permanent part of its host’s genome.
As a host cell manufactures new viruses, it sometimes accidentally adds some of its own genes to
them. The new viruses carry the genes of their hosts as they swim through the ocean, and they insert
them, along with their own, into the genomes of their new hosts. By one estimate, viruses transfer a
trillion trillion genes between host genomes in the ocean every year.
Sometimes these borrowed genes make the new host more successful at growing and reproducing.
The success of the host means success for the virus, too. While some species of viruses kill Vibrio,
others deliver genes for toxins that the bacteria use to trigger diarrhea during cholera infections. The
new infection of toxin-carrying viruses may be responsible for new cholera outbreaks.
Thanks to gene borrowing, viruses may also be directly responsible for a lot of the world’s oxygen.
An abundant species of ocean bacteria, called Synechococcus, carries out about a quarter of the
world’s photosynthesis. When scientists examine the DNA of Synechococcus samples, they often find
proteins from viruses carrying out their light harvesting. Scientists have even found free-floating
viruses with photosynthesis genes, searching for a new host to infect. By one rough calculation, 10
percent of all the photosynthesis on Earth is carried out with virus genes. Breathe ten times, and one
of those breaths comes to you courtesy of a virus.
This shuttling of genes has had a huge impact on the history of all life on Earth. It was in the oceans
that life got its start, after all. The oldest traces of life are fossils of marine microbes dating back
almost 3.5 billion years. It was in the oceans that multicellular organisms evolved; their oldest fossils
date back to about 2 billion years ago. In fact, our own ancestors did not crawl onto land until about
400 million years ago. Viruses don’t leave behind fossils in rocks, but they do leave marks on the
genomes of their hosts. Those marks suggest that viruses have been around for billions of years.
Scientists can determine the history of genes by comparing the genomes of species that split from a
common ancestor that lived long ago. Those comparisons can, for example, reveal genes that were
delivered to their current host by a virus that lived in the distant past. Scientists have found that all
living things have mosaics of genomes, with hundreds or thousands of genes imported by viruses. As
far down as scientists can reach on the tree of life, viruses have been shuttling genes. Darwin may
have envisioned the history of life as a tree. But the history of genes, at least among the ocean’s
microbes and their viruses, is more like a bustling trade network, its webs reaching back billions of
years.