Why biology (and chemistry) is not physics

In the Wall Street Journal, the physics writer Jeremy Bernstein has a fine review of a new joint biography by Gino Segre of George Gamow and Max Delbruck named "Ordinary Geniuses" which I just started reading.

Gamow and Delbruck were not as well known as some of their more famous peers but as Segre demonstrates, both made very important contributions to cosmology and molecular biology through direct experimentation and theorizing as well as by inspiring others' research. In addition the two were colorful and engaging characters which makes the book a pleasure to read.

But it was the first paragraph of the review that really caught my eye:

Some sciences are more unruly than others. Here's a parable to illustrate what I mean. Imagine that when the first life form appeared there was a superintelligent freak. If this freak had had a complete knowledge of the laws of physics, what could it have predicted? Quite a lot. All atomic nuclei consist of neutrons and protons, and the number of protons determines each element's chemical nature. Knowing this, the freak could have predicted all the elements that could possibly exist, along with their respective characteristics. Suppose that it also knew all the laws of biology, including the "central dogma," which explains how genes are expressed as proteins. Even so, it could not have predicted the existence of giraffes, nor even the fact that my brother and I share only half our genes. Both of these are evolutionary accidents. If it had not been for random mutation there would be no giraffes, and my brother and I might have shared all our genes, as male bumblebees do. Biology is not like physics.

This paragraph succinctly pretty much nails down the fundamental limitations of physics-based reductionism and it's a point that applies to chemistry as well. It's a very important point. The problem is that reductionism will never be able to account for the role of historical contingency and accident. Even if an all-powerful being could account for all biological scenarios emerging from an initial state of the universe, it could never tell us why one particular scenario is preferred over others. As Bernstein says, evolutionary accidents by definition cannot be predicted from starting conditions because they depend on chance and opportunity.

In addition function can never be uniquely derived from reductionism even if structure is. For instance in his book "Reinventing the Sacred", the complexity theorist Stuart Kauffman makes a powerful argument that even if one could derive the structure of the human heart from string theory in principle, string theory would never tell us that its most important function is to pump blood. The function of biological organs arose as an adaptive consequence of the countless unpredictable constraints that molded them during evolution. In addition the evolution of both structure and function was a mix-and-match process that depended as much on chance encounters as on strict adaptation. All this can never be captured in a reductionist worldview.

The same principle applies to chemistry. For instance the supreme being would never have been able to tell us why there are only twenty amino acids, why there are alpha amino acids instead of beta or gamma versions (which have extra carbon atoms), why amino acid stereochemistry is L while sugar stereochemistry is D, why there are four DNA bases with their unique structures, why nature chose phosphates (although Frank Westheimer comes close), why a given protein folds into only one unique functional structure, why water is the only solvent known to sustain life, and in general why the myriad small and large molecules of life are what they are. In retrospect of course one could provide several arguments for the existence of these molecules based on stability, function and structure but there is no way to predict these parameters prospectively.

The problem is that there is nothing in the nature of these molecules that dictates that their presence should have been uniquely determined. For instance we now know from synthetic studies that beta and gamma amino acids can also fold into the kind of helices and (less so) sheets that are ubiquitous for alpha amino acids. In addition these "higher order" amino acids provide extra handles for functional group attachment (see top figure). Yet for some reason they were discarded during evolution. Why? We could come up with several arguments. 

For instance because of their floppiness, maybe the higher order versions had to pay an unacceptable entropic penalty that could not compensate for their folding propensity. Or maybe the Strecker reaction that is thought to produce alpha amino acids could never be superseded by other chemical reactions for forming beta amino acids. Or perhaps alpha amino acids shield hydrophobic side chains much better than their longer chain counterparts. Cogent reasons, all of these, and yet I am sure we could find an equal number of arguments against alpha amino acids if we searched hard enough. The ultimate failure to find an explanation for the existence of alpha amino acids is a powerful reminder of the importance that chance and circumstance played in the evolution of both biomolecules as well as living organisms.

This role of contingency and accident is one of the most important reasons why the reduction of chemistry and biology to physics won't work. In addition as I have described before, reductionism cannot account for variety in chemistry. Yet another reason why chemistry and biology are not physics.