If it seems a bit unfair that humans have periods when other animals don't, you should know that most of our closest cousins, the great apes and chimpanzees, also menstruate, just not as obviously as human beings do. What is surprising is that aside from primates, only two other species experience a menstrual cycle. The other two lucky animals are bats and a type of shrew. Now, despite the fact that primates, bats, and elephant shrews make up a pretty odd lot, it does seem strange that humans would develop menstrual cycles when virtually all other mammals are period-free.
Scientists have puzzled for years over why humans have menstrual cycles. Though several hypotheses have been put forward, there's no general consensus as to what evolutionary advantage or biological purpose this serves. Some biologists believe that shedding and rebuilding the uterine lining (having a period) may be more energy efficient to the body than constantly maintaining it as some other animals do. However, other mammals still can re-absorb their discarded uterine lining instead of shedding it as humans (bats, and shrews) do. So it's a mystery yet unexplained.
You may be wondering why some animals, such as dogs, seem to have periods if they're not actually menstruating. When a female dog goes into heat, her vulva swells and subsequently bleeds. She isn't shedding her uterine lining, she's simply bleeding (albeit from her genitals), so it's not a period.
So what happens to a mammal's egg if it isn't fertilized? Unlike humans, who expel their unfertilized eggs with the uterine lining, most animals simply re-absorb them. This leads us again to the frustrating question of why we have menstrual cycles when so many other animals get to avoid them.
One theory that's currently gaining some interest is that the uterine lining of primates (including humans) is thicker than nearly all other mammals, which makes re-absorption far more difficult and energy consuming. If this is the case, then we can blame the monthly visits from old Aunt Flo on the evolutionary advanced development of the human fetus. As primates became more intelligent and evolutionary superior in some ways, gestation time and childhood became much, much longer. And one of the ways this affected the evolution of the womb was with a thicker uterine lining.
So, as we got smarter and began to take up more room in the womb, the womb needed to accommodate. And thus (if this yet-unproven theory is correct), we developed menstruation.
As uncomfortable and messy as they can sometimes be, when you put periods into the perspective of millions of years of successful evolution, it seems less like a “curse” and more like an amazing testament to human development. We really are amazing creatures.
Well, us, the great apes, the elephant shrew, and bats at least. If nothing else, we're definitely part of an exclusive club.