Why is DNA the Only Library?
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All life depends on the very particular molecule that is DNA. Why should this be? If upward causation were true, then self-replicators can in principle evolve spontaneously and replicate themselves. The fact that only one form of self-replicator (the one that is now proposed in the ancestral forms of the DNA or RNA molecule) has ever arisen is then a problem. If it can in principle evolve spontaneously, why hasn’t it evolved spontaneously multiple times, resulting in many lineages of life forms based on different molecular self-replicators?
I have heard two forms of explanation for this. The first is that DNA-based life has outcompeted all the other forms of self-replicator and they are now all extinct. This argument seems pretty weak to me. If life can have emerged fairly quickly after the conditions of the early earth stabilized, perhaps less than a billion years after the earth was formed, then in the more stable conditions afterward the stage should have been set for many more spontaneous occurences of self-replication. The natural prediction of this theory would be something like what we observe in lineages of life form generally that compete for resources. It seems to predict that many lineages would arise and branch out, and during extinction events they would be pruned, but the major lineages would all leave some select descendants, so that over time the pool of self-replicator lineages would become richer and more diverse. This theory of the competition of self-replicators should leave us with a world full of many forms of self-replicating, informational molecules. Yet we have only one.
The second idea, which I find more plausible, is that somewhere in the universe, perhaps in many places, this indeed has happened, but life reached our planet through panspermia. This is the idea that there was a bottleneck through which only one self-replicator system ever passed, and the bottleneck was interplanetary migration. Life came about on a different planet, and since the likelihood of surviving on a trip across space clinging to an asteroid is so unlikely, only one form of living system ever arrived on our planet. If this theory were true, it would explain neatly and elegantly why there is only DNA-based life, and not many other molecular self-replicating life forms. Yet this explanation also has a similar weakness, the fact that this interplanetary journey, if it happened once, should likely have happened more than once. A more obvious problem is that we have not yet discovered life on other planets, so we can’t be sure that it even exists out there. This is not really an explanation for how life arose so much as an explanation for why the data is inaccessible about how life arose.
If however, upward causation is false, this question is more tractable. If downward causation is the true motivational force that is organizing life forms, then any informational molecular system is more useful to the extent that it can be shared between life forms. Like a spoken or written language, the informational system can carry inventions from one being to another, useful innovations for making novel proteins. Languages compete as well, but it is a winner-take-all competition. Distinct languages seem to have evolved in isolation and with intergroup suspicion and violence, during the conditions of human prehistory. When languages truly compete in an atmosphere of peaceful interchange, like the present day, rare languages quickly go extinct and the most common languages become more widespread. This is because if you know the dominant language you can access many more informational resources than if you do not. The language that has the most speakers has a tremendous advantage because this means it has the most centers of creativity that generate knowledge. There is a strong network effect. For this reason it seems evident to me that the whole world will end up speaking either English or Mandarin. Rare languages may persist. I hope they do, because they certainly enrich our intellectual world and inform our knowledge of history. But languages are in the end pragmatic means of gaining information, and for this reason everyone will eventually converge on one system of exchange.
If DNA is a means of information storage and not a source of instructions, this convergence on one system of information is the natural result. The evidence that this may be true is clear. Lineages of unicellular prokaryotes and viruses commonly exchange DNA across lineages with no respect for anything that would resemble a species boundary. This means that one type of bacteria, for instance, can “mate” with a totally different type, resulting in novel combinations of genes and phenotypes. Horizontal gene transfer is so common in simple life that it is nearly impossible to identify exact lineages of unicellular forms at all. They cross-fertilize with each other promiscuously. This horizontal transfer is the main source of antibiotic resistance in infectious bacteria, a problem that is currently creating a health crisis worldwide and many researchers suggest it may be presenting a fundamental challenge to medicine. We ourselves are estimated to contain nearly 8% DNA from viral origins.
Viewing DNA molecules as libraries rather than as groups of self-replicators answers the question of why life has only been found that is based on DNA. In the downward causation story, life which may have initially spoken more than one “language” has now converged on DNA as the most transmissible and useful form of informational molecule. Otherwise it is not clear how the alternate forms of informational system that should have arisen have been so systematically eliminated. Since so little of the prokaryotic world has been explored, it is entirely possible that some day we will discover some alternate informational systems out there in nature. If we do, it might be easier to distinguish between the alternatives I just presented. Since I believe strongly that downward causation is true, I am already fairly satisfied with the DNA convergence idea, but I would be very curious to know if there are alternate systems out there that are still as yet unconverged with DNA systems. Or perhaps there really are self-replicators, and they are indeed arising and competing still today in the vast unexplored realm of the microscopic.
We will have to test epistolution to find out.
Copyright March 1, 2023 by Charles S. Munford
