Unlike most viruses, which infect, replicate, and then leave their host, retroviruses elbow their way into their host's genome where they are copied and passed on to daughter cells for the life of the host. This retrovirus, however, managed to sneak its way into one of our ancestor's sperm or egg cells, able to be passed on to every cell in every subsequent generation. Virus and host had become one.

The viral DNA used its own genes to copy itself, inserting those copies elsewhere in the hosts genome. These copies could be expressed in different parts of the body at different points in time, a symbiotic relationship that gave the shrew some extra raw materials with which to develop new functions.

''Viral proteins already have functions. Its much easier to borrow these than to evolve them from scratch,'' says Aris Katzourakis, an evolutionary biologist at the University of Oxford.

In that would-be mammal living 160 million years ago, a symbiotic retrovirus enabled it to evolve a placenta over many generations. In order to let a fetus mature inside a mother's uterus, an animal needed a way to provide oxygen and nutrients while removing waste and keeping both blood supplies separate.

Early mammals used the spare viral parts left in the junk drawers of the genome to use a viral gene to help create the placenta, and other symbiotic viruses help turn us from a ball of cells into a fully-formed squalling infant and protect us from pathogens.

Scientists are discovering that the so-called ''junk DNA'' - a significant portion of which is from symbiotic viruses - is actually a potent force in the evolution of new species. Although the evolution of pregnancy via the placenta might be some of the most persuasive evidence that viruses stashed deep within the genome can help give rise to new species, it's not the only proof. New studies revealing the role of endogenous retroviruses in the more recent evolution of humans show that these snippets of DNA are helping to blur the boundary between human and virus. Humans are, in a very real sense, part virus.

''The boundaries between organisms are a bit more merged now, a bit more shadowy. We need to break down those boundaries,'' says University of Queensland virologist Paul Young. ''The more we look, the more we find overlap,''

Viruses can infect all organisms, from the largest blue whales to the tiniest bacteria. To a host, infections can range from unnoticed to deadly. To the virus, however, infection is an opportunity to unleash its incredible genome copying abilities. Within hours, it can make millions or even billions of copies of itself.

Retroviruses, however, use a slower, stealthier approach. After entering the cell, the retrovirus uses an enzyme called reverse transcriptase to turn its RNA into DNA before making its way to the nucleus. Once in the nucleus, it inserts its DNA into the host's genome.

Most of the time, when a virus integrates its genome with the host's, the new hybrid genome dies when the cell and its descendants do. Sometimes, however, a virus will infect a sperm or egg cell. If fertilization occurs, the offspring will have a copy of the viral genome in every single one of its cells. It can pass the hybrid genome on to its offspring, creating what scientists call a fully endogenous retrovirus - a fancy term for a virus that comes from within.

The process requires an astonishingly rare set of circumstances be met, Katzourakis says. ''Although endogenous retroviruses make up a pretty large proportion of our genome, in terms of the number of times they've infiltrated our genome over the past sixty or so million years, it only comes down to about 30 or 40 distinct occasions,'' he says.

In humans, even the most recent of these infiltrations happened tens of thousands of years ago. In domestic sheep and koalas, however, retroviruses are currently establishing themselves, which gives researchers the opportunity to watch the process in action. In the 1980s, Queensland veterinarian Jon Hanger noticed something odd in a breeding colony of about 70-80 koalas..

Every year, the colony lost around 10 percent of its members to immunosuppression or cancer, especially leukemias, a cancer of white blood cells.

About 60 to 70 percent of the deaths were from cancer. ''That's an unusually high number of cancer deaths in any animal, including ours,'' Young says.

Although Hanger was working full-time as a veterinarian, he couldnt shake his curiosity. The mystery drove him back to school to get a PhD on the topic. By 2000, Hanger and Young had identified the full-length retroviral genome that was causing disease in koalas. The virus, however, wasnt actively transmitted from koala to koala. Instead, a retrovirus had embedded itself in the koalas' germline and passed from parent to child.

In other words, the virus that was causing disease was located in the koala genome itself.

''This was the first time anyone had seen this happening in real time. All previous endogenous retroviruses had embedded themselves in host genomes many, many millions of years ago,'' Young says.

At the breeding colony that Hanger initially studied in Queensland, he and Young found the retrovirus in the genome of every single koala they tested. As they moved northward, toward the city of Cairns, they found a similar picture: every koala carried the retrovirus. Moving south, however, the number of infected koalas dropped. On Kangaroo Island, off the southern coast of Australia near the center of the continent, only a handful of koalas were infected.

Examination of koala pelts from museums showed that the retrovirus has been in koala DNA for at least 200 years, according to a 2012 study in Molecular Biology and Evolution, although they think it was present for a few thousand years - the blink of an eye in evolutionary terms.

''It's amazing that it would have spread through the germline so quickly,'' Young says.

Because the introduction of the retrovirus was still so new, the virus hasn't accumulated many mutations, and the koala's genetic machinery still actively turns the viral DNA into active virus. That healthy animals also have this virus continues to stump researchers - what's making the other koalas sick? Nor can they explain how the retrovirus spread throughout the koala population so quickly, especially when it seems to create deadly problems for a significant number of the animals.