The menstrual cycle is a rare phenomenon in nature. In fact, it turns out that human beings are just one of a handful of species on Earth in which females experience it. So why is that? Why did the menstrual cycle evolve in the first place?

The TED-Ed video below explores what we know about the origins of the menstrual cycle—and why human females experience menstruation (i.e., have periods) more often than any other species. The short version is that it’s thought to be evolutionarily adaptive—it’s a protective process designed to clear the body of unfertilized eggs and unhealthy embryos. These would pose a serious risk to women if the uterine lining did not shed on a regular basis because, if they could continue to develop unabated, they would draw resources from her body at the expense of her health. In other words, you can think of menstruation as a “screening process” for embryos that is ultimately to the benefit of women’s health.

To learn more check out the full video below.