Viruses leave no fossils, so nobody can dig up the answer. What we do have is the DNA they left behind in us — about eight per cent of the human genome is old viral wreckage. That, and the fact that flu changes fast enough to need a new vaccine every single year.
📚 What you need to know
Viruses infect nearly all living species and are thought to have been on Earth about 3.5 billion years before humans evolved.
They have coevolved with other species. Around 8% of the human genome is viral DNA left from ancient infections, called endogenous retroviruses (ERVs).
Their origin is still debated, partly because viruses are not found in fossils.
Three theories: escape, regressive (reduction) and virus-first.
Different viruses may have had different origins, and shared features may be the result of convergent evolution.
Viruses evolve very fast because of high mutation rates, large population sizes and short generation times. Influenza and HIV are the named examples.
Antigenic drift is the slow accumulation of small changes. Antigenic shift is a large, sudden change when two virus types infect the same cell and combine their genetic material.
Vaccines must be updated, and work better against drift than against shift. Isolation of infected individuals may also be needed.
How old are they?
Viruses can infect nearly every living species, which suggests they have been around for as long as there have been cells to infect — an estimated 3.5 billion years before humans appeared.
There is good evidence that they evolved alongside their hosts, a process called coevolution. The clearest sign of it is sitting in your own DNA. Roughly 8% of the human genome is made of small segments of viral DNA thought to be left over from ancient infections. These are called endogenous retroviruses, or ERVs, and they have been passed down and modified over millions of years.
Why the origin is still argued about. Viruses are not preserved in fossils, so there is very little direct evidence to work from. Everything has to be inferred from the viruses that exist now and from the traces they have left in host genomes — which is exactly the kind of limitation an evaluation question wants you to mention.
Three theories
Read each panel left to right. Escape moves genetic material out of a cell, regressive strips a cell down, and virus-first has no cell at the start at all.
Theory
The idea
What it explains well
Escape theory
Viruses arose from genetic elements such as DNA and RNA that gained the ability to move between cells, and later became surrounded by an outer boundary
Why viral genomes resemble host genes, and why some viral sequences look like host DNA
Regressive or reduction theory
Viruses are the remnants of cellular organisms — once small cells that became parasites of larger cells and shed the structures they no longer needed
Why viruses are so completely dependent on host cells
Virus-first theory
Viruses predate their current cellular hosts. Since evolution generally moves from simple to complex, the simplicity of a virus could mean it came first
Why viruses are so simple, and why they are found in every branch of life
Why the argument is not over
Viruses are extremely diverse, which suggests there may have been different origins for different viruses. All three theories could be right, each for a different group.
It is also possible that none of them is right and a different process took place.
Some features are shared by almost all viruses — a capsid with no cytoplasm inside it, genetic material as DNA or RNA using the same genetic code as other organisms, and a fully parasitic lifestyle. Shared features in groups with separate origins point to convergent evolution.
The genetic code point is the strongest clue that viruses are tied to cellular life rather than being something separate. A virus uses the same codons for the same amino acids as you do. Whatever their origin, they have been reading the same instruction set for a very long time.
Why viruses evolve so fast
Influenza and HIV are the two examples the syllabus names, and they share three features that speed evolution up enormously.
Feature
Why it speeds up evolution
High mutation rate
Both have RNA genomes. Mutations can occur when viral RNA is converted into DNA during replication, and this copying is far less accurate than normal DNA replication
Large population size
Enormous numbers of particles mean even rare mutations turn up often
Short generation time
New generations appear within hours, so useful mutations spread quickly
Put together, these mean a virus population can change fast enough to evade the immune system of its host. By the time the immune system has learned to recognise a surface protein, the protein has changed.
Antigenic drift and antigenic shift
Follow the spikes. On the left they change one at a time, so a vaccine can be adjusted to keep up. On the right the whole surface changes in one step, which no vaccine can anticipate.
Antigenic drift
The accumulation of small changes to the viral genetic material over time.
Variation in the virus’s surface proteins appears slowly.
Eventually the host’s immune system cannot recognise the virus any more.
HIV undergoes antigenic drift.
Antigenic shift
A major change in the viral genetic material over a short period.
Two or more virus types infect the same cell in the host and combine their genetic material.
This produces rapid variation in the surface proteins, so a new virus is created that the host’s immune system does not recognise at all.
Influenza undergoes antigenic shift.
🧠
Drift is slow, shift is sudden
Something that drifts moves gradually. A shift happens all at once. Pair them with the examples: HIV drifts, influenza shifts.
Treating fast-evolving viruses
Vaccines have to be changed and updated yearly for rapidly evolving viruses, so that they stay effective.
This works reasonably well against antigenic drift, because the changes are small and not hugely rapid.
HIV is the awkward case. It undergoes drift, but at such an unusually rapid rate that a successful vaccine has not yet been produced.
Against antigenic shift, vaccines are less successful, because the changes are large and not predictable.
Fast-evolving viruses may therefore need to be handled by isolating infected individuals to stop the infection spreading.
The reason a flu jab expires. A vaccine trains your immune system to recognise particular surface proteins. Change those proteins and the training no longer applies. So the vaccine is not wearing off — the virus has changed its surface faster than your immunity can follow.
Worked examples
WE 1
Outline a theory of viral origin
Outline the regressive theory for the origin of viruses, and state one reason why the origin of viruses is hard to establish. (3 marks)
Point 1: the theory
Viruses are the remnants of cellular organisms — once small cells that became parasites of larger cells.
Point 2: what changed
Over time they shed the cellular structures they no longer needed, leaving only viral structures behind.
Point 3: the difficulty
Viruses are not found in fossils, so there is very limited direct evidence for their evolution.
Reduced parasitic cells, with no fossil record to test the ideathe “no fossils” point appears in almost every origin question — keep it ready
WE 2
Explain rapid viral evolution
Explain why influenza and HIV are able to evolve rapidly enough to evade the immune system. (3 marks)
Point 1: mutation rate
Both have RNA genomes and a high mutation rate, since mutations can occur as viral RNA is converted into DNA during replication.
Point 2: numbers and speed
They have very large population sizes and short generation times, so new variants appear constantly and spread quickly.
Point 3: the consequence
Their surface proteins change, so the host’s immune system can no longer recognise them.
RNA genome, huge numbers, fast generations — so the surface keeps changinglink the three features to immune evasion; listing them alone rarely scores full marks
WE 3
Apply drift and shift to vaccination
Explain why the influenza vaccine has to be updated each year, and why this approach is less successful for viruses undergoing antigenic shift. (4 marks)
Point 1: what a vaccine does
A vaccine prepares the immune system to recognise particular surface proteins of the virus.
Point 2: why it stops working
Influenza changes those surface proteins, so an older vaccine no longer matches the circulating virus.
Point 3: drift is manageable
Where changes come from antigenic drift they are small and gradual, so the vaccine can be adjusted each year to keep up.
Point 4: shift is not
In antigenic shift two virus types combine their genetic material in one cell, producing a large and unpredictable change, so a vaccine cannot be prepared in advance.
Small predictable changes can be tracked; sudden combined ones cannotmention isolation of infected individuals if the question asks what else can be done
💡 Exam tips
Learn all three theories by name and be able to give one sentence on each.
Quote the figure: about 8% of the human genome is viral DNA, called ERVs.
Say coevolution when describing viruses changing alongside their hosts.
Pair the examples correctly: HIV drifts, influenza shifts.
Give all three reasons for fast evolution — mutation rate, population size, generation time.
For any origin question, add that viruses leave no fossils, so the evidence is limited.
⚠ Common mistakes
Presenting one theory as proven. All three are still debated, and different viruses may have different origins.
Swapping drift and shift. Drift is slow accumulation; shift is a sudden combination.
Saying antigenic shift is just a lot of mutations. It involves two virus types combining their genetic material in one cell.
Claiming the vaccine “wears off”. The virus has changed, not the vaccine.
Forgetting to mention RNA as the reason for the high mutation rate.
Saying viruses do not evolve because they are not alive. Their genetic material mutates and is selected, so populations of viruses do evolve.
That completes Viruses. The three notes run in order: what a virus is made of, how it uses a host cell to copy itself, and how something that is not even alive manages to keep outrunning our immune systems.
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