What Is Life?

A March 12 New York Times op-ed by Ferris Jabr presented the provocative view that herds of moving sculptures on beaches could be thought of as "no more or less alive than animals, fungi and plants." That view is questionable. Most of us recognize that there is a fundamental difference between mechanical objects designed and created by man, no matter how sophisticated, and the naturally derived complexity of living things. In fact, my granddaughter, when she was just 2, already understood one basic difference. She loved toy dogs but was scared of real ones. Real dogs were unpredictable; she recognized that they had a mind of their own. All living things act on their own behalf, doggedly pursuing their agenda. That's true even for mindless bacteria -- no designer, no creative sculptor required. Somehow the entire cornucopia of life, spectacularly complex and purposeful, comes about and maintains itself through natural processes. No wonder we've been transfixed by this question for over two millennia.

Living and non-living things are different, even if there are those pesky borderline cases, like viruses, which make a definitional distinction problematic. The real question is how matter of any kind can have an agenda, and, no less tantalizingly, how the objective laws of physics and chemistry could have transformed "dead" stuff into spectacularly complex, agenda-driven living stuff.

Well, through a relatively new area of chemical research that deals with replicating molecules and the networks they establish, the answer to the "what is life?" question is coming into focus. Recent advances now indicate that abiogenesis, the process of life's emergence, and biological evolution are one continuous process with an identifiable driving force: the drive toward greater stability. However, the kind of stability that is operative is not thermodynamic stability, with its focus on energy, but one reflected in the system's persistence over time, termed dynamic kinetic stability (DKS). But how are two kinds of stability possible, and what is the nature of this other kind?

A most fundamental law of nature is the drive of the physical world toward more stable/persistent forms: Unchanging things don't change, and changing things do change, until they change into things that don't. That simple idea is axiomatic. It's as logical as one plus one equals two. But what hasn't been adequately appreciated is that stable/persistent forms can come about in two distinct ways: through either the thermodynamic drive toward energetically stable forms or a kinetic process that maintains persistence through self-replication. Let me explain.

The thermodynamic directive has been understood for over a century. Energetically stable things are also persistent over time, at equilibrium. That's the Second Law of Thermodynamics. And thanks to Ludwig Boltzmann we understand the mathematical underpinnings of that fundamental law. The drive toward more stable forms is just the drive toward more probable forms. The reason that water does not spontaneously convert to hydrogen and oxygen gases is the same as the reason that you are unlikely to win the lottery a thousand times in a row: It's possible but highly improbable.

But dynamic kinetic stability, the stability associated with persistent replicators, is quite different and unrelated to the system's energy. That alternative kind of stability derives from another mathematical principle: the mathematics associated with exponential growth. Take a dollar and double it every week, and in less than a year, you will be thousands of times richer than Bill Gates. Well, certain chemical systems, ones whose chemical reaction is to make copies of themselves, respond to that same powerful directive. Once some replicating entity -- say, a molecule or a replicating set of molecules -- happens to emerge, it can maintain a presence over time, even though the individual replicators may well be energetically unstable, and all because of that enormous kinetic power. As long as resources are available, they keep on making more of themselves, frantically. Consider cyanobacteria, an early life form. They have been around for more than a billion years. That's persistence for you!

But why did an early replicating system, relatively simple in nature, evolve to become so highly complex? For the same reason that picking up an object with two fingers (or, better, a whole hand) is a lot easier than doing so with just one. Complexity facilitates the replicative process, as was demonstrated by Gerald Joyce and Niles Lehman in some recent, beautiful RNA-replication experiments. In Joyce's experiment it was found that a single replicating RNA molecule was relatively ineffective at self-replication, but a pair of similar RNA molecules, neither self-replicating but each helping the other replicate, worked way better. Conceptually one can think of that step as the first on a thousand-mile journey toward that biological cell -- a highly complex cooperative entity, highly effective at making more of itself. And what governs which available materials will be chosen along the way to further facilitate the replication process -- amino acids, sugars, lipids? The title of a Woody Allen movie says it all: Whatever works.

As a final comment, an inherent property of DK-stable but thermodynamically unstable systems is that they must have a metabolic (energy-gathering) capability so that the thermodynamic books balance. That way, the replicating system can be energetically unstable (in order to keep making more of itself) yet persistently stable over time. That's nature's way of having its cake and eating it too.

The bottom line: Life's nature, the process by which it emerged, and how it relates to non-life can be understood through consideration of two long-established mathematical principles: the probabilistic one due to Ludwig Boltzmann, now over a century old, and the power of exponential growth, as applied by Thomas Malthus to his theory of population, now over two centuries old.

We may have been looking for the life coin everywhere but under the lamp post.