A Planet of Viruses by Carl Zimmer Read Free Book Online Page A

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 clear lines of identity blurred away.
     

    Microbiologists have been getting used to the viral roots of the microbes they study for decades now. And as long as microbes were the only organisms with much evidence of virus-imported genes, we could try to ignore this philosophical weirdness by thinking of it merely as a fluke of “lower” life forms. But now we can no longer find comfort this way. If we look inside our own genome, we now see viruses. Thousands of them.
     
    We have the jackalope to thank for this realization. The myth of the jackalope was one of the clues that led virologists to discover that some viruses cause cancer. In the 1960s, one of the most intensely studied cancer-causing viruses was avian leukosis virus. At the time, the virus was sweeping across chicken farms and threatening the entire poultry industry. Scientists found that avian leukosis virus belonged to a group of species known as retroviruses. Retroviruses insert their genetic material into their host cell’s DNA. When the host cell divides, it copies the virus’s DNA along with its own. Under the certain conditions, the cell is forced to produce new viruses—complete with genes and a protein shell—which can then escape to infect a new cell. Retroviruses sometimes trigger cells to turn cancerous if their genetic material is accidentally inserted in the wrong place in their host’s genome. Retroviruses have genetic “on switches” that prompt their host cell to make proteins out of nearby genes. Sometimes their switches turn on host genes that ought to be kept shut off, and cancer can result.
     
    Avian leukosis virus proved to be a very strange retrovirus. At the time, scientists tested for the presence of the virus by screening chicken blood for one of the virus’s proteins. Sometimes they would find the avian leukosis virus protein in the blood of chickens that were perfectly healthy and never developed cancer. Stranger still, healthy hens carrying the protein could produce chicks that were also healthy and also carried the protein.
     
    Robin