Women today tend to have far fewer children- fewer than two on average in the United States, according to the CDC. Things get complicated, however, when those pregnancies don’t actually occur. So instead the mother’s immune system ramps up in other ways throughout adulthood, Wilson and her colleagues think, so as to remain vigilant against germs even when some of its parts become dormant during pregnancies. Turning down the immune system too much, though, risks leaving women sensitive to pathogens, which would also be bad for the fetus. This might even mean turning down the immune system in some ways, or for some periods of time. When the placenta grows during pregnancy, the organ sends signals to the mother’s immune system to change its activity so that the mother’s body doesn’t eject the placenta and the fetus. Though bearing so many babies might sound grueling, women’s bodies evolved to cope. In modern hunter-gatherer populations, Wilson told me, it’s not uncommon for women to have eight to 12 children each. Before the advent of birth control, that was pretty much the fate of the female sex. Here’s how the theory goes: Women-and all other placental mammals-evolved such that they would be pregnant for many of their adult years. In a paper published last week in the journal Trends in Genetics, Melissa Wilson, an evolutionary biologist, along with her colleagues from Arizona State University, put forward an explanation called the “pregnancy-compensation hypothesis.” It suggests that women’s immune systems are engaged in a fierce tug of war with placentas, even when the organs aren’t actually present. Some scientists now think the placenta itself might be the reason why women are so disproportionately affected. The singer Selena Gomez underwent a kidney transplant after suffering complications from lupus, which is eight times more common in women than in men. Sjogren’s forced Venus Williams to drop out of the U.S. Women are 16 times more likely than men to get Sjogren’s syndrome, in which the immune system goes after the glands that make tears and saliva, and nine times more likely to have Hashimoto’s thyroiditis, in which it sets its sights on the thyroid. In the United States alone, women represent 80 percent of all cases of autoimmune disease. And today, the placenta might hold the key to one of the most enduring mysteries in human medicine: Why do women suffer much higher rates of autoimmune disease than men do?Īutoimmune diseases turn people’s own immune systems against their bodies. The rodentlike thing would become the common ancestor of the world’s placental mammals, with descendants that include whales, bats, dogs, and humans, among many other species. And it had a placenta, an organ that grows deep into the maternal body in order to nourish the fetus during pregnancy. It looked, according to artistic renderings, like an especially aggressive New York City rat. This “ scampering animal,” as researchers described it, was likely small, ate bugs, and had a furry tail. About 65 million years ago, shortly after the time of the dinosaurs, a new critter popped up on the evolutionary scene.