04/16/2014 01:44 pm ET Updated Jun 16, 2014

Why Are Atoms So Small?

Atoms are small, really small -- there are some thirty thousand billion of them in the head of a pin. But why are atoms so small? It took a great physicist, Erwin Schrödinger, to even think to ask such a question, as he did some 70 years ago in his classic text "What is life?" Already then he took a first step toward its resolution by pointing out that the real issue is not the small size of atoms, but rather the big size of us living creatures. Atoms are small in comparison to us. So the true question becomes: Why are so many atoms -- zillions of them -- needed to make up even the very simplest living thing? Why are we so complex?

Well, thanks to a new area of study, systems chemistry, we now have the answer.

Living things are highly complex chemical systems, with one central characteristic -- they are fundamentally replicative. All living things are structurally dedicated to that primal agenda, to make more of themselves (though that doesn't mean that all living things are able replicate). But, of course, living things did not appear all of a sudden. The process took millions of years and is generally believed to have begun with the fortuitous emergence of some simple molecular system endowed with the unusual capacity of self-replication. And that primal system then began to evolve.

One central lesson of systems chemistry is that abiogenesis (the process by which life emerged from non-life) and biological evolution are two stages of a single evolutionary process, a process that has an identifiable physical driving force -- the drive toward greater stability. But the relevant stability kind is not the traditional one, thermodynamic stability, but rather another kind, termed dynamic kinetic stability (DKS). That's the stability associated solely with replicating populations and their persistence over time. Replicating populations tend to evolve in the direction of increasing DKS -- from less persistent to more persistent. The mechanism is primarily Darwinian natural selection, though when the process takes place at the molecular level, we chemists call it kinetic selection, but the bottom line is the same: More effective replicating systems, whether chemical or biological, tend to drive less effective ones into extinction. So over time there is a natural tendency for replicative function to improve. That's a law of nature, just like the law of gravity.

But why did that evolutionary process toward greater DKS lead to such staggering complexity? Though complexity is frustratingly hard to quantify, we all have a pretty good idea of what the term means. Broadly speaking more complex things are made up of a greater variety of different components, and more of them. Certainly the evolutionary process from molecular replicator to cellular replicator was unmistakably one of inordinate complexification. That transformation resulted in a huge increase in molecular diversity and a dramatic increase in mass, some twelve orders of magnitude. In mass terms that's like going from a few grains of sand to several hundred elephants. So complexification by evolution is the answer to how living things became so big. But that still leaves Schrödinger's why question unanswered. Why such inordinate complexity?

The answer centers on the concept of function. In the world of functional design there is an unambiguous connection between function and complexity, even if that connection cannot be quantified. Function is almost invariably enhanced by complexity. Modern cars have power brakes and power steering, even though those extra components add cost to the car's manufacture. But that extra level of complexity enhances the car's function. Significantly, that relationship applies to all functional entities, from cars and computers to toothbrushes and disposable nappies. It's not just a technological issue. A chair made of just one material, say wood, is not as functional as one made of several materials -- say wood, rubber (for padding), upholstery, and so on. Any engineer or designer intent on improving function will almost invariably need to increase complexity.

Now think of replication as a function. If so, then complexity will likely enhance that function as well. And so it is. Nature has discovered and continually utilizes the complexity principle with regard replicative function. Molecular sized replicators are fragile and temperamental. It takes skilled chemists in well-equipped labs considerable time and effort to get such entities to replicate. Ask any systems chemist. But those highly complex replicating entities - bacterial cells - are supremely functional. They happily replicate pretty well anywhere, quite indifferent to the vagaries of the environment. The almost incomprehensible complexity of the biological cell, honed over billions of years of evolution, has resulted in a highly stable entity that can deal with even extreme variations in environmental circumstances. But now we understand why. All its componentry, molecular or supramolecular -- proteins, sugars, lipids, ribosomes, mitochondria, flagella, whatever -- are analogous to the car's power brakes and power steering. All are there solely to facilitate and enhance the cell's replicative function. The cell is nature's answer to the ultimate design question in chemical replication.

And how did that long evolutionary journey from molecules to cells take place? Step by step. Gerald Joyce and Niles Lehman have each carried out beautiful experiments on replicating RNAs that demonstrate that single molecules are less effective in replication than two- or three-molecule networks. So, already at the molecular level we see that more complex replicators are more functional than simple ones, just as two chopsticks are more functional for eating purposes than a single one. In the world of replicators, functional complexification is the name of the game.

Yes, atoms seem so small, because we living things are so big. Living things, whose very essence it is to replicate, must be big to incorporate all of the complexity needed to create highly stable replicating entities. The answer to Schrödinger's provocative 'why atoms are small' question has turned out to be satisfyingly simple.