It is a central thesis of biology that every example of specialised animal development from the morphology and physiology of the human retina to the aerodynamics of a bird’s wing has been exquisitely tailored by evolution. Much less thought and study has been given to the development of the mechanisms whereby humans adapted their reproductive capacity to match their environment even though from an evolutionary standpoint, this adaptation must have been just as important as developments in our sensory or locomotive systems.
Human beings are large, non-seasonally breeding animals closely related to the great apes. The last century has seen a quadrupling of the human population and today’s unprecedented explosion of human numbers poses serious problems for thousands of millions of those who live on the fringe of modern society. Man’s excessive population growth has already eliminated many species of animals and plants. If uncontrolled it may overwhelm the world’s biosphere. But it is in the social aspects of unintended fertility where the most immediate need for understanding ‘natural’ fertility lies. In the developed world young people are cruelly squeezed between the apparently irresistible force of a declining age of puberty and a seemingly irrepressible desire for sexual intercourse resulting in the anguish associated with unintended teenage pregnancy. In the developing world, human population grows by one million every five days. Numbers of this magnitude are difficult even for scientists to grasp and it may be useful to remind ourselves that there have only been two-thirds of a million days since the birth of Christ. It is a dramatic fact that homo sapiens, the slowest breeding animal in the zoo, has become the cause of an unprecedented population explosion. Failure to understand ‘natural’ human fertility has been one of the causes for the inadequate human response to twentieth-century population growth. Professor Short in his Galton lecture (Chapter 1) deals with this failure by modern man to appreciate, much less to deal with, this overwhelming biological threat. Since the problem has developed with such startling rapidity, an adequate evolutionary response is impossible and our survival as a species depends upon our political, social and technical ability to control our population growth. Sadly the magnitude and urgency of the threat is almost universally under estimated.
When we come to consider the development of Man’s sexual and reproductive patterns, historical time covers such a small part of human evolution that it is of little significance. Our deductions about the reproductive characteristics of prehistoric man have to be based largely upon current studies of ‘Traditional Societies’, combined with a knowledge of the probable physiological mechanisms involved. In Chapter 3, Campbell and Wood provide a scholarly account of the techniques used in such studies and of the basic data obtained. Since the primary interest of the symposium and of this book is in the natural or physiological factors influencing fertility, this data is of great importance and comparative studies of other primates provide much corroborative evidence which helps in unravelling the mechanisms involved. The studies reported by Stewart, Harcourt and Watts on wild gorillas demonstrate clearly that primitive man and these higher primates both possess physiological mechanisms capable of adjusting their fertility to their environment.
The human 28-day cycle is merely the minimum time it takes to select, nurture and release one egg from a dominant follicle and, should fertilisation occur, to prepare the uterus for invasion by the trophoblast. Repeated menstruation, as this volume will show, is largely the product of modern living. Although the menstrual cycle is almost universally of the same duration, race, culture and environment influence the cycle of puberty, pregnancy, lactation and menopause by mechanisms as yet imperfectly understood.
In Chapter 2, Stewart and her co-workers show that in the wild state gorillas reach sexual maturity at 7.6 years of age (range 6.1-8.5). Most mammals develop late but humans have taken this characteristic to an extreme. In Chapter 3, Campbell and Wood found that women of the Gainj tribe in New Guinea begin to menstruate at a mean age of 20.9 years. Western girls, as Wu describes in Chapter 5, begin to show breast growth at 10.5 years (range 8-13) and menstruation at 13 years (range 11-15). Since pregnancy, and most especially lactation, make great energy demands upon the female, it is likely that changes and improvements in nutrition is the important factor associated with the dramatic fall in the age of puberty. Studies in malnourished societies, summarised by Lunn in Chapter 9, emphasise the role of nutrition in all aspects of fertility. The exact mechanism responsible for a seven-year reduction in the mean age of puberty between ‘natural’ and ‘modern’ man is one of the least understood aspects of human fertility. The earlier puberty takes place, the more rapidly potentially fertile, ovulatory cycles appear to occur. Gainj women do not, on average, deliver their first child until they are 25.7 years old. In contrast many Third World women and a considerable number of Western women will have had several pregnancies by that age; they may have completed heir families and in extreme cases may be seeking voluntary sterilisation. As Smith emphasises in Chapter 4, differences in fertility in Europe prior to the nineteenth century were determined more by variations in the age at marriage than in patterns of child spacing after marriage. Nowadays it is recognised that this demographic variable is one of the most important explanations of differential fertility.
The age at first pregnancy, influenced by the age at menarche and the average age at marriage, is of prime importance in fertility but birth spacing remains of significance and the natural mechanisms which affect it deserve careful study. With regard to animals, Stewart shows that wild gorillas deliver at intervals varying between three and seven years unless an infant is stillborn or dies soon after birth in which case the interval may be as brief as one year. Howell’s (1976) important work on the Dobe !Kung and evidence from other preliterate societies, as well as the studies from Bangladesh presented in Chapter 3 by Campbell and Wood, demonstrate that human societies can have birth intervals averaging from 33 to 36 months after a live birth, even in the absence of contraceptive use. This volume establishes from comparative animal data, from historical studies and in particular by Fildes’s contribution (Chapter 7) together with studies of traditional societies, that the primary mechanism controlling birth spacing is lactation. In Chapter 6 McNeilly outlines the current neurophysiological theory which helps to explain how suppression of ovulation is affected not only by lactation itself but also by subtle variations in the pattern of breast feeding; something which is demonstrated by gorillas as well as by women. The fuller elucidation of these mechanisms represents one of the most important areas for future research. Meanwhile it is significant that there is close correlation between the evidence from such diverse sources.
Regular and continued menstruation has become so accepted that we tend to think of it as a natural condition. A modern woman is likely to experience about three hundred menstrual cycles in her lifetime Her hunter gatherer ancestor who probably commenced menstruation at about 20 and had four or five pregnancies, dying usually before her menopause but ceasing to menstruate by say, 35 years of age if she survived so long, would have experienced only between twenty and fifty cycles. The huge investment in time and energy which the human female must make in the process of pregnancy, lactation and rearing means that teleologically she should be incapable of reproduction until fully mature and that she should lose her reproductive powers before she becomes too old to bear and raise more children. All obstetricians are aware of the particular dangers of teenage pregnancy and of pregnancy in the older woman. In Chapter 11, Pike shows how repeated menstruation is statistically related to ovarian, uterine and possibly to breast cancer. His contribution shows us that there is a price which women must pay for departing from the patterns of puberty and child bearing laid down by their evolution.
In fact, Pike shows that one of the most important epidemiological discoveries of the 1980s has been the unambiguous demonstration that the use of oral contraceptives significantly reduces the risk of developing ovarian or uterine cancer in later life. It would seem that the Pill recreates, albeit in a clumsy way, some of the long intervals of anovulation associated with the natural patterns of fertility as observed among the Gainj or the !Kung or as documented by historical demographers. Pike, who has made important contributions to the study of contraceptive usage and breast cancer, shows that at present the data is confusing but he holds out the hope that eventually a combination of synthetic steroids may give a contraceptive pill capable of reducing the breast cancer risk as has already occurred for cancer of ovary and uterus.
At the symposium, Professor Pierre Jouannet from Paris gave a lecture on the male contribution to fertility. Unfortunately, this paper is not available for publication. One point that he made was that paternal age is an important factor in the transmission of genetic defects. He said that achrondroplasia increases in incidence just as significantly with paternal age as does Down’s syndrome with the age of the mother. Since men remain potent and fertile into old age this would seem to be at odds with evolutionary development; probably one may assume that in prehistoric times the younger and stronger members of the tribe would have effectively prevented the elderly from passing on their now defective genes.
The zygote/embryo/fetus takes half its genes from its father and thus its tissue is genetically foreign to the mother. Why the mammalian mother does not immunologically reject her offspring remains an enigma. In Chapter 8, Mowbray deals with our present understanding of this problem and reports on new and highly significant research into the problem of unexplained recurrent abortion and unexplained fertility which is now undergoing clinical trials and of which we undoubtedly hear more in the medical press. The whole problem of subfertility and infertility lies mainly outside the scope of this book but Cooke (Chapter 10) presented a global view of the main causes of infertility based upon the World Health Organization’s survey of human fertility in seventeen countries conducted in the 1970s.
Malthus, in the second edition of An Essay on the Principles of Population (1803), suspected that family size was determined by more than mere patterns of human copulation: ‘It would be a most curious, and to every philosophical mind a most interesting piece of information’, he wrote ‘to know the exact share of the full power of increase which each existing check [to population] prevents; but at present I see no mode of obtaining such information.’ One hundred and eighty-three years later the Eugenics Society called together specialists from many disciplines to discuss this same problem. This book presents the papers delivered.