The Evolution of the Human Placenta

8. Modern Gestational Challenges

8 Modern Gestational Challenges Modern human beings are an evolutionarily successful species. We have spread across the earth, and our population continues to increase at an impressive rate. It would seem that our reproductive adaptations are quite effective. Yet the processes of conception, implantation, gestation, and finally parturition in humans is not without its dangers and setbacks for both mother and fetus . Childbirth is still a leading risk factor for mortality and morbidity among women of childbearing age. Maternal mortality has recently slightly increased in the United States, to  deaths per , live births. In much of subSaharan Africa, the maternal mortality rate is in excess of  per , live births (Hogan et al., ). A number of authors have commented that human gestation is fraught with several possible diseases and poor outcomes for both the mother and fetus that appear to be, if not unique to human beings, at least far more common in human pregnancy than in the gestation of other mammals. These conditions include: high rates of early pregnancy loss, nausea and vomiting of pregnancy, preeclampsia and related maternal hypertension syndromes, and preterm birth. All of these conditions involve the placenta to a greater or lesser extent. For example, the placenta is intimately involved in the etiology of preeclampsia , which is essentially a placental disease. Assuredly, many of these pathological events are related to placental development and function, and placental pathologies are associated with many poor birth outcomes. What evolutionary advantage might such a seemingly inefficient and pathology-ridden reproductive system have conveyed to our ancestors? Or are many of these reproductive problems incidental outcomes of adaptive changes to human reproduction that persisted despite the increased risks of ancillary disadvantages? The proximate mechanisms that underlie these placenta-related diseases and conditions are not well known. There has been much recent speculation and theorizing on possible ultimate explanations for these states. An important question to consider is to what extent these diseases are truly pathological and to what extent Modern Gestational Challenges  they reflect normal adaptive function that is being inappropriately, or possibly excessively, expressed. In this chapter we examine human reproduction from an evolutionary perspective, focusing on pathology, especially as related to placental development and function; we also attempt to discern adaptive functions that may predispose us to vulnerability to pathology. For example, a vulnerability to preeclampsia would appear unlikely to have had adaptive function per se, but the adaptive changes necessary to support a deeply invasive placenta may have favored a developmental pattern that included an increased possibility for preeclampsia as a maladaptive consequence, outweighed by the selective advantages. Fertility, Efficiency, and Child Rearing Human female fertility is remarkably low, at least when calculated by the number of copulations often needed to produce a viable pregnancy. The mean time to pregnancy for sexually active couples not using contraception is more than six months. The number of ovulations required before pregnancy occurs appears to be intrinsically higher in women compared to our nonhuman primate kin. Estimates of the chances of a clinically recognized pregnancy occurring in any given cycle (assuming no contraception and a fertile male partner) are about % (Macklon et al., ). Thus, human reproduction appears to be inefficient. Of course, no one is suggesting that human reproduction overall is compromised by these conditions; human beings are the most fecund of hominoids, the phylogenetic unit that includes us and our nearest relatives, the great apes. We have populated the entire planet. However, our high level of reproductive failure begs for an evolutionary examination. First of all, evolution does not always favor efficiency. Efficiency is a human construct; in many cultures (but not all, and not over all time) efficiency has been viewed as a desirable quality of human endeavors. In evolution, reproductive fitness is the currency of most import. Often, the more efficient reproductive strategy will provide the greatest fitness, but not always. Being efficient at getting pregnant is not necessarily adaptive. Evolution rewards the genetic lineages with the greatest reproductive fitness, in other words, those that produce the most members of future generations. Pregnancy is certainly a necessary step in that process, but the goal of reproduction is not  The Evolution of the Human Placenta pregnancy. Rather, the goal is to produce offspring that survive to become sexually mature adults, capable of having young of their own. For primates, that is a long, involved process with many steps that the offspring must successfully accomplish. It does certainly...