| Compared to other families of viruses, coronaviruses recombine unusually often and are unusually likely to produce viable new strains that way. Those traits make them unusually capable of producing zoonoses, which is one of the reasons why they're a subject of particular interest among infectious disease researchers. Those same traits also produce a high degree of similarity between a lot of genomically differentiable strains of coronavirus, especially since spike proteins tend to be strongly conserved
for their direct effect on virulence - a significant mutation there is likely to be maladaptive, if it changes the protein structure enough for cells to no longer uptake a virion that expresses it. So it's not really a surprise to see that two different strains have similar spike proteins, and doesn't really give much basis for inference about relatedness between them. Another important point is that coronaviruses are so common specifically in bats, which are both extremely plentiful and unusually accommodating hosts for many kinds of viruses including coronaviruses, very often with multiple strains infecting a single host at once. That gives coronaviruses - which, remember, are already good at recombining to produce new strains, every one being possibly able to jump species - more chances at that kind of recombination than they would have otherwise. Too, bats shed virus in feces the same way humans do, and bat guano is so effective a fertilizer that wars have started over access to supplies of it. So it's not difficult or unlikely to postulate a chain of events like this: - a recombination event in a bat produces a virus capable of infecting humans, - which is then deposited in feces used to fertilize a human food crop, - which is then harvested and taken to a city market to be sold, - and all it takes for someone to get sick from there is not washing their hands often enough while they're cooking. To be clear, I'm not saying this is what happened. Nobody knows that yet. But precisely because nobody knows that yet, it's important to consider the relative likelihoods of various ways that the pathogen might have developed into a form that can infect humans, and then reached the point of actually doing so. That's what I'm doing here. Each stage of this postulated chain of events relies only on things that are already known to frequently occur, and have been so known since long before SARS-CoV-2 was even known to exist. The result is unprovable, of course, just as with every other theory of this disease's origin that anyone has advanced so far. But it doesn't rely on any unusually small probabilities, and so it seems a lot likelier to me than the combination of a lab accident of a kind known to have happened only a few times in the last two decades, and a simultaneous superinfection of a single researcher with two different strains of coronavirus. |