Within approximately 10 million years of the dinosaurian demise, mammals diversified into all of the existing mammalian orders—rodents, bats, carnivores, primates, etc. Some took to the air while others returned to the water—each group evolving and passing on its own suite of adaptations, like wings or fins, to supplement basal mammalian characteristics like hair and bigger brains. Many of these species went extinct themselves. Others thrived, eventually outcompeting many of the non-mammalian vertebrates that had also survived the great die-off. And except in isolated regions like Australia and South America (which were effectively isolated from the expansion of the terrestrial placentals), the eutherians even outcompeted the older, non-placental mammals—the marsupials and the egg-laying monotremes.37
The organ that gives placental mammals their name is transient in nature, undergoing its entire rapid development only after conception. The tissue is derived from the fetus, as opposed to the mother, and in humans it has an average diameter of about nine inches. Thickest at its center (up to an inch), it thins out toward the edges and weighs in at just over a pound. The placenta functions as an interface between the mother and the developing fetus, connecting it to the mother’s uterine wall but acting as a buffer as well. The organ itself is richly vascularized, which gives it its dark reddish-blue to crimson color, which relates to the placenta’s life-support function: carrying oxygen and nutrients from the mother to the placenta and then from the placenta to the fetus via the umbilical artery. Structurally, most of the placenta is composed of cells called trophoblasts, which have a dual role. Some form small cavities that fill with maternal blood, thus facilitating the exchange of nutrients, waste, and gases between the fetal and maternal systems. Other trophoblasts specialize in hormone production. Waste products and carbon dioxide travel from the fetus back to the placenta via the umbilical vein. A sheath of connective tissue binds and protects both umbilical vessels, and together the entire structure is known as the umbilical cord.38
The placenta has additional functions, which include the production and release of several hormones, including estrogen (which maintains the uterine lining during pregnancy) and progesterone (which stimulates uterine growth as well as the growth and development of the mammary glands). It also prevents the transfer of some, but not all, harmful substances—bloodborne pathogens for one—from the mother to the developing fetus. Finally, the placenta secretes several substances that effectively cloak the developing fetus from the mother’s immune system—similar to the way in which immunosuppressant drugs prevent the body from rejecting a transplant.
Given its essential role in fetal development, what the human placenta experiences after delivery must surely be the most precipitous fall from grace in all of Organdom. Expelled by the uterine contractions associated with childbirth, this complex and amazing structure goes from revered mammalian namesake to biohazardous “afterbirth” faster than you can scream “PUSH!”
In the vast majority of mammals, though, the newly delivered placenta serves one last purpose.
In 1930, primatologist Otto Tinklepaugh took a break from his groundbreaking study on chimpanzee vaginal plugs to coauthor an article on the birth process in captive rhesus macaques (Macaca mulatta). He noted that the monkeys, and just about every other terrestrial mammal except humans and camelids (camels, llamas, alpacas, and their relatives) consumed their own placentas after giving birth. More recently, the behavior in the animal kingdom has been studied in rodents, lagomorphs (rabbits and their kin), carnivores, primates, and most artiodactyls (hoofed mammals).
Mark Kristal is the world’s foremost authority on placentophagology, and until recently, he may have been the only expert on the topic. A SUNY Buffalo professor emeritus of psychology, Kristal’s research began more than four decades ago. His work supports the hypothesis that, since placenta-eating has been observed in such a variety of mammals, it probably evolved independently and in response to one or several survival-related problems.
Researchers initially posited that eating the placenta kept the birthing area sanitary while eliminating smells that might attract predators. The fact that chimps giving birth in the trees hung around to eat their placentas instead of simply moving off (or flinging them down on some cheetahs), suggested that a new hypothesis was needed. Answering the call, dietary researchers suggested that placentophagy replenished nutritional losses associated with late-stage pregnancy and delivery. Endocrinologists hypothesized that moms might be acquiring (and replenishing) hormones present in the afterbirth. Other researchers suggested that placentophagy sated a mother’s hunger after the delivery, or that placentophagy demonstrated the new mothers’ tendency to develop “voracious carnivorousness” after giving birth.39