Science Tuesday: What is a virus anyway?

As you may have observed, I like to step back sometimes and think about deep stuff. The origin of life is one of the Big, Hard Questions but a related question to which we have no clear answer is the origin of the viruses. Here's a good summary of the problem from Nature magazine.

Most people begin by asking whether viruses are even alive. I classify that as a semantic quibble. Personally, I find the word "life" useful to describe self-replicating entities that expend energy for the purpose (and whatever else it takes along the way). Can viruses be said to satisfy those criterion? They replicate but they require the machinery of a living cell to do so. On the other hand living cells have certain requirements of their own. Similarly viruses don't expend energy on their own, they take over cells that then invest energy in making more viruses. Viruses really consist of an informational medium that takes over cells. The computer virus analogy is actually pretty good. Viruses are like malware. Malware is not a computer, so viruses are not life forms. But you can say whatever you like.

So there are these bits of genetic material floating around, inside a protective protein package (often with other attached molecules). Cell walls incorporate what are called "receptors," gateways that selectively let molecules in and out. Viruses are configured to get into cells through particular receptors (which is why they preferentially infect particular kinds of cells and thereby cause different kinds of diseases), and their genetic material, by one means or another, is then read by the cellular machinery, which it instructs to make more viruses. (The words "instructs" is somewhat anthropomorphizing and seems to suggest volition. That's misleading but it's an easy shorthand.) In the process, the cell eventually destroys itself and all those new viruses burst out. That's what makes us sick.

That's all well understood but the hard question is, where the heck did these things come from? There are three proposed answers, all of which might be true for different viruses. In other words they aren't necessarily all the same thing. The first is the so-called progressive hypothesis. Properly functioning cells have machinery that copies elements of the genome and can insert the at new places in the DNA. This may not be adaptive at all, it may have arisen by accident, but once these so called retro-transposons exist they might get loose from the cell and go on to infect others. This could be the origin of the retroviruses such as HIV. The genetic material of these viruses consists of RNA.

The regressive hypothesis is that they are degenerate parasites. There are bacteria that can infect eukaryotic cells and maybe over time they lost the ability to replicate on their own and came to depend on host cell machinery. There are viruses that plausibly arose this way, including smallpox. These are DNA viruses.

Finally, maybe viruses are left over from the origins of life. Maybe they existed before cells came into existence, able to replicate under favorable environmental circumstances. Now their descendants depend on cellular machinery to replicate. This might best account for some single-strand RNA viruses.

There doesn't seem to be any way ever to answer this question. But as life on earth depends on the DNA-RNA information processing system, it's just a fact that units of information are out there, floating around, that can corrupt the system. These units can mutate and evolve fast, and living things evolve in turn. As a matter of fact the leading hypothesis for the ubiquity of  sexual reproduction is that it speeds evolutionary adaptation to pathogens by mixing the genes. Otherwise, speeding up evolutionary adaptation wouldn't be worth the substantial costs and risks of sex. So you can thank viruses for your very existence, not to mention what might be your favorite activity.