About Periods
Menstruation is a biological process that occurs primarily in humans, some primates, and a few other mammals, and it involves the shedding of the uterine lining (endometrium) when pregnancy does not occur. Most mammals, however, do not menstruate in the same way, raising the question of why this process exists in certain species and not in others.
One key reason for Periods in humans and some other species lies in the reproductive strategy. The uterus prepares for potential pregnancy by thickening its lining, creating a nutrient-rich environment for a fertilized egg to implant and develop. In most mammals, if fertilization doesn’t occur, the endometrial lining is reabsorbed by the body, conserving energy and resources. This is known as an “estrous cycle,” which differs from the menstrual cycle.
In humans, Periods is a consequence of the invasive nature of human embryos. Human embryos, compared to those of many other mammals, have a very aggressive implantation process. They deeply embed themselves into the uterine lining to establish a strong connection with the mother’s blood supply. As a result, the uterine lining has to be especially robust and prepared for this invasive implantation. When pregnancy doesn’t occur, the thickened lining becomes unnecessary and is shed through Periods
Another theory is the “maternal-fetal conflict” hypothesis, which suggests that Periods evolved as a way for the mother’s body to safeguard itself against potentially harmful embryos. By developing a thick uterine lining and shedding it if no viable embryo implants, the body can eliminate defective or low-quality embryos, ensuring that only healthy pregnancies proceed.
Menstruation evolution has led to greater body proportions and more sophisticated reproductive systems in primates and bats. They can simply reabsorb it, which is more efficient from an energy conservation standpoint. Menstruation evolution has also been related to more complicated reproductive systems in species such as primates and bats.
