Quantum mechanics plays dice with our DNA

I realize that my speech yesterday, on the unpredictability of biological evolution even downstream of a perfect understanding of its generative mechanisms, has aroused in the readers a certain bewilderment and in some cases a clear misunderstanding as regards the role played by chance in the Darwinian process. Some clarification is therefore in order. First of all, let’s start by eliminating a persistent confusion that arises between phenomena with an unpredictable outcome and random phenomena.. We differentiate properly, that is, the meaning of “chance” from that of “not determinable”. The outcome of a phenomenon may not be determinable for two reasons. First, we can have the circumstance where our measurements of its initial state are insufficient; that is, its future evolution could depend in such a convoluted and detailed way on every smallest fluctuation of a multitude of initial parameters, that it is not possible for us to experimentally determine with precision the starting point of its evolution, and therefore not even its outcome, even for small time intervals at will. There could even be the circumstance that some variables are not measurable, because they are “hidden” or in any case obscured from our senses and our instruments. In the situation described, the evolution of the system will be unpredictable, even if perfectly determined by the initial conditions, due to our insufficient knowledge of these. This type of physical system does not present a random evolution, but a perfectly deterministic one, ie perfectly determined by the initial conditions and by the laws of physics; to a certain state, another invariably follows, even if we, knowing only imprecisely the initial state, cannot know what is the final state in which we will find our system.

We then have another type of systems whose evolution is unpredictableif not on a statistical basis, in the sense that to a starting state, two or more outcomes of a successive measure can follow for these systems, not necessarily with the same probability. These are non-deterministic systems, i.e. in which perfect knowledge of the state does not uniquely determine one and only one possible outcome of a future measure.

Now, strange as it may seem, the foundations of reality established by quantum mechanics have precisely this characteristic: they are non-deterministici.e. they present characteristics of intrinsic randomness in the sense just defined, and not only apparent due to some measurement imprecision or the failure to measure “hidden variables”.

Now, at the scale of our world, the random effects connected to quantum physics are not generally evident, because they rapidly disappear as the complexity of the bodies considered increases; but the world in which mutations occur in the DNA molecules, which is what interests us for the purposes of the spontaneous generation of biological variety on which natural selection can act, is the world of atomic dimensions.

The quantum processes that can lead by pure chance, in the strict sense that we have illustrated, to a mutation of the DNA are many; here I will mention one of those described more recently and confirmed on an experimental basis, which involves the exchange of a proton by tunnel effect between cytosine and guanine of a pair of DNA bases.

Cytosine and guanine, when two strands of the double helix are paired, constitute a quantum system, in which three protons, engaged in three hydrogen bonds, are delocalized between the two bases. However, when, due to the beginning of the copying process of each strand, the double helix opens, separating the two strands, the starting quantum system corresponding to each pair of guanine and cytosine collapses in a totally random way in one of two states possible, with one of the three protons attributed in the “usual” way to the starting base or, anomalously, to the opposite base. In every second, this process happens in hundreds of sites of every single double helix of the DNA that composes us; if this happens while the DNA is being replicated, a proton positioned in the “wrong” place generates a copying errorfor which in the new double helix generated a different base pair will be found in place of the original one.

Note well: in the absence of mutagens, oxidizing radicals or UV rays, thanks to these phenomena, however, the DNA mutates, and the sequence of bases in the resulting copies changes, which implies a continuous flow of variety generated at each replication, variety which, under the scrutiny of natural selection, can take on an adaptive meaning and therefore contribute to the evolution of a species.

But – this is the point – this mechanism underlies an important role of pure chance, derived from the collapse of a quantum system, and is only one of several proposed and investigated with varying depth: that is, chance is at the root of the possibility of biological evolution , because it contributes to substantially determine the rate and type of mutations. Even canceling any effect of the incredible number of fortuitous circumstances that we don’t know, that is even if we want to assume our perfect knowledge of the starting point of the evolution of a living species, we don’t know which and how much variety will be generated within the generations, and , until we prove the indeterminism of quantum mechanics false, we can never know.

Quantum mechanics plays dice with our DNA.