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is that marine viruses have so many hosts to infect. Each lineage of viruses has to evolve new adaptations to get past its host’s defenses. But diversity can also evolve by more peaceful means. Temperate phages merge seamlessly into their host’s DNA; when the host reproduces, it copies the virus’s DNA along with its own. As long as a temperate phage’s DNA remains intact, it can still break free from its host during times of stress. But over enough generations, a temperate phage will pick up mutations that hobble it, so that it can no longer escape. It becomes a 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.

Our Inner Parasites

Endogenous Retroviruses

The idea that a host’s genes could have come from viruses is almost philosophical in its weirdness. We like to think of genomes as our ultimate identity. We know who our biological parents are because they gave us our DNA. In our DNA are not just the instructions for the color of our skin or our susceptibility to diabetes. Our very nature lurks there. That’s why the idea of cloning is so abhorrent: no one should have to carry secondhand genes.

But if most of an organism’s genes arrived in its genome in a virus, does it even have a distinct identity of its own? Or is it just a mishmash of genes, cobbled together by evolution? It’s as if the world was filled with hybrid monsters, with