Question

What are the major hypotheses for the origin of viruses? Describe these hypotheses in detail. What evidence exists that supports or does not support these hypotheses?

Answer 1

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. They can't survive on their own but reproduce themselves by using surrounding environments.Out side the living cell usually viruses are not living. It consists a genetic material (Either DNA or RNA) and a protein coat (Capsid). Some time it posses an outside envelop of lipids. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity of virus.

The origins of viruses in the evolutionary history of life are not clear. But there are mainly 3 hypothesis regarding the origin of virus. they are

1. Escape Hypothesis (Progressive hypothesis or vagrancy hypothesis)
2. Virus first hypothesis
3. Reduction hypothesis (degeneracy hypothesis or regressive hypothesis)

1. Escape hypothesis.

According to this hypothesis viruses are originated through a progressive process. They arose from small genetic elements (Either DNA or RNA). And this small genetic elements are escaped from the genes of larger organisms. Later that escaped genetic material gained the ability to move between cells and became capable of obtaining coat protein that they need to protect themselves from the outer environment.

This hypothesis doesn't explain the structures that are unique to viruses. And that structure is not seen anywhere in cells. It also does not explain the structure of complex capsids and other structures of virus particles. Different viruses uses totally different hosts and their activity areas are also different. This hypothesis fails to explain that also.

One thing that give little preference to this hypothesis is the resemblance of  their gene structures with messenger RNA (mRNA, Result of transcription) in cells.

2. Virus first hypothesis

The virus-first hypothesis states that viruses co-evolved (together evolved) with their current cellular hosts.In this views viruses and cells have different origins but have mutually influenced their evolution. According to this hypothesis viruses existed in a pre-cellular world as self-replicating units. So viruses evolved from complex molecules of protein and nucleic acid before cells first appeared on earth and it contributed to the rise of normal cell.

Virus need a host to exists. So this hypothesis violates the definition of virus. Before the host, the existence of virus is not possible.

The supporting factor for this hypothesis is, protein encoded by the virus is not homologous to cellular proteins. So may be they came from entirely different lineage.

Another supporting factor is Reverse transcriptase. It is an enzyme found in retroviruses. It catalyzes the transcription of retrovirus RNA into DNA. From this there will be a possibility of the contribution of virus in the rise of normal cell.  In existing cellular organisms, the activity of reverse transcriptase is extremely limited.

3. Reduction hypothesis

According to this hypothesis Viruses were once small cells that parasitized larger cells or they are remnants of cellular organisms. Viruses may have originated via a regressive, or reductive, process. In this hypothesis ,viruses stem from normal cells that have atrophied to genes and protein coat.

Hypothesis does not explain why even the smallest of cellular parasites do not resemble viruses in any way. It fails to explain the origins of RNA viruses that account for more half of the entire viruses, while cells use just DNA to store genetic information.

Supporting factor is, like human DNA, the genes of poxviruses and herpes viruses have double-stranded DNA. Giant viruses with similar genetic material to parasitic bacteria also supports this hypothesis.