Past Pandemics Are in Our Genes

What koalas can teach us about human evolution and disease.

A female koala cuddles her joey.
When koalas infected with the koala retrovirus reproduce, they pass the viral DNA along with their own to their offspring.Photo by Torsten Blackwood/AFP/Getty Images.

To understand what it means to be human, you have to understand koalas. Or, to be more precise, you have to understand how they are dying from a bizarre viral outbreak that has been raging for the past 150 years or so. The koalas are now going through something our ancestors experienced 31 times over the past 60 million years. And those ancient viral outbreaks have helped to make us who we are today.

Australian biologists discovered the koala outbreak in 1988. They were examining the blood of a koala dying of leukemia when they came across a virus infecting its white blood cells. The koala retrovirus, as it is now known, made its hosts sick in much the same way the feline leukemia virus sickens cats. It inserted its genes into host immune cells, which then produced new viruses. The infection also caused the cells to replicate at a frenzied rate. Once the scientists had found the koala retrovirus in one koala, they looked for it in others, and they soon found it all over the koala’s range—the entire eastern side of Australia.

Koalas had long been known to have terrible health. One survey estimated that leukemia and lymphoma were responsible for up to 80 percent of koala deaths. The discovery of the koala retrovirus made sense of this cancer epidemic. It also explained why koalas were getting devastated by chlamydia, a sexually transmitted bacterium. By turning koalas’ immune systems cancerous, the virus was leaving the koalas open to infection by other pathogens.

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Scientists who went rummaging through koala skins in museums found genes from the retrovirus as far back as the mid-1800s, but that still makes it a fairly young outbreak. To figure out where the koala retrovirus came from, scientists have compared its genes to those of other species of viruses. One of the most similar viruses to it infects the grassland mosaic-tailed rat of Australia. The virus may well have been carried from the rats to the koalas by a mosquito or a tick.

Once the koalas picked up the retrovirus, they began to spread it among themselves. After young koalas are weaned but before they start dining on eucalyptus leaves, they feed on their mother’s feces—feces that may be laden with koala retroviruses. The epidemic appears to have started in northern Australia, where today the virus is ubiquitous. Since then, it has spread southward, even jumping to islands off the coast of Australia—presumably in the guts of mosquitos.

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