“Are you sure they’re not twins?” The question came from a woman I’d never spoken to before, both of us standing on our pedals in the middle of class in a dark spin studio that smelled like clove deodorant and someone else’s protein shake. I had been pregnant with my second daughter for six months and my shirt had ridden up on a stomach that had stopped checking my due date and was starting to make its own decisions. I laughed, especially because crying into my water bottle seemed like the worst option, and I told him no, I was just a very ambitious baby. She didn’t seem convinced and, honestly, she wasn’t wrong to be surprised. He just got the wrong culprit. What was really happening there had nothing to do with twins but with a virus. Approximately eight percent of the human genome is made up of traces of retroviruses. Richard A. Stein, a scientist at New York University, put the figure clearly in a 2023 review of research: “Approximately 8% of the human genome […] It comprises sequences of viral origin that are known as human endogenous retroviral elements (HERVs).” Those sequences entered through the oldest trick a retrovirus has. It infects a cell, inserts its own genetic code into the host’s DNA, and if that cell turns out to be a sperm or egg, the virus’s instructions are copied into each subsequent generation. No longer like an infection. As inheritance, it is passed on in the same way as eye color. Most of that viral code is simply inherited. It sits there, mutated and no longer useful, taking up space doing nothing. But every once in a while, evolution took a stolen gene and put it back to work, and the placenta is the clearest case of this happening. Somewhere in the deep history of mammals, an ancestor contracted a retrovirus whose outer shell protein was built for one function: to fuse with a cell wall so that the virus could enter. and their parents’ bloodstreams sit close enough to exchange oxygen and nutrients without mixing. The gene is called syncytin, and molecular biologist John McCoy, who worked on some of the early syncytin research at the Institute of Genetics, described the discovery to NOVA in terms that still sound like they belong in a laboratory thriller more than a pregnancy: “This was a genuine retroviral envelope protein that had somehow been captured during evolution and had been trained to operate in human biology.” The don’t mix part matters more than it seems. A fetus is, immunologically speaking, half foreign tissue, carrying genetic material from two people instead of one. If left unchecked, the parent’s immune system would treat that the same way it treats anything else it doesn’t recognize. The same NOVA report on syncytin describes the layer it builds as a barrier that prevents maternal and fetal blood from completely mixing, since mixing them carries the risk of an immune reaction. fatal. A protein whose original purpose was to help a virus overcome a cell’s defenses ended up, generations later, helping a parent’s body tolerate the one intruder it’s supposed to love. Say “all mammals” out loud and a biologist will want to add an asterisk. Marsupials build a much simpler placenta, and egg-laying mammals, like the platypus, skip the structure entirely. everything that most people imagine when they imagine a mammal, the pattern becomes stranger the closer you look. Different lineages captured and reused their own separate viral genes at different points in evolutionary history. Rabbits ended up with one, primates with another, rodents with a third, each of them extracted from an unrelated viral infection and each of them landing in roughly the same solution. version for rabbits in the journal Retrovirology, called it evidence that “retroviral infections have resulted in the independent capture of genes that have been positively selected for convergent physiological function.” The weight gain that made a stranger say “twins” was real, as was the moment. self-conscious about a body that, in a matter of months, had stopped looking like the one I recognized. Knowing that didn’t make it any less humiliating to see my own reflection mid-pedal and not recognize the shape staring back at me. I’m not a biologist, and here I don’t pretend otherwise. A less generous version of this story might. be interpreted as disturbing: a virus silently rewrites what the body is for. I don’t experience it that way. Every family I know, on every continent I’ve lived on, is based on inheritances that no one asked for permission first: a grandmother’s temper, a father’s stubbornness, a language learned before anyone could consent to learn it. A viral gene stolen from tens of millions of years ago turns out to be the inheritance furthest back in line. My daughter arrived in July, with syncytin and. everything, the same way all his ancestors did, going further back than anyone bothered to name. He has no idea that his existence passes by a virus. Honestly, neither did I until I went to look for him. It doesn’t speed up the recovery or shorten the newborn’s nights. It just means that somewhere underneath all that, something old and uninvited is still silently doing the only useful thing it ever learned to do. We use artificial intelligence tools in the newsroom. See our editorial standards and masthead.
