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Convergence in Biology
Genetic TakeoverA. G. Cairns-Smith, in vis book Genetic Takeover, makes the case for a new theory of the origin of life, an alternative to the conventional primordial-soup theory. As is necessary in any serious attempt to refute conventional wisdom, the book is balanced, full of details, and slow to reach conclusions … all of which makes it hard to find pithy quotes. So, I will start with my own biased and oversimplified view instead.
As I learned it, the conventional theory goes something like this. After its formation as a planet (a whole separate issue, which I imagine to be well-understood), Earth was lifeless, with only the simplest chemical compounds present. Lightning bolts arcing through the atmosphere caused the formation of simple biochemical molecules, which accumulated and formed a “primordial soup”. In this soup, the biochemicals reacted with one another and gradually produced more and more complex molecules, culminating in chains of nucleic acids much like the ones we use today.
There are some experimental facts that make this plausible, notably, that electrical arcs can in fact produce amino acids and other compounds, and that RNA, in addition to storing information, can also act as a catalyst. The latter is important because it means we don't have to imagine a coupled system of nucleic acids and proteins magically popping into existence all at once—the catalytic role of the proteins could originally have been filled by the nucleic acids themselves. However, there are also some problems with the conventional view, which are discussed in detail in the book but which are not of particular interest here.
According to the genetic takeover theory, the original replicators were not biochemicals at all, but instead were minerals—clays, to be specific. A substantial portion of the book is spent explaining the chemistry of clays, so that the reader can understand why replication in clay is plausible. Here's my quick summary. Clays can grow by accretion, and can contain information in the form of defect patterns. Replication occurs because the pattern of defects on the growing face propagates itself into the succeeding layers.
What's more, it is well known that surfaces can act as catalysts, so different defect patterns should be able to catalyze different reactions in the chemicals that flow over the surface. It is in this way that the first biochemicals would have been formed. If, for example, a particular defect pattern catalyzed the formation of a protective organic coating for the clay, then that pattern would possess an evolutionary advantage over other defect patterns.
As the clays formed more and more complex biochemicals for their own purposes, the biochemicals became capable of replicating on their own, and did so, superseding the old clay-based replicators. This was the genetic takeover. As an aside, it is interesting to wonder whether the clay-based replicators were necessarily completely superseded—maybe we could find some of them still alive as mud on the sea floor somewhere.
Here is Cairns-Smith's own summary of the theory of genetic takeover.
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@ April (2000)