Life › The microbial world › Bacteria & archaea: the microbial world
Horizontal gene transfer
throughout microbial history· Evolutionary innovation· Microbes & cells
Well establishedStrong fossil, genetic or geological evidence.
Microbes passing genes sideways to unrelated cells, not just down to their offspring.
See it on the Life map →What happened
Bacteria and archaea swap DNA three main ways: picking up loose DNA from dead cells (transformation), receiving genes carried by viruses (transduction), and passing small DNA loops called plasmids through direct contact (conjugation). A cell can gain a complete ready-made ability in one step, such as resisting an antibiotic or digesting a new food. It is so common that, near its base, the tree of life looks more like a web. Plants and animals do it far less.
Why it matters
It lets microbes adapt far faster than mutation alone, and explains how antibiotic resistance spreads between unrelated bacteria in hospitals and farms.
Think of it like
Like buying an established business unit instead of building one in-house: the capability arrives finished.
The evidence (fossils, genes, rocks)
Frederick Griffith's 1928 pneumonia experiment; Joshua Lederberg and Edward Tatum's 1946 bacterial mating; genome studies finding many genes whose ancestry does not match their host's.
Still debated
How much it erases the deep branching pattern of early life, and whether a single tree is even the right picture for microbes.
Connections (4)
▸Bacteria & archaea: the microbial world share genes through Horizontal gene transferWell established
The connection
Their DNA floats free in the cell, with no nucleus around it, and each cell reproduces by splitting, so a gene picked up today passes straight to its descendants. The result is a shared gene pool: two strains of E. coli can differ by more than a thousand genes.
When and how
Throughout microbial history; first shown by Frederick Griffith in 1928 and by Joshua Lederberg and Edward Tatum in 1946.
The evidence
Lab experiments show transfer directly; genome comparisons find many genes whose family tree does not match their host's, a sign they were borrowed.
Debates and open questions
How much transfer blurs the deep tree of life; some researchers think most microbial gene families have moved between lineages at least once.
▸Horizontal gene transfer spreads Antibiotic resistanceWell established
The connection
Resistance genes often ride on plasmids, small loops of DNA that bacteria pass to each other, even across species. One resistant microbe can hand protection to unrelated neighbors in a gut, hospital ward or farm. Japanese researchers found in the late 1950s that dysentery bacteria were swapping resistance to several drugs at once with gut E. coli this way.
When and how
Japan, late 1950s (Tomoichiro Akiba, Kunitaro Ochiai and colleagues); since then worldwide, as with the NDM-1 gene spreading from 2008 and the colistin-resistance gene mcr-1 found in China in 2015.
The evidence
DNA sequencing finds identical resistance genes on plasmids in unrelated bacteria, and lab experiments watch plasmids pass from cell to cell directly.
Debates and open questions
How much transfer happens in human guts versus livestock, soil and sewage, and whether drugs could be designed to block plasmid sharing.
▸Horizontal gene transfer tangles the roots of Common descent & the tree of lifeStrong evidence
The connection
A tree assumes genes pass only from parent to offspring. But bacteria and archaea also swap genes sideways, between unrelated lineages. Over billions of years this blurred the lines between early branches, so different genes can tell different family histories. Near its base, the tree of life looks more like a tangled web.
When and how
Recognized in the 1990s–2000s as whole microbial genomes were sequenced; W. Ford Doolittle's 1999 'net of life' diagram made the point famous.
How sure are we?
Genome comparisons find genes in a microbe that closely match those of distant species but not its relatives; antibiotic-resistance genes are seen moving between bacteria today.
Debates and open questions
Whether a single tree still makes sense for microbes, or whether a core of rarely swapped genes, such as ribosome genes, records true ancestry; most researchers favor the latter.
▸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.