In a recent post I said that the replication of cells is required for evolution and the origin of modern life as we know it. While it’s true that the daughter cells of a parent dividing cell end up containing all the elements and features of the parent (assuming replication accuracy is very high, as it is in all life forms), the fact is that cells don’t actually replicate all of their components. In other words, each protein doesn’t produce a faithful copy of itself, and neither do membranes, filaments, flagella, or ribosomes. There is only one component of living cells that is replicated with high accuracy, and that is the DNA molecule. The reason that works is that DNA is not only capable of being copied almost exactly, it is also the part of the cell that is responsible for guiding the production of all the other components of the cell – all the proteins, enzymes, membranes, and cell organelles. (One exception is the energy-producing mitochondria in eukaryotes, but we won’t go there now). This means that if the DNA of the parent cell is faithfully copied, then the entire cell will be faithfully copied in the two daughter cells.
There is a scientific consensus that the modern life system of DNA replication and DNA control of the synthesis of all subsequent cellular components is so complex that it could not have been present in very early life forms. The idea that earlier life forms might have used RNA (similar to DNA) as the original replicating molecule has become known as RNA world. This RNA world is proposed to have predated our current DNA world (along with LUCA, the last universal common ancestor of all life today).
RNA can probably be copied (although the details of how this happens are not yet completely worked out. See previous post). In addition, RNA can act as a catalyst, so all the protein enzymes present in modern cells might not be needed. How RNA world turned into DNA world is a question that has not yet even been addressed.
Aside from the existence of some kind of replicator molecule like DNA or RNA (there are one or two other possible candidates), there is no known possible mechanism for a cell to replicate itself into two daughter cells that are just like the parent. This is the reason that replicator-first proponents insist that cells that have plenty of metabolism but no replicators could never evolve to produce anything like the life we know.
But even if primitive early cells had replicators that were able to manage the production of an accurate copy of the parent cell phenotype (meaning all the components and characteristics of the cell), there is a potential problem that has only rarely been addressed. That problem can be stated simply: How did the accuracy of replication become so high?
One would expect lots of errors in primitive replication systems, no matter what chemical composition they had. It’s likely that replication became more accurate over time thanks to the only biological process we know that makes things better over time – evolution. But wait, if accurate replication is required for evolution, and evolution is required for accurate replication…? We have a problem.
And that is the problem I have been working on. I will present some preliminary results in Part 3. Stay tuned.