Reading Darwin gets my own brain working, and I’ve been thinking about the necessity of accurate copying in lifelike self-replicators.
While googling Stuart Kauffman, I came across a concept of “Lipid World” which proposes that life arose by the development of lipid vesicles. Lipids can self-assemble into cell-like enclosures with lipid bilayer walls. This is highly suggestive. But the Lipid World FAQ points out that lipid vesicles “give rise to somewhat imperfect copies of themselves” and concludes that “Life then moved on an axis of increasing FIDELITY rather than increasing complexity.”
It seems to me that in any given environment, it will either be the case that a large proportion of possible structures are “fit” – that is, self-sustaining through stability and/or replication – or only a few possible structures are “fit.”
If a large proportion of possible structures are fit, then those structures will arise easily, and each spontaneously appearing structure will evolve (without even needing replication) to a very fit state; then they will persist in that state, acting as an energy sink that precludes further advances and probably precludes the development of self-replication. Note that this is not intended as a description of life as we know it; rather, it is a description of non-living “natural processes.”
If only a small proportion of possible structures are fit, then any given accumulation of random variation from a fit structure is likely to be unfit. Thus, in a population reproducing by imperfect copying, the population’s fitness will tend to devolve.
Therefore, self-replicators, from the start, must have been based around a high-fidelity copying process – indeed, a digital copying process. Covalent chemistry is digital. If I understand correctly, lipid vesicle copying is not digital, even though it can transmit information.
To put this theory into concrete terms, it seems to me that the Lipid World would quickly converge to a sea of optimal vesicles, and then improvement would stop. These vesicles would be as “natural” and as non-living as clay particles. Either vesicles or clay particles might then have provided a substrate for the development of self-templating covalent polymers.
Since covalent chemistry is digital, self-replicating polymers would be able to explore thin and perilous pathways in the space of all possible molecules. As long as a sufficient number of “offspring” were identical to the parents, an arbitrarily large fraction of the variants could be less fit; competition would remove the less fit ones, and preserve the good-enough ones until the rare improvement came along.
For another writeup of this theory from a slightly different point of view, see my post on the origins of life on my “Responsible Nanotechnology” blog.