PDF Summary:A Brief History of the Female Body, by Deena Emera

Our bodies are not only shaped by biology and evolution, but also by the relentless push and pull between mothers and their offspring. In A Brief History of the Female Body, Deena Emera explores the evolution of distinctly female traits like breasts, menstruation, and orgasms—their origins, development, and vestiges of primordial conflicts still playing out in modern women.

Emera traces the roots of these features to ancient genetic power struggles between fetal, maternal, and paternal interests. Beginning before birth with invasive placentas battling uterine defenses, continuing through infancy with both parties vying to set the terms of weaning, and persisting into adulthood as mother and child negotiate redistribution of resources, Emera reveals how ubiquitous biological conflict has sculpted every stage of the female experience.

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• Some studies have found no significant relationship between breast size and actual reproductive success, suggesting that breast size may not be a reliable indicator of fertility.
• Prum's focus on the lack of evidence connecting estrogen and breast size might not consider the full spectrum of scientific literature, including more recent studies that could provide evidence for such a connection.
• The role of cultural influences on the perception of beauty may have significantly shaped the preference for breast size, rather than an innate evolutionary preference.
• The feedback loop described by Darwin's theory might not be as strong or as universal as suggested, with some species showing little to no preference for exaggerated traits, indicating that other factors are at play in the evolution of species.
• The evolution of large breasts could be due to a combination of factors, including but not limited to sexual selection, such as the potential for fatty tissue to serve as an energy reserve for women, particularly during times of food scarcity or while nursing.

The Evolution of Menstruation

Natural Decidualization and Endometrial Shedding

Emera turns her attention to menstruation, an enigmatic aspect of female biology that baffled Darwin and continues to confuse women today. Why do we menstruate when the majority of mammals don’t, especially when menstruating species can successfully become pregnant and reproduce without it? To address this question, Emera first describes how menstruation works in people. The clearest indicator of menstruation is the period, the shedding of the uterine lining that begins on the first day. The primary hormone that initiates this process is progesterone, which the ovaries produce. When progesterone levels drop, the endometrium—the uterine lining—begins to break down, a process that lasts for three to seven days.

Emera then details the series of hormonal changes that occur throughout the cycle, emphasizing communication between the uterus, ovaries, and brain. The key hormone in this interaction is gonadotropin-releasing hormone (GnRH), made in the brain's hypothalamus region and sent to the pituitary gland, another area of the brain. During the initial phase of the cycle, GnRH signals the pituitary to secrete follicle-stimulating and luteinizing hormones (FSH and LH). FSH then travels through the bloodstream to the ovaries to signal the development and maturation of multiple follicles; these follicles pump out estrogen, which stimulates the uterus to prepare for a pregnancy. One follicle eventually outcompetes the others, signaled by an increase of LH. Around day 14, the follicle erupts, releasing an ovum from the ovary—ovulation. After ovulation, this follicle structure (now termed the yellow body) produces progesterone, signaling the endometrial lining to change into a form capable of supporting a pregnancy. If a pregnancy does not occur, the structure known as the corpus luteum dies off and progesterone levels drop. This withdrawal of progesterone from the endometrium causes the tissue to break down and bleed, starting the cycle again.

Practical Tips

• Create a menstrual kit tailored to your specific needs that you can carry with you, ensuring you're prepared for your period no matter where you are. This kit could include items that you've found personally helpful but are not commonly suggested, such as a particular type of tea that eases your cramps, a heat patch, or a stress ball for tension relief. The idea is to go beyond the typical contents of a menstrual kit and customize it based on your own experiences and discoveries.
• Create a menstrual phase-based exercise plan to optimize your workouts. During the first half of your cycle (follicular phase), when estrogen levels are higher and you might feel more energetic, schedule more intense activities like HIIT or running. In the second half (luteal phase), when you might feel more fatigued, focus on lighter, restorative exercises like yoga or walking. This approach respects your body's hormonal fluctuations and can improve your overall fitness experience.
• Create a simple journal to note changes in your body and mood throughout the month. This can help you become more attuned to the physical signs that may indicate the dominance of a follicle and the approach of ovulation, such as mid-cycle pain or increased libido, which can often correlate with the LH surge.
• Incorporate foods rich in magnesium and vitamin B6 into your diet during the luteal phase, the period after ovulation. These nutrients are believed to support progesterone production and may help alleviate premenstrual symptoms. Examples include spinach, bananas, and avocados.
• Create a peer support group to share experiences and strategies for managing menstrual health. This can be a space where you and others exchange information on how different supplements, stress management techniques, or sleep patterns may impact the menstrual cycle, particularly the effects of progesterone withdrawal.

Theories on Why Women Menstruate: Embryo Screening and Mother-Fetus Conflict

After describing how menstruation works in humans, Emera reviews the different evolutionary theories about its existence. She begins with the oldest idea—that it rids the system of impurities—which was posited by Aristotle. More recent, "adaptive" hypotheses suggest menstruation is functional, increasing female reproductive success. One such hypothesis is “menstruation as defense” – that it evolved to clear the uterus of pathogens taken in during intercourse. Another proposes that it saves energy by discarding the modified uterus rather than maintaining it indefinitely. After presenting these theories, the author systematically critiques each one, highlighting the inconsistencies and lack of sufficient evidence, in part because most placental mammals don't menstruate at all.

Emera then delves into her own hypothesis, which focuses on the fact that only menstruating mammals exhibit “spontaneous decidualization,” the monthly transformation of the uterine lining in preparation for pregnancy. The link between menstruation and spontaneous decidualization was first noticed by biologist Colin Finn, who hypothesized a causal connection. In other placental animals, the transformation happens exclusively when a pregnancy occurs. Yet in creatures that menstruate, this occurs every month, regardless of pregnancy. So instead of asking, “What’s the reason for menstruation?”, Emera asserts that we should ask, “Why do we undergo a monthly transformation of our uteruses when this isn’t common among mammals?” She theorizes that this transformation—spontaneous decidualization—evolved as either an early maternal defense against the fetus's invasive placenta cells or as an embryo screening strategy to swiftly eliminate embryos of lesser quality.

The defense hypothesis relies on the fact that the mother’s and baby’s genes have different agendas, which may cause conflict. Fetal genes will try to extract as much energy as possible from the maternal body, compromising the well-being of future siblings that might carry those same genes. Meanwhile, maternal genes attempt to balance investment in a present child with her own wellbeing and that of future offspring. In some primate species, including humans, the fetus produces a placenta that has evolved to be highly invasive, burrowing deep into the uterine lining. Emera proposes that spontaneous decidualization evolved in females as a reaction to this invasion, offering an initial protection from a baby determined to take too much.

The other hypothesis, which is compatible with the invasion hypothesis, is that the early transformation evolved to screen out embryos that haven’t developed properly. Compared to their original, untransformed form, uterine decidual cells are uniquely able to identify embryos of poor quality, which might be more common in species, like humans, with frequent sex. This interaction would lead to an early miscarriage, which is preferable to a later miscarriage or the birth of an unhealthy baby for the mother and fetus—a miscarriage is a relatively small initial investment compared to months of gestation. Because of this, one would expect natural selection to have favored genes in females that allowed decidual cells to effectively weed out the losers.

Practical Tips

• Create a myth-busting blog or social media page dedicated to sharing accurate information about menstruation. Use this platform to discuss historical misconceptions and present current scientific understanding, aiming to educate and reduce stigma. You could collaborate with healthcare professionals to provide credible content and engage with your audience through interactive posts, such as quizzes that debunk old myths.
• Consider adjusting your diet to support your body's processes during menstruation. Research foods that are rich in iron and other nutrients that can be lost during menstruation. Incorporating these into your meals can help replenish your body and potentially reduce the energy expenditure associated with rebuilding the uterine lining.
• Start a journal documenting any pregnancy experiences, noting any health challenges faced and strategies used to address them. This personal record can be valuable for healthcare providers in understanding your unique health profile and can also serve as a resource for other family members who may face similar health issues in the future.
• Engage with a local school or community group to create an educational program that illustrates the concept of evolutionary adaptation through interactive activities. Design a simple game where participants have to "evolve" traits to survive in changing environments, which can help them grasp how species like primates might have developed specific reproductive strategies over time. This could involve role-playing scenarios where they have to choose traits that would help them thrive in a hypothetical ecosystem.
• Consider participating in a citizen science project related to reproductive health. These projects often seek non-expert contributions to large-scale studies and can provide insights into factors affecting embryo quality. By contributing your own health data, you can be part of advancing scientific understanding while also potentially learning more about your own reproductive health.
• Educate yourself on reproductive health by subscribing to a monthly newsletter from a reputable medical organization. This keeps you informed about the latest research and recommendations for maintaining a healthy pregnancy, which can help you make informed decisions about your reproductive health.
• You can explore your genetic heritage to better understand your reproductive health by using direct-to-consumer genetic testing services. These services can provide insights into your genetic predispositions, including potential reproductive issues that may be influenced by natural selection. By understanding these factors, you can make informed decisions about family planning and discuss any concerns with a healthcare provider.

Menstrual Experience Variations Across Populations, Throughout History, and the Lifestyle-Menstrual Biology Mismatch

Emera turns her attention to the menstrual experience, highlighting how women's menstruation in westernized societies today differs from that of women in earlier evolutionary eras. Emera cites anthropologist Beverly Strassman, who studied women from Mali's Dogon people and determined that the average woman has about 100 reproductive cycles in her lifetime. This is because women who forgo contraception typically spend most of their childbearing years in a state of pregnancy or lactation, both of which naturally suppress ovulation. The typical woman in the U.S., however, will go through roughly 400 reproductive cycles during her life.

Emera describes the variation in the number of cycles as a "discrepancy" between how our biology evolved and our current lifestyles. She discusses this mismatch in relation to a variety of women's health issues today, such as breast cancer. With each instance of breast tissue proliferation as the body prepares for pregnancy, there’s a chance for a cancerous mutation to develop. Because American women cycle more than their ancestors did, they undergo additional cell divisions, which increases their likelihood of developing breast cancer. Although many treatments can effectively combat this illness, increased hormones throughout a woman’s life may contribute to a faster-growing and more aggressive breast cancer.

Emera details how and why hormone levels have increased compared to the past. The typical American woman has more estrogen and progesterone circulating in each cycle compared to women with different lifestyles, likely because of higher-calorie diets. Increased progesterone levels could account for why modern women experience longer periods. Increased dietary fat and sugar are also correlated with a younger age of menarche—when girls have their first period. While the reasons for earlier puberty are not entirely clear, wealthier nutrition and insufficient exercise are believed to play a role.

Emera points out that modern dietary influences on puberty timing align with evolutionary theory—if resources are plentiful, why wait? But the environmental and cultural conditions in which we now live don’t appear to have those limits, resulting in a range of health problems, including earlier puberty, increased risk of breast cancer, and a higher incidence of PMS and menstrual cramps. Emera concludes by acknowledging that we aren’t going to revert to our ancestors' nutrition and exercise, though we can apply these insights to change our current lifestyles to lower the likelihood of these negative health outcomes.

Practical Tips

• You can track your dietary intake using a food diary app to monitor calorie consumption and potential hormone influences. By logging everything you eat and drink, you can identify patterns that may contribute to higher hormone levels. For example, if you notice a correlation between consuming certain high-calorie foods and feeling hormonal fluctuations, you might consider adjusting your diet accordingly.
• Create a "Healthy Eating Challenge" for your household where you set goals to gradually reduce the consumption of high-fat and sugary foods. For example, you might aim to replace dessert with fruit three times a week or cook with healthy fats like olive oil instead of butter. Track your progress with a chart on the fridge and celebrate milestones with non-food rewards, such as a family outing.
• Experiment with a new physical activity every month to find enjoyable ways to increase your exercise levels. If you typically don't exercise much, this can be a fun way to discover activities that you like and that fit into your lifestyle. Whether it's dancing, hiking, swimming, or a team sport, trying different activities can keep exercise interesting and may lead to a more active lifestyle.
• Engage in a community-based study group to discuss and observe dietary habits and puberty timing within your local area. Gather a small group of interested individuals and collectively share observations about dietary patterns and puberty development in your children or adolescents in the community. This grassroots approach can provide anecdotal evidence and foster a support network for making informed dietary choices.
• You can monitor your exposure to artificial light after sunset to potentially delay the onset of puberty. Since light exposure can influence hormonal activity, using dimmer switches in your home and reducing screen time in the evening could help mimic natural light conditions. For example, try reading a book instead of watching TV an hour before bed.
• Engage in regular fasting or time-restricted eating to emulate the feast and famine cycles experienced by early humans. Start with a 12-hour eating window and gradually reduce it to an 8-hour window, eating your last meal earlier in the evening. This can help regulate metabolism and improve overall health markers. Remember to consult with a healthcare provider before starting any fasting regimen, especially if you have underlying health conditions.

The Evolution of the Female Orgasm

Theories About the Origin of Female Orgasms: By-Product, Good Genes, and Ovulatory Models

Emera continues her investigation of female biology by unpacking the contentious debate surrounding female orgasm. What makes it so puzzling? Unlike male orgasm, which is critical for reproduction and happens consistently, orgasm for females—particularly when having vaginal intercourse—varies widely. Some women have it regularly, while others only sometimes, and for some, it never occurs at all. However, the intensely pleasurable sensations of climax seem too complex to have developed without any purpose.

Discussion on female orgasm began in 1979 with anthropologist Donald Symons’s book, The Evolution of Human Sexuality. Emera summarizes Symons's "by-product" account, which proposes that orgasms in women lack function; they are simply "a sort of biological fluke," a consequence of men's requirement for them. Because the tissues that make up female clitorises and male penises are identical during the initial stages of embryo formation, females get orgasms simply because of shared development with males. Years later, evolutionary biologist Stephen Jay Gould backed Symons’s theory, further legitimizing and strengthening the "byproduct" explanation.

Though seeming straightforward, Symons’s by-product theory was met with resistance by researchers who are reluctant to classify a complex trait like female orgasm as accidental. Emera describes the range of "adaptive" explanations that arose in response to Symons's by-product theory. The first set of explanations focused on the development of the pair bond—the prolonged phase during which females and males raise children together, which is exhibited by some primate species, including humans. The argument here is that women's orgasms evolved to fortify pair bonds. However, the pair bonding theory was later discredited because pair-bonded individuals, including humans, are known to mate outside those bonds, especially males.

Emera then summarizes a few more “adaptive” explanations that were proffered. One posited that women's orgasms developed because the contractions assist men in achieving sexual climax (which is offensive to me!). A different hypothesis is that orgasm developed as a way to indicate to males that a female is satisfied, thus reducing aggression from the male. And a final group of explanations centers on the theory that muscle contractions during female orgasm push sperm up toward the egg, increasing the likelihood of fertilization and allowing the woman to choose when to upsuck during sex with the most desirable male.

Practical Tips

• Participate in online forums or social media groups dedicated to the topic. This allows you to connect with a diverse community of individuals interested in the same subject. Share your insights, ask questions, and learn from the experiences of others. For example, you might join a group focused on sexual health or women's experiences and contribute to threads discussing the nature of female orgasm.

Other Perspectives

• Gould, while a respected evolutionary biologist, was not a specialist in human sexuality or female reproductive biology, which could suggest that his endorsement may not carry the same weight as that of a specialist in those fields.
• The rejection of the pair bonding theory might be premature if based on limited or biased data; further research could potentially reveal nuances that support a modified version of the theory.
• The suggestion that female orgasms assist men in achieving climax could be criticized for being anthropocentric and male-focused, as it implies that the female sexual response is primarily a tool for male benefit rather than an independent aspect of female sexuality.
• The "upsuck" theory, which suggests that muscle contractions during female orgasm help move sperm toward the egg, lacks robust empirical evidence; studies have not consistently demonstrated that orgasm significantly affects sperm retention or transport.

Clitoris, Ovulation, and Orgasm in Ancestral Mammals: Modifications in Spontaneous Ovulators

Emera then describes the work of philosopher of science Elisabeth Lloyd, who in 2005 published a book called The Case of the Female Orgasm, which deconstructs each of the “adaptive” explanations for the female orgasm that have been presented. Her major argument is that these adaptive accounts fail to acknowledge or address how female orgasms vary, both within and between women. This variability of orgasm weakens the assertion that it developed as a reproductive adaptation, which would be consistently experienced if it actually conferred reproductive benefits. Lloyd systematically dismantles each adaptive account and, like Symons and Gould, backs the idea that it results from a by-product.

Emera introduces another theory regarding female orgasm that doesn’t fit neatly into either the “adaptive” or “by-product” accounts. A recent model proposed by Günter Wagner and Mihaela Pavlicev argues that orgasm in early mammalian females had a reproductive function that has since been lost. It was once an evolutionary adjustment, but has changed such that it no longer functions reproductively, resulting in traits that are confusing and frustrating to modern women.

To support this theory, Emera discusses how ovulation evolved, which is when an egg is released from the ovary. The signaling cascade from the brain to the ovaries that directs ovulation is ancient – we share it with every vertebrate species. Nonetheless, the process begins differently among animals. In many animals, including those with external fertilization, such as fish and amphibians, external cues, such as temperature or the duration of daylight, initiate the process. Internal fertilizers, such as avians, reptiles, and mammals, often use a mate's presence or copulation itself as the trigger. Early mammals, like various present-day mammals, including rabbits, cats, and squirrels, were likely “induced ovulators", meaning the physical stimulation of the vagina and clitoris by a male’s penis during copulation signaled ovulation. In some mammalian lineages, including the one that gave rise to humans, this trigger transformed once more, resulting in spontaneous ovulation. In animals with spontaneous ovulation, hormones inside the female body, not physical stimulation by a male, initiate ovulation.

Emera highlights the connection the researchers make between a mammal's ovulation method—spontaneous or induced by copulation—and the position of the clitoris. They argue that in ovulators by copulation, the clitoral glans—which contains the nerve endings responsible for sensory stimulation—was located inside or very near the vagina to ensure that copulation would trigger ovulation. Because spontaneous ovulators no longer need signals during mating to initiate ovulation, the clitoris's position became unconstrained, and it drifted away from the vagina. In humans, the clitoris is located a considerable distance from the vagina, a consequence of its lost function, and the reason that many women find it difficult or impossible to reach orgasm from vaginal intercourse.

Practical Tips

• Start a personal journal to track your experiences with sexual pleasure, noting the variability and any factors that might influence it. By doing this, you create a personalized record that can help you understand your own patterns and preferences, which may not align with generalized explanations. For example, you might find that stress, diet, or exercise have more of an impact on your experience than previously thought.
• Engage in open conversations with your partner about the nature of sexual pleasure. Discussing the concept that the female orgasm might not have a direct reproductive function can lead to a more relaxed and exploratory approach to intimacy. This could involve sharing what you've learned about the non-functional aspects of pleasure and encouraging your partner to also prioritize mutual enjoyment over goal-oriented sexual encounters.
• Volunteer to participate in surveys or studies conducted by sexual health researchers, which can contribute to the broader understanding of human sexuality and its evolutionary aspects. Your participation can provide valuable data that helps researchers explore the current functions and significance of sexual pleasure in humans.
• Track your menstrual cycle using a smartphone app to better understand your body's internal hormonal signals. By monitoring your cycle, you can observe patterns and changes that may correlate with your mood, energy levels, and physical symptoms. This personal data can help you plan activities around your cycle, potentially increasing your productivity and well-being.
• Explore the evolutionary biology of reproduction by starting a casual book club focused on the topic. Gather a group of friends or like-minded individuals interested in science and evolution. Select a range of books that cover different aspects of reproductive biology, ensuring they complement the idea of induced ovulation in early mammals. Discuss the concepts, compare them, and consider how they have shaped current reproductive strategies in animals and humans.
• Engage in stress-reduction activities like yoga, meditation, or deep-breathing exercises to potentially influence your hormonal balance. Since hormones trigger ovulation, managing stress might have an indirect effect on your reproductive system, possibly leading to a more regular cycle.
• Use educational models or apps to visualize and learn about the reproductive anatomy of different mammals. By comparing these models to human anatomy, you can gain a deeper appreciation for the diversity of reproductive strategies in the animal kingdom and how they may relate to physical structures. For instance, you could use a 3D anatomy app to compare the placement of the clitoris in various mammals and consider the implications for their mating behaviors.
• Explore different sexual positions and techniques that prioritize clitoral stimulation, such as the Coital Alignment Technique (CAT) or using hands and toys during intercourse. By experimenting with these methods, you can discover what works best for your body and potentially enhance your sexual experiences.

Source of Orgasmic Contractions and Sensations Traced to Primitive Vertebrate Spawning

Emera reminds us that orgasms involve a multifaceted sensory-motor reaction: The clitoris receives sensory input that initiates involuntary muscle contractions in the pelvis. Wagner and Pavlicev focus only on the hormonal increase that happens during orgasm, which is similar to what happens in some species during copulation. They don't, however, address the contraction of muscles.

Emera delves deeper into orgasm's evolutionary past by explaining the origin of the muscular contractions. She takes us back to our fish predecessors. In fish, the act of spawning—releasing eggs and sperm into the water for fertilization—involves the pelvic area and reproductive tract muscles contracting in sequence. She theorizes that once internal fertilization evolved in our amniote ancestors, requiring males to develop a penis to deposit sperm, the muscular reflex of spawning was modified based on sex. Males linked the reflex to the penis, their new sensory organ, while females connected it to the clitoris. As internal fertilization evolved, selection also favored the hooking-up of this reflex to pleasure/reward centers of the brain, which incentivized copulation. In early mammals, copulation prompted egg release and spawning, both of which relied on the same hormonal surges and muscle contractions. In humans and animals that ovulate spontaneously, clitoris-mediated orgasm lost its role in ovulation. But relics of this orgasm-related reflex—the hormonal surge, muscle contractions, and pleasure—remain.

Practical Tips

• Engage in regular physical exercise that mimics the physiological effects of orgasm, such as high-intensity interval training (HIIT). HIIT can induce hormonal changes similar to those experienced during orgasm, such as the release of endorphins. By incorporating this type of exercise into your routine, you can explore and potentially harness the positive effects of these hormonal surges in a non-sexual context.
• Create a simple animation or drawing series that illustrates the muscle contractions during fish spawning. Use free online tools or drawing apps to visualize the process, which can help solidify your understanding of the muscular activity involved. Share your creation with friends or on social media to educate others about this aspect of fish biology.
• Implement a 'gamification' approach to mundane tasks. Turn routine chores or tasks into a game where you can earn points or rewards for completion. For example, assign point values to household chores and once you accumulate a certain number of points, you can exchange them for a treat, such as a spa day or a special meal. This taps into the pleasure/reward mechanism to make less enjoyable tasks more motivating.
• Create a relaxation routine before bedtime that includes muscle relaxation techniques. Since muscle contractions played a role in the reproductive processes of early mammals, understanding how muscle tension and relaxation affect your own body could provide insights into your overall well-being.

Conflicts and Compromises in Pregnancy, Placentation, and Mother-Child Interactions

Evolution of Invasive Human Placenta and Maternal-Fetal Conflict

Fetal-Maternal Genetic Tug-of-war Over Resource Allocation During Pregnancy

Emera delves into the intricacies of gestation, beginning by comparing different mammalian strategies to develop their offspring. The monotremes, which lay eggs—platypus and echidna—represent the oldest mammalian strategy. Platypus mothers lay two eggs at a time and incubate them for about ten days before hatching; after hatching, mothers nurse their young until they are ready to leave the nest. The next approach is the pouch-forming marsupials’—an embryo hatches from its eggshell while still in the mother, relying on a simple and short-lived placenta for nutrient transfer. Finally, the eutherian or placental mammals evolved an increasingly sophisticated, aggressive, and durable placenta to provide nourishment for their developing offspring, allowing placental babies to advance further in their development while in the womb. The evolution of the placenta was clearly a winning strategy, as reflected in the large number of placental species on the planet – roughly 5,000 compared to ~250 marsupial and only 5 monotreme species!

Emera details the evolutionary history of the placenta. In amniotes that lay eggs, the embryo is nourished by a sizable yolk, and waste products remain in a sac until hatching. This sac, called the allantois, additionally participates in gas exchange. The allantois and another membrane from the amniote egg, the chorion, are co-opted to form the placental organ in live-bearing amniotes. But how did mammals develop invasive placentas? A critical stepping stone was the emergence of a new kind of placental cell, the trophoblast, with the unique capacity to invade maternal tissue to gain access to the mother’s circulatory system. To restrain the aggressive placental invasion in mammals, the mother’s uterine cells evolved alongside the placenta, creating a new type of cell, the decidual cell. These two types of cells—the placental trophoblast cells and uterine decidual cells—interact so closely in mammals that it is unclear where the placenta of the baby ends and the uterus of the mother begins.

Emera describes the fascinating work of Charlie Loke, an expert in reproductive immunology, who says in Life’s Vital Link that the placenta varies greatly among placental mammals. Though organs such as hearts, brains, and lungs look similar across different mammals, placentas are vastly diverse in appearance. This incredible diversity in how placentas are built and operate is believed to result from parental genetic conflict. As with sexual selection, mate choice preferences, and the evolution of beauty, the conflicting interests of parents over the allocation of maternal resources to the fetus have influenced the development of placental diversity.

Practical Tips

• Create a themed art project that represents the life cycle of monotremes, using materials like clay for eggs and fabric for a nest, to visually explore their unique reproductive process. This hands-on activity can deepen your understanding of monotremes by engaging with the concept in a tactile and creative way.
• You can observe and document the development of chicken embryos by incubating store-bought fertilized eggs. By carefully monitoring the eggs' temperature and humidity in an incubator, you can witness firsthand the role of the yolk and the waste sac in the development of the embryo. This hands-on experience will give you a deeper appreciation for the process described.
• Explore the development of amniotes through creative writing by crafting a short story or poem that personifies the evolutionary process. This can deepen your understanding of the subject matter in an engaging way. For example, you could write a narrative from the perspective of a developing embryo, detailing the formation of the placenta as a pivotal moment in its journey.
• Start a journal to track and reflect on the parallels between mammalian evolution and human development. Write entries that connect stages of personal growth to evolutionary milestones, such as the development of invasive placentas. This can deepen your appreciation for the complexity of life and the interconnectedness of all species.
• You can deepen your understanding of cell interaction by observing plant grafting in your garden. Similar to the way placental trophoblast cells and uterine decidual cells interact, plant tissues must communicate and integrate when grafted. Try grafting a tomato plant onto a potato plant and monitor the changes, noting how the cells might be interacting at the graft junction.
• Engage children in learning about biology by creating a simple matching game with cards that show different mammals and their respective placental types. This hands-on activity can help younger generations grasp the concept of biological diversity in a fun and interactive way.

Other Perspectives

• The term "short-lived" may not fully capture the complexity of the marsupial placenta's function, as it is adequately adapted to support the marsupial reproductive strategy, where the young continue to develop externally in a pouch after a brief gestation period.
• The concept of a "winning strategy" implies a goal-oriented process, whereas evolution is driven by natural selection without any predetermined outcomes.
• The language suggests a one-way evolutionary pressure, whereas it is possible that the evolution of decidual cells was influenced by multiple factors, including but not limited to the need to control placental invasion.
• The diversity in placental structure and function may also be influenced by environmental factors, such as the availability of resources or the mother's diet, rather than solely by genetic conflict.

Co-Evolution of Intrusive Placentae and Maternal Decidual Cells Restraining Invasion

The evolution of placental invasiveness is a fascinating example of conflict, cooperation, and compromise. Emera describes the dynamic interplay between maternal and fetal genes during pregnancy, likening their interactions to a tug-of-war. Fetal genes continually try to draw more nutrients from mothers, disadvantaging both the mother and potential future siblings. Conversely, maternal genes resist fetal demands to balance investment in current and future children with her own needs and health. This tug-of-war has influenced how placental and uterine tissues have evolved. In mammals such as horses and cows, maternal genes seem to have won the battle. In these species, fetal tissues are forbidden from penetrating the uterus lining. But in humans and apes, which have the most invasive placentas, the fetus appears to have the advantage. These are just two ends of a distribution that encompasses all sorts of strategies for placentas and uteruses across mammals.

Context

• This condition can arise when the fetus's demand for glucose leads to increased maternal blood sugar levels, illustrating the potential health impacts on the mother due to fetal demands.
• Different species have evolved varying levels of placental invasiveness based on their reproductive strategies and ecological niches. This diversity reflects the balance of evolutionary pressures faced by each species.
• Specific molecular pathways, such as those involving hormones and growth factors, mediate the interactions between maternal and fetal tissues, influencing how resources are allocated during pregnancy.
• Horses and cows have a type of placenta known as epitheliochorial, where the fetal tissues do not deeply invade the uterine lining. This contrasts with the hemochorial placenta in humans, where fetal tissues invade more deeply.
• The evolution of the invasive placenta in primates is thought to have been a key factor in the development of larger brain sizes and complex social behaviors, as it supports the extended development period required for these traits.

Role of Parasitic DNA in Placenta and Uterine Evolution

Though this maternal-fetal conflict can explain the diversity of placentas across placental mammals, Emera turns her attention to an even more curious dimension to the pregnancy story—parasitic DNA elements that were harnessed by parents in their battles over resources. These elements, called transposons, have clashed with genetic material for eons. Emera describes transposons as "genomic parasites," whose primary goal is to spread their sequence around a host genome, just as pathogens like viruses and bacteria spread their sequence to new hosts. She details the dynamic that exists between transposons and the genomes they inhabit. Transposons enter genomes and exploit host cellular mechanisms to replicate their DNA, increasing their copy number and likelihood of being passed on to the next generation. However, if they leap into crucial sections of the host's genetic material, such as areas that include genes, they can be deleterious, negatively impacting the survival and reproduction of a host individual. Therefore, hosts have developed methods to deactivate jumping transposons.

Because of this, most transposons in our own genome are inactive remnants from ancient invasions. However, inactive doesn’t mean useless. In fact, transposons, with their unique capacity to rapidly spread new genetic sequences throughout the genome, have been co-opted into many useful functions. Emera describes how transposon recruitment has played a huge role in mammalian placental evolution, contributing to the amazing variety of placental structures we observe today across placental mammals. There is robust proof of transposon recruitment in this context. Many transposons, including those whose ancestors were infectious viruses, have the ability to help cells fuse together, avoid the body's immune response, and invade tissues, all of which are necessary functions of placentas in mammals.

There is also considerable proof that in the mother, transposons were recruited by uterine tissues, causing transformations that were pivotal in the development of pregnancy and giving birth to live young. The co-evolution of uteri and placentas in placental mammals is closely tied to progesterone, the hormone that establishes and maintains a pregnancy. A significant amount of research has shown that placentas produce progesterone and other molecules in an attempt to hijack gestation away from the mother, furthering the interests of the fetus. In response, maternal adaptations developed to counter the placenta's production of progesterone. Interestingly, many of the gene regulators in uterine tissues that are responsive to progesterone come from transposon sequences. In fact, transposons are thought to have been critical in the origin of maternal decidual cells that restrain trophoblast invasion in mammalian placentas.

Emera wraps up by reminding us of a major theme—conflict. She's intrigued that the very factors enabling the parasitic bond between fetus and mother while pregnant are themselves parasitic relics of evolution. Transposons became tools in the mother-fetus battle over resource allocation that still unfolds during the pregnancy of every placental mammal today, including ourselves.

Context

• Transposons move through a "cut and paste" or "copy and paste" mechanism. DNA transposons typically use the "cut and paste" method, while retrotransposons use "copy and paste," involving reverse transcription.
• The discovery of transposons was groundbreaking, earning Barbara McClintock a Nobel Prize in 1983 for her work on maize, which revealed the dynamic nature of genomes.
• These are a class of small RNAs that specifically target transposons in animal germ cells, helping to maintain genome integrity by preventing transposon mobilization.
• While most transposons are inactive, some can still cause genetic disorders if they insert themselves into or near essential genes, disrupting their function. This can lead to diseases such as hemophilia and certain types of cancer.
• The placenta must evade the maternal immune system to prevent rejection. Transposons have contributed to the evolution of mechanisms that help the placenta modulate immune responses, ensuring the fetus is not attacked as a foreign body.
• The presence of transposons in mammalian genomes is the result of ancient invasions by these elements, which have been retained and repurposed over millions of years, contributing to the diversity and adaptability of mammalian reproductive strategies.
• Co-evolution refers to the process where two or more species influence each other's evolutionary trajectory. In the context of uteri and placentas, this means that changes in one (e.g., the uterus) could drive changes in the other (e.g., the placenta), leading to mutual adaptations over time.
• The ability of the placenta to produce hormones like progesterone is seen as an evolutionary adaptation to optimize fetal development, sometimes at the expense of maternal resources.
• These specialized cells in the uterine lining help regulate the invasion of the placenta into the uterus, controlling the extent of resource extraction by the fetus.
• The transformation of uterine cells into decidual cells, a process influenced by progesterone, is essential for successful implantation and pregnancy maintenance. Transposon-derived gene regulators play a role in this transformation.
• The process by which organisms take existing structures or genes and repurpose them for new functions. Transposons, originally parasitic, have been co-opted for beneficial roles in pregnancy.
• This is a genetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner. It plays a role in maternal-fetal conflict, as paternal genes may promote fetal growth, while maternal genes may limit it to conserve resources.
• The mother-fetus resource allocation conflict refers to the evolutionary tug-of-war over nutrients and energy. The fetus, through the placenta, seeks to maximize resource intake from the mother, while the mother aims to balance her own survival and future reproductive success.

Mother-Child Conflicts of Interest Throughout Gestation and Beyond

Divergent Evolutionary Interests of Mothers and Children: Gestation, Nutrient Sharing, Weaning

Emera turns her attention to how these genetic clashes in gestation and into later life have impacted the physiology and behavior of moms and their offspring today. The tension between a mother and her fetus begins in the womb and continues after the baby is born, but the power dynamic changes—the baby is in control inside, but the mother outside. A mother can withhold her milk, leave the child with other caregivers, or, in extreme cases, abandon the infant. In contrast, the child needs the mother's care and commitment. According to biologist David Haig, an ongoing tension between children and their mothers—then and now—has shaped numerous characteristics and behaviors we observe today in mammals, including humans.

Context

• This theory explains that parents and offspring may have conflicting interests regarding the amount of investment a parent should provide, with offspring typically seeking more than the parent is optimally willing to give.
• In many societies, cultural practices and social structures influence how maternal and infant care is managed, affecting the balance of power and resource distribution.
• Hormonal changes after childbirth can affect a mother's behavior and her ability to produce milk, impacting her interactions with the infant.
• Studies have shown that consistent maternal care is linked to better physical and mental health outcomes in children as they grow.
• In humans and other mammals, behaviors such as crying or clinging in infants can be seen as evolved strategies to ensure maternal investment, while maternal behaviors may include strategies to manage or mitigate these demands.

Genetic and Physiological Mechanisms in Mother-Child Disputes: Hormonal Signaling and Genomic Imprinting

Emera revisits the concept of genomic imprinting, in which children’s genes recall which parent they came from. Although most of our genes are expressed regardless of whether they came from the mother or the father, roughly 200 genes in humans have been identified as imprinted, with a significant number operating in the brain and placenta. She points out that imprinted genes align with what we would expect from the conflict theory—father’s imprints in children push for greater maternal resources, while the mother’s imprints tamp down demands to balance investment in their current child while ensuring her own health for the next.

The effects of imprinted genes are illustrated in imprinted gene disorders, in which imprinted sections of DNA malfunction. In Beckwith-Wiedemann syndrome, paternal imprints dominate, and children exhibit symptoms aligned with genetic greediness. Conversely, in babies with Prader-Willi, Silver-Russell, and Temple conditions, maternal imprints dominate, and their symptoms reveal the more moderate goals of the mother. According to Haig, these genetic disagreements among family members influence many aspects of a child’s life, such as weaning and when they go through puberty.

Practical Tips

• Encourage your children to participate in activities that stimulate both sides of their brain. Since genomic imprinting can affect how genes from each parent are expressed, engaging in activities that require analytical and creative thinking, such as playing musical instruments, coding games, or doing art projects, might help in the balanced development of their inherited traits.
• You can increase awareness by sharing simplified information on social media about imprinted gene disorders. Create infographics or short posts that explain the basics of these conditions in layman's terms. This could involve using free graphic design tools like Canva to visually represent how imprinted genes work and what happens when they malfunction. By doing this, you contribute to public knowledge and potentially help individuals recognize symptoms or seek genetic counseling.
• Observe and note the developmental progress of children in your extended family. If you have nieces, nephews, or cousins, take note of their growth and development, paying attention to any variations that might align with the family health timeline you've created. This doesn't require any special skills, just a keen eye and perhaps a journal to record your observations. Over time, you may notice trends that could be related to the genetic factors discussed in the family health timeline and dialogues.