Invited review article
The negative legacy of human evolution
2023 Volume 22 Issue 2 Pages 63-68
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2023 Volume 22 Issue 2 Pages 63-68
Humans, standing up about seven million years ago, have evolved independently by freeing their hands. However, many physical inconveniences, called negative legacies, are caused by the physical structure of modern humans. This review focuses on the structural causes of dystocia and how people respond to it.
Upright bipedal locomotion is one of the defining physical characteristics of human beings. Our ancestors, who stood up about seven million years ago, evolved uniquely by freeing their hands and came to have the most expansive distribution area on earth. However, many physical inconveniences, called negative legacy, are the consequences of the physical structure of modern humans that resulted from human evolution, and many people suffer illnesses associated with these changes. This study focuses on the negative legacy, particularly dystocia.
Humans are primates and the closest genetically related species to the ape chimpanzee. Humans and chimpanzees differ by only approximately 2% in their genes but are distinguishable from each other in appearance, as there are significant differences in their anatomy. A distinctive feature of humans is that they are the only primate that walks upright on both legs. This is also a defining physical characteristic of humans. Specifically, the structures that play essential roles in walking, the pelvis, knees, and feet, are different. In 1924, only the skull of the Taung Child of Australopithecus was found, the first representative ape-man fossil discovered in South Africa. Here, the location of the foramen magnum in the occipital bone was the decisive factor in determining whether it was human. The foramen magnum of quadrupeds, including primates, is located posterior to the skull base, but that of humans is located in the middle of the base of the skull (Fig. 1). The foramen magnum of this fossil is situated in the central part, although it is an infant human bone. In addition, the pelvis of the Australopithecus afarensis, whose bones accounted for approximately 40% of the whole body, was discovered in 1974, confirming that the ape-men were walking upright on two legs and were closer to humans than the chimpanzee. The pelvis has remarkably developed the gluteus minimum and medias muscles that balance the left and right sides of the body, so it does not shake when walking on both legs. Also, the gluteus maximus supports the upright body, so these muscles are therefore well developed. The iliac wings of the hip bone become wider, and at the same time, they stand upright so that the pelvic organs can be inserted. Thus, when the hip bone is viewed from the front, it looks like an open morning glory flower. The point to note in human evolution is that the pelvis and legs have changed to a structure suitable for bipedal walking in the ape-man, however, the brain size was approximately 400cc, which is almost the same as that of gorillas and chimpanzees. Therefore, the first step to becoming a human being was not the development of intelligence but standing up. In other words, ape-man has the appearance of an ape, but its body shape is that of a human. An ape-man can be defined in simpler terms as a monkey standing up. Humans are thought to have originated approximately seven million years ago in Africa, where many fossils of ancient humans have been found. There was no brain expansion over the next few million years.
Humans freed up their hands while standing up. After that, no significant changes were observed for a while, but around 2 million years ago, humans began to feed on the bone marrow of dead animals in the grasslands of Africa and, by actively adopting a carnivore diet, secured enough energy to develop their brains, which required large amounts of energy. In this process, a stone tool for extracting bone marrow was invented. The adroit use of freed hands to make tools has paved the way for brain enlargement through increased stimulation and interaction with carnivores. Even though upright bipedal locomotion is at the root of human evolution, standing up had adverse effects. Several diseases are closely related to or caused by the upright position, extending from head to toe. Cerebral anemia, sleep apnea syndrome, aspiration, stiff shoulders, heart disease, gastroptosis, lumbago, dystocia, hemorrhoids, inguinal hernia, sciatic nerve, knee arthritis, lower extremity venous retention, and flat feet are diseases that plague modern humans that are closely related to the structure of the human body.
This review focuses on dystocia among the characteristics of the human body and the causes of the diseases.
Childbirth used to be very dangerous. In 1900, maternal mortality was 436.5 per 100,000 births in Japan, with 6200 deaths. It is no exaggeration to describe Japan in the 1900s as being in a state of war of pregnancy and childbirth for women, while the number of deaths due to traffic accidents in Japan exceeded 10,000, and it was called a war of traffic. Similarly, it would not be an exaggeration to say that the women were in a fertility war. Before modern medicine was established, the situation could have been worse. In Heikemonogatari, there is an expression, ’’It is said that 9 out of 10 women will surely die during childbirth.’’ which may seem an exaggeration. Considering that 11 out of 47 multiparous women who appear in Eigamonogatari, a historical tale from the Heian period, died during childbirth, the expression might have captured the state of mind at the time. Despite the rapid decline in pregnancy-related death rates since the end of World War II, which has led to childbirth being recognized as mostly safe, 60 to 70 pregnant women die each year in Japan, and unfortunately it is not completely safe yet.
One concrete evidence supports the notion that childbirth imposes a considerable physical burden on women. It is a bony depression or groove found in the hip bone called a pregnancy scar. In particular, the anterior auricular groove behind the iliac section of the hip bone is prominent (Fig. 2). This sulcus is frequently observed in middle-aged and older women, suggesting its association with childbirth. During childbirth, women try to move the pubic symphysis and sacroiliac joints, which are immobile, to widen their birth canal. It is believed that the bone at the attachment point of the ligament deforms as the number of births increases.
Why do humans develop dystocia? The main factors are body structure, and brain size adapted to upright bipedal locomotion. There are significant differences in the morphological structure of the birth canal between humans and other animals. A mammal’s birth canal can be described as a straight cylindrical tunnel. On the contrary, humans have a complex structure with a sharp L-shaped curve, cuboid entrance, and cylindrical exit (Fig. 3). In addition, because it became necessary to support the internal organs with the pelvis, the gluteal muscles developed significantly to enable upright bipedal walking, which differs from quadrupedal walking (Fig. 4). A wide ilium was required as the attachment. It is impossible to walk upright on both legs simply by changing the bones, as the muscles that support and move the body must also change. Even gorillas with muscular upper bodies have surprisingly small buttocks. The gluteus maximus muscles do not develop in individuals who do not always need to stand. In tetrapod, the base of the legs plays a crucial role in supporting body weight. In contrast, in humans, the pelvis is the central part of the body that supports the upper body and is positioned at the center of gravity. Furthermore, regarding the soft birth canal, human internal organs were stored in the pelvis by walking upright on both legs. In quadrupeds, the viscera sit on top of the stomach muscles like a hammock; therefore, there is no concern that they will slide down toward the anus, and the pelvic floor muscles are not well developed. However, human internal organs are bucket-like without a bottom plate. Muscles that support the internal organs from falling out of this “hole” were developed and blocked this hole. The pelvic floor is originally a muscle that tightens the anus but also hinders childbirth, and the development of this muscle is one of the causes of dystocia. In addition, at the exit under this hole are the sacrospinous ligament and the Sacro tuberous ligament, further hindering childbirth. Underneath these ligaments, the muscle of the urogenital diaphragm covers the triangular portion of the subpubic angle. In other words, a human fetus must pass through a netted cage of solid bones.
In addition to the pelvic structure, another significant difference between humans and other mammals is the relationship between the birth canal and the head size of the fetus. Most primates have a head size smaller than the diameter of the birth canal, whereas humans with enlarged brains are approximately the same. In other words, with an enlarged head, human fetus has pass through a curved birth canal which necessitates deforming its shape. Thus, human birth requires the fetus to change its posture when existing the birth canal.
Let us consider how humans have dealt with dystocia from the standpoint of fetuses, women, and the people around them.
The head of the fetus narrows its width by 4 cm to pass through the birth canal, which is about 10 cm wide. This is possible as the skull bones are not yet fused. The shape of a newborn’s head is often distorted rather than symmetrical. These distortions usually disappear immediately after birth. Naturally, the head of a newborn born by a caesarean section that does not pass through the birth canal has an oval shape with no distortion. An infant gorilla and chimpanzee are born with the same tight skull joints as an adult. Premature physiological birth is a characteristic of humans. As the human brain expands, it becomes increasingly difficult to give birth. To overcome this issue, human fetus deforms the skull when passing through the birth canal, even if it means giving up the role of protecting the brain.
As the human brain expands, so does the fetal brain. In the evolutionary process of growing discordance with the birth canal, one of the ways women responded was to widen the caliber of the birth canal and develop the birth canal with as few obstacles as possible. As a result, females evolved a long pubic ramus of the hip bone, a large sciatic notch angle, a vertically long rectangular sacrum in males, and a wide square in females. In contrast to the heart shape, females have a circular shape, males have the sacrum and coccygeal part curved forward, and females have a straight coccyx so as not to protrude in the direction of the birth canal.
Examining the pelvic bones, an expert can predict the sex of an individual with certainty. This is because, in the human body, the pelvic bone is where the sex difference is more explicit. In other mammals that give birth, there is no significant difference in the pelvis, and sex cannot be determined from the shape of the hip bone.
Why did women not change the shape of the pelvic bones to widen the birth canal until birth? If women changed their pelvises to the point where they could safely give birth, there would be differences in the ability of men and women to walk and run. In modern times, it may be possible to maintain society even if a significant difference in athletic ability between men and women exists. However, humans acquired bipedal locomotion more than 5 million years ago. At that time, people lived by hunting, gathering, and searching for food in the fields and mountains and were always moving. It is thought that the difference in athletic ability between men and women prevented them from maintaining a cooperative living.
Foragers and hunters like the Bushmen of modern Africa and even pregnant women go out searching for root crops until the last minute of childbirth. Humans have a difficult birth, but pregnant women can walk around just before giving birth.
In addition, women lose the sacroiliac joint ligaments and fibrocartilage of the pubic symphysis -which do not move in daily life- during childbirth, trying to make the size of the birth canal slightly more flexible to facilitate the passage of the fetus.
It is challenging for women to deal with childbirth alone, and many societies have caregivers and various childbirth methods. Studies on childbirth in populations worldwide show that different ethnic groups have different ways of giving birth. If childbirth were purely a biological and physiological phenomenon, it would be expected that the easiest childbirth method would be prevalent worldwide; however, there are many known childbirth methods. People’s trial and error can be seen as a symbol of human childbirth, which was difficult. It is not known precisely how pre-Homo sapiens gave birth, but it is believed that society dealt with it at the same time as the emergence of dystocia.
Most of the landscape paintings that deal with childbirth show people gathering around a woman giving birth and taking care of her. Some may argue that such paintings and records are difficult to preserve in societies where women give birth alone; however, in most communities, women do not give birth alone.
Many human societies have been familiar with childbirth for a long time since the start of upright bipedal walking. As humans spread over most of the earth, the cultural aspects of childbirth have increased and diversified in addition to physiological aspects such as method, place, and caregiver, depending on the environment and society. The physical structure of Homo sapiens, which was born 200,000 years ago, has remained unchanged. Until modern medicine was established, the alleviation of dystocia was a cultural accumulation that could be called the wisdom of life based on the human experience. For example, the vaginal delivery of breech babies, which any obstetrician in the 20th century was skilled at, would have been communicated in many societies, and the technique of returning breech babies to an upright position during gestation was also known. The methods of instruction may have been mother-to-child, group-experienced women, or local midwives.
In addition to the transfer of knowledge, the presence of friends who encourage each other during childbirth is also essential. The number of childbirths being attended by a husband has increased in the recently. Even though a husband is having minimal role during delivery, presence of the husband at delivery decreases the anxiety associated with the fear and pain of childbirth.
Birth attendants are also known to ease childbirth. In the latter half of the 1970s, in the United States, doulas, a Greek word meaning female servants, came to be used for women who helped mothers unable to produce milk and who eventually received the necessary training to attend childbirth. People who support women’s desire to give birth and help them adapt to actual childbirth have come to be established as professionals. In many societies, people like doulas are known, but people with extensive experience in childbirth are disappearing from around the world, especially in urban areas. Therefore, the need for people around mothers to encourage them has been increasingly recognized.
Humans experience dystocia due to bipedal locomotion. Fetuses and women have remodeled their bodies to eliminate dystocia but have yet to be able to solve it. Even today, with the advances of modern medicine, childbirth is a big event for women. Generous support from people and the environment is effective in alleviating dystocia. This warm thoughtfulness may have helped human evolution. Thus, the negative legacy of dystocia may have been necessary for humans.
