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"And another minor point, we know of divergent evolution for things as complex as large animals, like echidnas and hedgehogs. And that article talks about all virii sharing the proteins to make their capsule, and implies they all inherited it from a common ancestor. But isn't that an extraordinary claim? Isn't it just as likely for there to have been multiple origins and we're just looking at convergent evolution? For things that simple, how many ways to build a capsid could there be?"IIRC there are a few different mechanisms for capsuling a cell, however it's all controlled by all the environments it can be expected to combat. One of the main things that inhibits viruses is high temperature, which is the main reason why you often get a fever when you're sick with a strong variant of the flu.
In viruses it appears the protein layer serves the function perfectly, it's capable of replicating fine at human body temperature, it can deal with the acidity present in certain parts of our body (however, most cannot deal with direct exposure to stomach acid, they usually take the trip inside a chunk of food) and in certain viruses allows easy modification of their surface markers by a change in their amino acids. IIRC the protein surface also enables the easy stealing of the lipid layer from a human cell, which they use as camouflage. The protein capsid can easily attach to the lipid layer and also has a higher rigidity than a human cell, possibly to enable its passage through the cell membrane... I mean a virus wouldn't be very good if it just bounced off of a human cell or if it got crushed when trying to pass through the lipid layer.
It appears the protein layer serves a wide range of purposes quite expertly, which would suggest that viruses would likely evolve to use the same protein without connection. The true test of this theory will be the trip to Europa (if it ever happens) because 1 milliliter of sea water can contain 250 million bacteriophages, so it will be extremely easy to tell if Europa has life (unlike Mars) and I bet $1000 that any virus-like life on Europa will contain a protein capsid and any bacteria-like life will contain a lipid layer (due to the fact that the lipid layer can actively move nutrients into the cell, just like humans can actively put food in our moves... I don't think we'd have survived if biology had trusted luck in getting food into our moves and not hands).
BTW A side note: The plural of virus is viruses, I don't intend to sound rude but I love the English language and the reason behind is rather complex, and no one really knows why someone tried to use virii because it isn't talking about a virus. The use of virii to denote multiple viruses appears to be an attempt by some to sound more educated by using a more Latinized plural. However, the ironic thing is that Viruses is in fact the correct pluralization for the English language. Any argument that in Latin that multiple viruses would be virii is patently false, because it's double pluralized for a male word: truly the plural of virus in latin should be virusi, vir is man in latin, so viri is men and virii is a plural of a plural! However in latin the plural of virus would likely be, virus because it's not a masculine word and plurals were exceptionally rare. It's like one cannon and two cannon, you have one virus and two virus, however the English language says two viruses to make it fit.