Viruses - Hard to Categorize

We think of viruses as being tiny, much tinier than bacteria. But as they say about mutual funds, past performance is not indicative of future results! In 2003, scientists discovered a “giant” virus. A typical virus has around 100 genes, this one had 1262. More giant viruses were found from that point onwards. Pranay Lal points out something startling in Invisible Empire:

“Some viruses are so large that they can be parasitized by smaller viruses.”

Sounded like Russian dolls.

 

The discovery of giant viruses reopened the debate on how viruses came to exist. Broadly speaking, there are two schools of theories. The first one says that viruses got started just before or around the same time as life on earth (remember how viruses are said to be on the border of living and non-living? And that viruses need to insert themselves into living cells to kick into action? That is why, in theory at least, they could have gotten started before life got started). The second one says that viruses could have started only after cellular life came into existence.

 

The second school (cellular life followed by viruses) is further split into two branches. The first sub-theory says that when genetic material gained the ability to move across cells, some of them then learnt to build a cell membrane around themselves, and thus the virus came into existence. The second sub-theory says that viruses are organisms that kept reducing their genome size and cellular functions until they were at a size and level of complexity where they could exist but not function until they hijacked the cellular functions of some larger organism –viruses came by reducing functionality, in other words.

 

The first theory (virus before cellular life) has few proponents today because it is uncommon for parasites to evolve before the host. Also, if viruses had a common ancestor, at least some genes should be common across most viruses, but that is not the case. In fact, there is very little commonality in genes across viruses. Even related viruses retain very few genes in common as time passes.

 

If some cellular life kept reducing in size until it became a virus (the second sub-theory), then by logic, there must be viruses of a variety of sizes. The discovery of giant viruses certainly fits with the second sub-theory.

 

As Lal says, viruses are fascinating, whether you consider them “living or dead or the living dead”.

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