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An Evolutionary Perspective On Human Reproduction

Posted on: Tuesday, 30 March 2004, 06:00 CST

AN EVOLUTIONARY PERSPECTIVE ON HUMAN REPRODUCTION On Fertile Ground: A Natural History of Human Reproduction. By Peter T. Ellison. Cambridge, MA: Harvard University Press, 2001, 358 pages. Paper, $ 19.95.

This is a very good book that will be particularly useful to those with general or scientific interest in the areas of human evolution or human reproduction. There are several factors that contribute to the book's quality.

First, the book is written by an author who has scholarly mastery of the area. Ellison draws from a variety of disciplines including biology, evolutionary theory, ecology, anthropology, physiology, endocrinology, and the history of science, which he integrates and brings to bear upon the basic argument of the book: Human reproductive physiology looks like it does because it was shaped by evolution under a particular set of ecological conditions. second, Ellison expresses enthusiasm in making the argument that unfolds across the chapters and in presenting the evidence which supports this argument. This enthusiasm stimulates the interest of the reader. The book is written in an engaging style, and the diverse material is made accessible to a broad range of readers. Unavoidably, the detail of some of the topics may not be of interest to all readers.

Third, Ellison does an excellent job of demonstrating the complexity of some of the scientific questions involved and the nature of problem solving and critical thinking in science. In this regard, the book would be a wonderful teaching tool for graduate students and advanced undergraduates. Ellison shows how hypotheses are formulated based on past knowledge and then tested, debated, and reconsidered in light of alternative hypotheses.

The book consists of nine chapters. In chapter 1, Ellison sets the stage for the evolutionary examination of human reproductive physiology. he contrasts the birth stories of two women: an African woman in the Ituri Forest of the Congo basin and his own wife in Boston. From the starting point of these two different stories he describes the universal aspects of human reproductive physiology and its speculated evolutionary origins.

Chapters 2 through 8 each treat a major aspect of human reproduction. Ellison sets up the chapters such that each has within it several scientific questions that have either been unanswered or answered unsatisfactorily. he skillfully uses critical, scientific analysis presented within an evolutionary framework to lead the reader through the questions to the likely answers.

Chapter 2 examines conception and implantation of the embryo in the mother's uterus. Two major questions here are "Why do human females tend to have single rather than multiple pregnancies?" and "Why do human females menstruate?" The chapter describes the complex steps and precise timing involved in successful implantation. A very large percentage of embryos fail to implant, which is interpreted as selection against embryos with some genetic flaw. The question then raised is why human females do not release numerous eggs at once so that viable embryos may be immediately available to replace defective ones. According to Ellison, human reproductive physiology has been shaped by two competing risks: the risk associated with too much investment in genetically defective embryos and the risk associated with multiple births. The evolutionary solution has been a tendency for single births and mechanisms which filter out defective embryos as early as possible, thus minimizing lost biological investment.

Ellison presents several competing theories on why human females menstruate when no implantation has occurred after ovulation. One theory holds that the shedding of the outer layer of the endometrium occurs to cleanse the female reproductive tract of harmful bacteria which may have entered with sperm. Another holds that it is metabolically cheaper for the female to shed the old layer and regenerate it during the next cycle than to reabsorb it or maintain it in a state of readiness. A third is that successful implantation of the embryo requires the endometrium to be in a particular form that is present only at the beginning of the cycle. Thus, when implantation does not occur, growth of a new endometrium is necessary, and the old endometrium is too bulky to be reabsorbed efficiently.

Chapter 3 covers the topic of birth. Ellison presents a fascinating description of the social nature of birth in humans and evidence of an evolved mechanism to forestall delivery if social support is not available. Two questions addressed in this chapter arc "Why is birth so difficult in humans?" and "What triggers birth in humans?" According to Ellison, childbirth is difficult in humans because of competing selection pressures. During the course of evolution human ancestors became bipedal, requiring a shift in the angle of the pelvis. The brain also became larger, requiring the infant to be born more underdeveloped so the head would pass through the birth canal, albeit just barely. An evolutionary change in the size of the pelvis in human females resulting in easier childbirth would have come at the price of impaired bipedal locomotion, which would have adversely impacted female survival. In the long run the costs of impaired locomotion outweighed the benefits of easier births.

Ellison argues that, unlike other animals, there is no simple "trigger" for birth in humans. Rather, he states that a more complex process may be involved. An increasing fetal demand for energy eventually outstrips the mother's ability to supply it. Gestation ends when the fetus begins to starve. The fetus may then set in motion the process of parturition which allows it to move into another environment in which the mother can supply a highly effective alternative to placental energy to meet its nutritional needs: milk.

Chapter 4 examines the topic of breast-feeding. An important question addressed in this chapter is "What is the relationship between breast-feeding and suppression of pregnancy?" This is a fascinating chapter in terms of scientific problem solving. Although there was much historical anecdotal evidence that breast-feeding suppressed pregnancy, the relationship lacked solid empirical evidence. Ellison traces the history of the question, beginning with conceptual assumptions laid down by Thomas Malthus. he shows how these assumptions precluded early scientific investigation in certain directions that later proved to be fruitful. he presents a number of studies that ultimately did identify a relationship. Once the relationship is empirically established, Ellison takes the reader to the next level: Which aspect of nursing suppresses pregnancy? Ellison's evolutionary explanation is that breastfeeding reduces the mother's metabolic energy, and it was metabolically untenable for early human females to gestate and nourish another child in this state. Thus, breastfeeding extends the duration of amenorrhea. Once the child becomes metabolically independent of the mother, the mother's metabolic energy is freed for the potential conception of another infant.

In chapter 5 the topic is physical growth. A focal question is "What initiates menarche in girls?" The most widely recognized and accepted explanation is that a certain proportion of fat in the body initiates menarche. However, this appears to be an excellent example of an unsubstantiated scientific myth. According to Ellison, neither data from the original study nor data from other studies support this explanation. Ellison presents a compelling alternative hypothesis that pelvic size is probably the critical factor initiating menarche. Ellison argues further that this makes sense from an evolutionary perspective since pelvic size would be the best predictor of capacity for successful childbirth.

Chapter 6 treats the topic of ovarian function in women. The question here is "What factors have influenced ovarian function in human females?" Ellison argues that there are two constraints on human female reproduction: time and energy. Thus, the availability of resources, energy intake, time spent working, and energy expenditure in the evolutionary environment shaped common aspects of ovarian function. Ovarian function continues to respond to these factors, which contributes to variability across populations. Ellison reports that the human tendency to store fat suggests we evolved in a boom or bust environment where resources and the energy expenditure necessary to gather them varied greatly. Similarly, contemporary women in different cultures show variability in ovarian function and fertility resulting from changes in the balance of energy intake and expenditure.

Chapter 7 examines female fertility across the lifespan and menopause. A central question in the chapter involves the evolutionary meaning of menopause in human females in view of the fact that females of other species can reproduce through old age. Ellison examines several hypotheses for menopause in human females. The most well-known of these is the grandmother hypothesis: Menopause evolved in human females because, ultimately, older females increased their fitness more effectively through care of grandchildren than through their own direct reproduction. Ellison debunks this explanation as the cause ofmenopause, and he turns the argument on its head. he presents material supporting his contention that grandmaternal investment did not select for menopause. Rather, menopause is a biologically predictable phenomenon in humans, and a long postreproductive life in human female ancestors may have selected for grandmaternal investment.

In chapter 8, Ellison presents reproductive issues related to the human male. The limiting factor for male reproduction is access to females. The ultimate biological mediator of access and reproductive behavior is testosterone. Ellison does a good job describing the complex biological, psychological, and social issues that interact to create the ultimate expression of sexual and aggressive behavior in the human male.

Finally, in chapter 9, Ellison provides a brief review of the path of human evolution and the selective pressure on human reproduction. Overall, this is a very interesting book that readers will find informative and stimulating.

Reviewed by Frank Muscarella, Ph.D., Barry University, Department of Psychology, 11300 NE second Avenue, Miami Shores, FL 33161; e- mail: fmuscarella @mail.barry.edu.

Copyright Society for the Scientific Study of Sex Feb 2004

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