The big idea: A virus is about as simple as biology gets: a short stretch of nucleic acid (DNA or RNA) wrapped in a protein capsid. That very simplicity makes its origin hard to trace.
Where did viruses originally come from?: We are not certain — and the evidence suggests viruses did not all arise once. Their origin is most likely polyphyletic: viruses probably arose several separate times, in different ways.
A virus is just a nucleic-acid genome wrapped in a protein capsid (some also have a lipid envelope studded with glycoprotein spikes). The origin hypotheses ask where that simple genome-plus-capsid package first came from.
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- Polyphyletic
- Having more than one independent evolutionary origin — not all descended from a single common ancestor. Viruses are thought to be polyphyletic.
- Capsid
- The protein coat that surrounds and protects a virus's genetic material.
- Mutation rate
- How often errors are introduced when a genome is copied. Viruses — especially RNA viruses — have very high mutation rates.
- Natural selection
- The process by which variants that survive and reproduce best become more common over generations. It acts on virus populations exactly as it does on any other population.
'Uncertain' is the correct answer: For the origin of viruses, a full answer says it is uncertain / not fully known, and that viruses are probably polyphyletic (multiple independent origins).
Confidently claiming one definite origin would actually be wrong — the honesty is part of the mark.
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There are three leading hypotheses for where viruses came from. They are not rivals to choose between — because viruses are polyphyletic, different viruses may have arisen in different ways, so more than one hypothesis can be true at once.
| Hypothesis | Core idea | A way to picture it |
|---|---|---|
| Escape (progressive) | Viruses began as host genes or mobile genetic elements (e.g. transposons, plasmids) that broke free of the cell and gained a protein capsid to travel between cells. | A piece of a cell's own DNA 'escapes', wraps itself in a protein coat, and starts moving from cell to cell. |
| Reduction (regressive) | Viruses descend from once free-living cellular parasites that, living inside hosts, lost gene after gene until only a genome and a coat remained. | A small parasitic cell strips itself down over time until it can no longer live on its own — only replicate inside a host. |
| Virus-first (co-evolution) | Self-replicating molecules existed before the first cells; some of these early replicators evolved alongside cells and became viruses. | Simple replicating genetic 'pre-life' came first, and cells and viruses both descended from that early molecular world. |
Why the three together point to 'polyphyletic': Each hypothesis explains some viruses well but not all of them.
Which viruses each route fits
- The escape route fits viruses whose genes closely resemble their hosts'.
- The reduction route fits unusually large, complex viruses.
- The virus-first route fits the idea that replication came before cells.
So: polyphyletic: No single story covers every virus — which is exactly why biologists conclude viruses had several independent origins (polyphyletic), rather than one.
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The big idea: Three pieces of evidence point the same way: viruses came from cells, and they probably did so many separate times.
The points to remember
- Viruses are extremely diverse: DNA or RNA, one strand or two, envelope or none. Such variety suggests several origins, not one.
- All viruses are obligate parasites: they need a host cell to be copied, so they arose after cells.
- Their shared plan, a capsid round a nucleic acid, may have evolved separately many times: convergent evolution for the same way of life.
- Viruses use the same genetic code as living organisms: evidence that they came from cells.
- Some look like pieces of cells: retroviruses resemble retrotransposons, jumping genes in cells.
Remember it as: Many kinds, one lifestyle, one code: several origins from cells.
Real example: a giant virus: Mimivirus, found in 2003 inside an amoeba, is bigger than some bacteria and carries genes for enzymes used in making proteins. It looks like a cell that lost most of its parts: the reduction hypothesis.
In a Justify or Discuss answer, link each piece of evidence to what it shows:
Evidence, then what it shows: Shared genetic code or obligate parasitism shows an origin from cells; diversity shows several origins. Stating a hypothesis without its evidence does not earn the mark.
How this is tested: The origin of viruses comes up as a short Discuss or Justify question.
Origin
- State that it is uncertain and probably polyphyletic.
- Then give the hypotheses (escape from host genes; reduction of a parasitic cell; virus-first co-evolution).
- Don't commit to one origin as definite.
Rapid evolution
- Explain it as natural selection powered by large populations + short generation times + high mutation rates (RNA polymerases lack proofreading).
- Then give a consequence — antigenic drift/shift in flu, new SARS-CoV-2 variants, immune escape, antiviral resistance, and why vaccines must be updated.
IB-style questionJustify[2 marks]
Justify the hypothesis that viruses evolved from living cells, and that this happened several times rather than once.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.