Life › Origin of life › LUCA, the last universal common ancestor
Viruses: on the edge of life
origin unknown; probably as old as cells (debated)· Organism or group· Microbes & cells
DebatedScientists disagree about when, how or why.
Tiny packages of genes in a protein shell that can copy themselves only inside living cells.
See it on the Life map →What happened
A virus carries DNA or RNA but none of the machinery to use it; it hijacks a cell to make copies. Many biologists therefore place viruses at the edge of life rather than inside it. They are the most numerous biological objects on Earth, an estimated 10³¹ particles, mostly infecting bacteria. Their origin is unknown: genes that escaped from cells, cells that shrank into parasites, or relics of a world before cells.
Why it matters
Viruses drive microbial evolution, move genes between species and kill vast numbers of ocean microbes daily; ancient viral DNA makes up about 8% of the human genome.
The evidence (fossils, genes, rocks)
Discovered through tobacco mosaic disease by Dmitri Ivanovsky (1892) and Martinus Beijerinck (1898); first seen by electron microscope in 1939. Viruses leave no fossils, so deep history comes from comparing genes and protein shapes.
Still debated
Are viruses alive? Did they arise before, alongside or after the first cells? Giant viruses, found from 2003, blurred the line further.
Connections (4)
▸CRISPR gene editing repurposes microbes' defences against Viruses: on the edge of lifeWell established
The connection
Bacteria and archaea store snippets of past viral invaders in their DNA, in repeated stretches called CRISPR. When the same virus returns, a matching RNA guides a cutting protein, such as Cas9, to slice it. Scientists turned that guided cutter into a tool that can edit genes in almost any organism.
When and how
Repeats noticed by Yoshizumi Ishino in 1987; Francisco Mojica linked them to viruses in 2005; Jennifer Doudna and Emmanuelle Charpentier showed the editing tool in 2012.
The evidence
Experiments on yogurt bacteria at the food company Danisco (2007) showed CRISPR snippets protect against viruses; the 2012 tool earned a 2020 Nobel Prize.
Debates and open questions
How ancient CRISPR defences are; a 2024 study suggests LUCA may already have had a CRISPR-like system.
▸Viruses: on the edge of life constantly infect and reshape Bacteria & archaea: the microbial worldWell established
The connection
Viruses that infect microbes outnumber cells perhaps tenfold. In the oceans they kill an estimated fifth of microbial life each day, releasing nutrients and shaping which species thrive. They also carry genes between hosts, and microbes evolved defences such as CRISPR in response, an arms race running for billions of years.
When and how
Bacteria-infecting viruses (phages) were discovered by Frederick Twort in 1915 and Félix d'Hérelle in 1917; their huge ocean role became clear from the late 1980s.
The evidence
Seawater counts of about 10 million virus particles per milliliter; microbial genomes full of viral DNA and CRISPR records of past infections.
Debates and open questions
How far back the arms race goes; viruses leave no fossils, so their deep history must be pieced together from genes.
▸Viruses: on the edge of life are a major route of Horizontal gene transferStrong evidence
The connection
When a virus packs new copies of itself inside a host cell, it sometimes grabs pieces of the host's genes by mistake and delivers them to the next cell it infects. Viruses can also insert their own genes permanently. That makes them major couriers of genes between microbes, and even into animals like us.
When and how
Virus-carried gene transfer (transduction) was discovered by Norton Zinder and Joshua Lederberg in 1952; about 8% of the human genome comes from ancient retroviruses.
How sure are we?
Lab experiments show transduction directly; genomes of microbes, plants and animals hold viral genes, including syncytin, a borrowed viral gene essential to the human placenta.
Debates and open questions
How much of microbial evolution was steered by virus-carried genes, and whether viruses helped create key cell machinery early on.
▸Rosalind Franklin later studied the structure of Viruses: on the edge of lifeWell established
The connection
After her DNA work, Franklin moved to Birkbeck College in London and used X-ray diffraction to study viruses. She showed that tobacco mosaic virus is a hollow tube of protein with its RNA wound in a spiral, embedded in the protein, and began work on the polio virus.
When and how
Birkbeck College, London, 1953–1958, with Aaron Klug and others; she died of cancer in 1958, aged 37.
The evidence
Her published papers; Aaron Klug continued the work and won the 1982 Nobel Prize in Chemistry, building partly on it.
Debates and open questions
Little dispute about the link; the open question is what more she would have achieved, and whether she would have shared later Nobel Prizes.