This is the placenta. And without it, you would never have been born. This amazing organ is a lifeline connecting mother and baby, as well as a defensive barrier between them. The placenta provides the baby with oxygen and nutrients, carries away waste, filters out harmful microbes, and pumps out hormones– all while keeping maternal and fetal blood supplies completely separate. But we might never have evolved a placenta like this if it wasn’t thanks to an infection… with a virus. Viruses are excellent at making more copies of themselves. Some viruses do this by inserting their own genes into the host’s DNA. This tricks the infected host into doing the virus’ dirty work: producing viral proteins to build more viruses to infect more cells. Very rarely, one of these viruses will infect a sperm or an egg cell. And as the the new organism grows, these viral genes are also copied into every single one of its cells, ready to be passed down to future generations. In other words, changing the whole genome of the host forever. In fact, if we looked into your entire genome, around 8% of it originally came from viruses. So, you could say that we’re all part virus. Most of these viral genes no longer work. They’re like dead viral fossils in our DNA. However, some of them have been resurrected, including one gene that’s fundamental in the formation of the placenta. The human placenta is a blend of maternal and fetal tissue. And the way it forms is incredibly invasive. After the embryo implants into the womb, finger-like projections burrow into the maternal tissue and alter its blood vessels so that they become bathed in a constant supply of the mother’s blood. This interface, which has a surface area of twelve square meters, is what allows mother and fetus to exchange nutrients and waste. And yet, such close contact means that the mother’s immune system could attack the developing embryo, which it sees as a foreign invader. And in a way, it kind of is. As a first line of defense, the fetal cells along this boundary fuse together, using a protein called syncytin. This removes any gaps where maternal white blood cells could squeeze through and launch an immune attack. It’s a clever strategy, but we didn’t come up with it entirely on our own. That’s because syncytin was originally a viral protein. That virus used syncytin to fuse with cells so it could infect them. And sometime in the past, during one of those ancient viral infections, a virus inserted the gene for syncytin into the genome of its host. Once it was inherited, that gene may have lain dormant for generations. But eventually it was repurposed by evolution to fuse cells together in the placenta. The interesting thing is that in some ways, a developing fetus is a little bit like a virus, in that it exists inside the body of another organism where it tries to avoid detection by the immune system. So perhaps it’s fitting that syncytin, which helps the placenta to invade the womb, originally helped a virus to invade its host.