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pregnant can be too easy.
I explain here why nature often seeks to limit fertility -- and why evolution has produced so much genetically determined infertility among humans. In doing so we will consider From Amoebae to HumansReturn to Top From single-celled animals, such as the amoeba, to man, successfully reproducing organisms that are part of expanding populations gather resources so that their progeny will outnumber their ancestors. In today's society, humans need to have an average of just over two children per couple to sustain the population. More than this will increase it. But back in the fourteenth century, six or more children were needed per couple to sustain the population. This was because of the high likelihood of death, not just of the children within families, but also of the adults, before their dependent children were grown up and able to have children of their own. (For a brief account of the transition, at least how it was recorded in public by contemporary European artists, see the box, The growing value of children in European history ...). For a species to succeed, it's not enough just to give birth to babies. For a human couple to be reproductively successful -- for their reproductive strategy to triumph -- they not only must have children, they must also survive long enough to rear their offspring to an age at which the children become fertile adults. Children need to be nurtured to the point of independence, otherwise the reproductive effort is wasted. This principle lies behind the reproductive strategy of all large animals (see the box, Of whales and women). In the course of our evolution towards bigger and better brains, the time from birth to maturity has also increased considerably - spectacularly so for animals of our size. The anatomy of the female pelvis eventually limited the size the fetus's head could be at birth without getting stuck on the way out, so a lot of brain development had to occur after birth. This in turn meant that infants were born less and less mature, more and more vulnerable and reliant on their mothers, so a longer time came to be devoted to dependent infancy and childhood. But childhood and adolescence also came to be extended with increasing ultimate brain size and the time needed for the person to learn living skills. In primitive human societies, menarche, the time of the first period (pronounced "menarckie"), is at about 16 and a half years. First pregnancy is not till about 18 years. (For more on the consequences on the growing gap between menarche and the age of readiness to have children in modern societies, see the box, 'Twixt sex and reproduction.) Pregnancies and bearing children wear a mother out. Having many children is extremely detrimental to a woman's health, and also to her ability to raise the children she's got. Put simply, too much pregnancy can be bad for you and bad for the survival of your genes, especially if the environment is harsh and challenging, as it has probaby been for most of human history and prehistory -- indeed as it has been right up to the most modern of times. So, interestingly, we have a reproductive strategy in which delaying sexual maturity among the offspring turns out to be an advantage for our species. (To understand how population scientists distinguish r-selection and K-selection population strategies -- and to see where we as humans sit in the scheme of these things -- click to the box, Of whales and women.) During this time it's important for the continuation of the species that childhood be secure: that parents in general, and mothers in particular, don't wear themselves out by getting pregnant too often. As a result, we have experienced much evolutionary pressure for curbing fertility, for the birth of children to be spaced. Putting a Brake on FertilityReturn to Top Spacing pregnancies in mammals -- humans as well as the largest of them, whales and elephants -- has come about in several ways. The first is through "lactational amenorrhea", the inhibition of ovulation, and for humans the inhibition of menstrual periods, that's caused by suckling the young at the breast. This is the chief natural contraceptive of the great apes and humans. To be an effective contraceptive, though, breast-feeding shouldn't be diluted by other feeding. The hunter-gatherer !Kung nomads of the Kalahari Desert in southern Africa feed their babies several times an hour for the first two or three years of life, and sleep with them at the breast; as a result, the average birth interval among the !Kung is over four years, despite their natural, apparently high, fertility. There's no way of knowing, though, for what time in human history and prehistory such intensive suckling has been usual. Modern but primitive societies such as the !Kung may have evolved considerably from the early tribes we once had in common as ancestors. Seasonal breeding among mammals is also associated with a lack of ovulation in the off-season. It helps space pregnancies among the lesser apes, such as the lar gibbon of the Malayan rain forest. But among humans, other than the observation that we have higher sperm concentrations in winter, seasonal changes in fertility are not conspicuous. (Probably men and women outside the tropics, through clothing, shelter, and sometimes migration, have successfully adapted the environment to provide rough constancy between the seasons.) Environmental constraints on human fertility are seen, though, in the stop that occurs to periods and to ovulation when a woman loses weight to below that at which, during adolescence, menstruation began (explained further on Web Page 10). An intriguing exception to this occurs with the polycystic ovary syndrome, in which fertility in the past might have been best during times of famine (see the box, Fertility in famine).Return to Top But over the last several hundred thousand years, in environments less harsh than the Kalahari Desert, suppressed ovulation from breast feeding might not have been contraceptive enough -- as steadily evolving higher intelligence and increasing brain size and complexity demanded co-evolution of longer and longer childhoods. So what was it, then, that nature could turn to for evolving early humans to extend their birth interval and to increase the number of young raised safely to maturity? How did evolving humans, pressed to reproduce, unconsciously slow down their fertility to achieve an ultimately successful reproductive strategy? Not by limiting sexuality -- that is, not by getting rid of the pleasure of sex. Desmond Morris, among others, has argued convincingly in his book The Naked Ape that sexuality (probably a million or more years ago) had become -- and remains today -- the enjoyable cement needed for human relationships to be durable in the long-term. Predictable sexual reward was presumably important to help make sure that females had the food supplied by their mates for themselves and for their dependent children. Today, the lessons of medicine tell us that fertility in humans is not only rather low by the standards of other primates, but that a number of conditions that diminish fertility have a genetic cause ... an evolutionary cause. Subfertility, a benefit ? Any or all of the clinical conditions we can identify today with subfertility might have acted beneficially to limit human fertility during our evolution, and thus to improve successive generations' chance of reproductive success. Any or all of these identifiable conditions we associate with clinical infertility might in fact be inseparable from our species' successful long-term reproductive strategy. Many miscarriagesReturn to Top A high chance of miscarriage can serve a useful purpose in long-childhood species, according to Roger Short, a reproductive biologist at the University of Melbourne, and co-author of Ever Since Adam and Eve. Short draws attention to the advantages brought by "a sprinkling of genetically defective gametes [to produce] a measure of non-recurrent infertility through early embryonic death". Short draws attention to the advantages brought by "a sprinkling of genetically defective gametes [to produce] a measure of non-recurrent infertility through early embryonic death". Evidence of high rates of failure of early pregnancy among humans shows that no more than 25 percent to 50 percent of fertilizations will produce a healthy pregnancy -- a lower proportion than among the other primates. We see this in our IVF programs today: a majority of the embryos of normal women we test as part our PGD/sex selection/Down-syndrome-prevention program at Sydney IVF have abnormal numbers of chromosomes, or aneuploidy. Aneuploidy is usually of maternal origin and the risk of it increases with maternal age - or at least with a decline in the population of eggs remaining in the ovaries. It is by far the commonest genetic cause of human reproductive failure. There appear to have been similar evolutionary pressures among other species with long childhoods because of the demands of huge adult size and hence a requirement for maternal longevity. According to Dr Short whales too have high rates of embryonic death. Low sperm countsReturn to Top The inheritance of low sperm counts, or oligospermia (WebPage 9), has a similar effect to defective eggs in systematically diminishing fertility. Roger Short and others have shown that variation in sperm counts and in the size of the testes in primates is determined genetically. Before I had known about Roger Short's work, I had an exciting encounter with the American urologist Sherman Silber in an airport lounge in Singapore. I was explaining my theories of evolutionary causes for the high prevalence of endometriosis in modern women (see the next section) when he drew my attention to the advantages that genetically determined oligospermia may paradoxically have for men (at least men in monogamous relationships). What we had in common -- and, it turns out, in common with Short -- was that independently we had all reached the conclusion that by limiting fertility without limiting sexuality, evolving humans (or more exactly groups of humans as societies) probably were able to achieve greater ultimate reproductive fitness. EndometriosisReturn to Top Endometriosis (explained in detail on WebPage 15) is a condition in which tissue the same as the lining membrane of the uterus, the endometrium, develops in locations outside the uterus, changing the environment for the woman's pelvic organs and producing subfertility. Endometriosis gets more common with age during the reproductive years, as the modern woman, generally neither pregnant nor breast-feeding (unlike her ancient predecessors), experiences an unprecedented number of ovulatory menstrual cycles -- each one increasing the chance of new lining developing in the wrong place. Endometriosis is rarely severe enough to be an undoubted cause of sterility, but several lines of evidence show that fertility is reduced even in mild endometriosis. Such information fits a theory that endometriosis reduces fertility in a manner that could be called "dose-dependent" -- the more endometriosis, other things being equal, the greater the reduction in the chance each month of getting pregnant. Endometriosis has been found in the other menstruating mammals, namely monkeys and apes, but only occasionally does it obviously disable them. Endometriosis could possibly have evolved independently in early humans, but more probably nature seized on this mostly harmless consequence of menstruation in primates, opportunistically bestowing it with an added responsibility. This would mean that endometriosis may date from at least 14 million years ago (which is when the old-world monkeys separated from the line destined to become humans). Whether my exercise in theoretical anthropology is right or wrong, endometriosis is so common in women that, given its hereditary basis, it's plausible that nature -- at the cruel expense of significant symptoms for many modern sufferers -- kept it and developed it for a purpose. That purpose could have been to improve reproductive fitness through diminishing fertility. But, today ... Today there are three big problems in the way nature dumps you with the legacy of this strategy. FIRST, whereas a mild dose of an inherited infertility condition such as endometriosis might not be much more than a nuisance, remember that you get half your genetic inheritance from your mother and half from your father. While a single dose of endometriosis genes might cause only minimal or mild endometriosis, every now and then, by chance, a woman will get a double dose of the genes, with crippling endometriosis and virtual sterility. SECOND, again while a mild dose of endometriosis by itself, genetically determined and inherited, might not be much more than a nuisance, every now and then one of the other genetically-determined fertility-restraints will be co-inherited (either by the woman herself, or by her partner). Thus oligospermia in the male and endometriosis in the female are a common combination among infertile couples. Or you might see endometriosis and polycystic ovaries occurring in the one person. And that indeed is then a whole new ball-game. It means that instead of say it taking 12 months to conceive instead of a more usual 4 months or so, it can take 5 or 6 years. (If the arithmetic of this migh interest you, it's on WebPage 28.) And THIRD, the physical or social environment may impact on what otherwise is a mild fertility-restraint and again put you behind the eight-ball. Examples include multiplying in the effect of delayed child-bearing. By the 30s, not only is the endometriosis likely to have developed somewhat further, but age-related effects on eggs are starting to show, and other age-dependent abnormalities such as fibroids are becoming more prevalent. So there are three ways that a single "natural fertility restraint", while of benefit to the species' reproductive strategy on a population basis, can wreak havoc with individuals' reproductive ambitions ... that if isolated and mild there would be no serious difficulty with conception, but the chance coming together of two factors, or the chance genetic exaggeration to a more severe form, or the impact of an added environmental factor, transforms the desirable restraint it puts on fertility for the population into the unwelcome burden of serious infertility for the individual. (On WebPage 8 I explore further the grim reality of nature's disregard for individuals' genetic well-being in the box, Heterozygous advantage.) Yesterday's AdvantageReturn to Top You can conclude from this web page that low human fertility is the result of a long and generally positive evolutionary sequence. What today we call unexplained infertility is by no means abnormal: it's a highly evolved human condition. But that's not to say it oughtn't to be treated. Having to wait about five years to get pregnant is no longer considered the pinnacle of human reproductive achievement that sociologically it once might have been. Just as we use contraception to minimize the social disadvantages of excessive fertility, so we're justified in employing assisted conception to cope with insufficient fertility. To appreciate the finer points of infertility and its effective treatment, WebPage 3 takes us into those normal workings of the body that lead to getting pregnant naturally.
Copyright © Robert Jansen, W.H.Freeman and Scientific American Books (New York) and Allen & Unwin (Sydney) |