: Living organisms are generally thought to be open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce and evolve.

This definition is definitely more complete and encompasses living things more closely. Homeostasis refers to the need to maintain biological equilibrium and was first described by the French physiologist Claude Bernard in 1849. In 1920, Walter Bradford Cannon coined the term to describe this biological necessity.

The homeostasis requirement probably rules out fire since fire really doesn’t have a moderating feedback loop to maintain a kind of static equilibrium. Nevertheless, it is quite reasonable to think of life, at least life that respires, as controlled burning. One of the surprising aspects of Earth is that there was no free oxygen early on in the history of the planet. This was fortunate, because the process of oxidation releases energy, just as oxygen is needed to fuel most fires. Had oxygen existed early on, many of life’s raw materials would have oxidized early on, releasing valuable stored energy necessary for the life that now covers our planet to begin, evolve, and grow.

Life exists somewhere between oxidation and fire. Unlike either, it moderates the intake of energy to maintain homeostasis, which is so crucial for survival, and ultimately reproduction.

But, speaking of reproduction, is this essential? What about viruses, like SARS-CoV-2, which has controlled all of our lives over the past few years? Viruses like SARS cannot reproduce on their own. They need to hijack the genetic machinery of other living cells to do so.

While they do not fulfill all the requirements to be alive according to the earlier definition, viruses definitely seem alive to me. They have a strategy that requires them to piggyback on other organisms, but they have hardwired the complex biochemical machinery necessary to reproduce in the environs of other living organisms. Moreover, I don’t like the idea that I have to wear masks and get vaccine injections to protect myself against an inanimate object. So, I say viruses may as well be alive. But I also say that Pluto is a planet…

Alive or not, it is quite possible that viruses helped life evolve into its present form. Some viruses do not have a deleterious effect on their hosts, in which case, we can consider them as symbionts. Ultimately their biochemical features may be incorporated in more complex cells to allow living systems to expand their capabilities.

Perhaps the most famous form of such merging is mitochondria. These are the parts of modern living cells that control the intake and processing of oxygen in respiration. As Lynn Margulis and others first postulated, it is quite likely that mitochondria were autonomous organisms that were assimilated into other cells, enhancing their ability to process energy (respiration allows more than 35 times more energy to be released from the processing of electrons than photosynthesis does).

This is not unlike the more sophisticated assimilation of the Borg collective, for Star Trek fans. The Borg are an advanced civilization that conquer other civilizations, adapting and utilizing the best features of those civilizations into their own biology and technology. The first Eukaryotic cell that engulfed a mitochondrion didn’t have the capacity to say, “Resistance is futile,” but it probably was.

Definitions are useful, but they are not the heart of science (although unfortunately that is the impression that is given all too often in elementary science classes). Science is about processes, about understanding dynamics—and that is what I want to focus on. And while the study of the evolution of diverse life on Earth is a rich and exciting field, involving its own puzzles, this is not where the dominant outstanding questions about life really lie. The big unanswered questions remain: How did life first begin? Is life on Earth unique? Is all life like life on Earth?

The Edge of Knowledge: Unsolved Mysteries of the Cosmos is due to be released on May 9th, 2023. Copyright © 2023 by Lawrence M. Krauss, and published by Post Hill Press. All rights reserved. This excerpt was published with permission from the author.