The fertility of a woman is dependent on two hormones: oestrogen and progesterone. They play a pivotal role during puberty, insemination, and the development of foetus. However, they also have a ‘darker side’: they’re often responsible for breast, ovarian, and uterine cancer. Did evolution play a cruel trick on us by making something so important for creating life dangerous, and sometimes even deadly?
Since the great synthesis of genetics and the theory of evolution in 1930s and 1940s we know that natural selection favours such physical attributes and behaviours that facilitate passing on one’s genetic material to the next generation. It’s a biological imperative that overrides other issues, even important ones, like longevity and health.
What are the consequences of this mechanism? How can we better understand it in order to combat cancer and create successful pro-health policies? The paper Human reproduction and health: an evolutionary perspective, written by a research group led by Prof. Grażyna Jasieńska from the JU Faculty of Health Sciences and recently published by Lancet, is devoted to this very topic.
The concentration of oestrogen and progesterone in the organism is determined not only by genetic factors, but also by environmental ones. We’re talking about a phenomenon called phenotypic plasticity: the more energy an organism has, the more hormones it produces, thereby increasing its chance to reproduce… and its chance to get cancer. According to researchers, the levels of progesterone in American women (good nutrition) are much higher than those in Congolese women (malnourishment).
The amount of energy stored in female body affects the chance of her bearing and bringing up offspring. Of all animals, this is particularly important in the case of humans, since our evolutionary adaptations – large brains and slow maturation – require a great deal of effort. Unsurprisingly, lack of adequate nourishment negatively impacts the chance of procreation and consequently, the levels of sex steroids. What is a bit more surprising is the fact that the amount of sex steroids is determined not only by a woman’s current state, but also the conditions she found herself in while growing up. The organism adjusts the speed of growth based on access to energy, frequency and severity of diseases, stress and overexertion etc.
When resources are limited, giving birth to a child is simply too risky. Nature doesn’t care about emotions and dreams. In the world of selfish genes (as described by Richard Dawkins), cold calculation is king.
It’s an established fact (sources available in the Lancet article) that sex steroids, especially oestrogen, contribute to development of various forms of cancer, such as breast cancer. In layman’s terms, they interact with substances responsible for processing energy (like insulin, leptin, inflammagens etc.). Could we assume, then, that the higher number of instances of cancer in affluent societies is related to high levels of oestrogen and progesterone? It seems very likely. Researchers cite the analyses of traditional models of growing up and giving birth to children. Such models are characterised by later menarche, earlier pregnancy, high parity, and longer periods of breastfeeding. This combination of factors causes breast cancer to become less likely to occur. The concentration of sex steroids throughout the entire life of a woman is much lower.
Paradoxically, changing the relation between fertility and cancer is not guaranteed to yield any benefit. The process of reproduction causes the energy that would normally be used for the benefit of the mother to be ‘transferred’ to the task of bearing living, healthy progeny. It results in increased chances of diabetes, obesity, and cardiovascular diseases. The authors of the paper give a number of examples to prove their claims. ‘Six or more pregnancies increase the risk of stroke by 70%’, researchers say, citing an analysis conducted on a group of nearly four thousand women between the ages of 45 and 74.
Physiology and society
These already complex interrelations are further obfuscated by socio-economic factors affecting each individual woman. In wealthy societies, where the sex steroid levels are much higher, families aren’t larger than in underdeveloped ones. This is caused by the clash of cultural and economic issues (employment, relationship stability etc.) with biological issues (hormones). It’s an additional piece of the puzzle.
Robert Sapolsky, American neuroscientist and science communicator, wrote about the relations between non-physiological factors and various diseases in his book Monkeyluv (2005). He analysed social economic status gradient (SES), observing that in some cases, when moving from the richest to the poorest members of society, the risk of death or disease increases over tenfold, with the situation getting progressively worse with each step away from wealth and towards poverty. There are, however, some exceptions: Sapolsky notes that wealthy people are more likely to suffer from certain afflictions. Such is the case of multiple sclerosis and melanoma. Various types of cancer caused by excessive amounts of sex steroids are also an example of this. Fortunately, thanks to a multitude of analyses (such as the one found in the paper written by Prof. Jasieńska and her colleagues), we’re able to learn more about the details of the pathogeneses of these diseases and introduce effective preventive measures.
Why is it that today women in affluent societies are exposed to much higher levels of sex steroids than they used to in the past?
Contemporary women have more menstrual cycles because:
- they experience their menarche earlier,
- they go through the menopause later,
- they bear fewer children overall,
- they breastfeed for a shorter period of time.
The production of hormones during every menstrual cycle is higher due to much larger amount of available energy (due to abundance of food) throughout all stages of life.
Obesity and hormonal therapy increase the concentration of sex steroids after the menopause.
Millions of neural connections, synergised senses, able hands, and efficient vascular system and skeleton. Isn’t our organism a perfectly working machinery? It isn’t – at least not in the light of our current needs. Biologists have known this for years. ‘None of our features are perfect. We could change that, but improving one trait means worsening another’, claim researchers in another paper, an introduction to studies on evolution and health. It’s important to look at our imperfections, their causes and their interrelations, and use our knowledge about them to improve our quality of life.
However, this will certainly be no easy task. Too high levels of sex steroids increase the risk of cancer, while too low levels impede our procreation, increase the likelihood of cardiovascular and bone diseases as well as adversely affects our psychological health. Sex steroids have positive and negative effects, and the key is to balance them.
Prof. Jasieńska provided us with an interesting example illustrating the purpose of implementing the results of evolutionary biology research in relief effort. In the 1990s, water wells were dug in the middle of several villages of the Arsi tribe in Ethiopia. It reduced the amount of time it took women to collect water from three hours to fifteen minutes a day. ‘This was, naturally, a very good thing. However, an analysis of later demographic surveys showed links between the digging of the wells and increased number of children in the families and worse health of older children. The authors of that research cited our studies of the Island Beskids area. In my PhD thesis, I've proved that intensive work decreases the level of sex steroids. When the Arsi women had to put a lot of effort into collecting water, their hormone concentration was probably rather low. The well allowed them to work less, which possibly increased the concentration and thus resulted in higher fertility and pregnancy rate’.
‘As to why the children were less healthy: the wells allowed the women to expend less energy, but they didn’t provide more food. It became increasingly more difficult to feed the children’, Prof. Jasieńska added.
Evolutionary biology tells us that our organism always strives to maximise its reproductive potential. In this case of Arsi women, the organism decided that the energy which was previously used to collect water can now be spent on reproduction. This means that relief effort, although it provides instantaneous benefits, may in the long run prove detrimental. If we’d known then what we know now, we might have predicted the outcome.
Prof. Grażyna Jasieńska is a biologist and the head of the Department of Health and Environment at the JU MC Faculty of Health Sciences. She received her PhD at Harvard University. During her academic career, she worked at or collaborated with higher education institutions such as Harvard University, Deakin University, Yale University, University of New Mexico, and University of Illinois. In 2005, she received the prestigious Radcliffe Fellowship at Harvard. She authored the book The Fragile Wisdom. An Evolutionary View on Women’s Biology and Health (2013), a result of several years of research on women’s fertility and health.
Original text: www.nauka.uj.edu.pl