Human Birthing Process - A Scar of Evolution?

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To understand how certain biological features developed over time it is helpful to take an evolutionary prospective. While evolution is often incorrectly thought of as being a progression toward the most optimal or complex set of features it is rather influenced only by the qualities of random genetic differences that give one member of a species an advantage over another in reproduction. The individuals with such favorable qualities are more likely to survive than those without them and, therefore, these individuals will be more likely to pass those traits on to future generations. Examining the chain of such an evolution in our own past can help us understand how and why many of the features in the human anatomy developed as they did. It also allows us to recognize the reasons why some of the unfavorable features developed and have remained in our biology to this day. One set of such features are those that cause the human birth process to be relatively difficult and painful; a problem W.M. Krogman would most likely have referred to as a "scar of human evolution."

The difficulty and pain associated with human childbirth arises at the lowest level from the fact that a baby's head needs to be large to accommodate a large brain but it still needs to fit through the mother's pelvis that cannot be to big or bipedal locomotion will become inefficient. This represents a competition between two conflicting evolutionary pressures. A woman's pelvis should be as wide as possible to allow the passage of large-brained infants while at the same time it shouldn't be too large or bipeal locomotion will be inefficient. These problems combine to make the human birth process difficult and, until modern medical practices, sometimes life-threatening.

The birth process is made especially difficult due to the special maneuvering the infant must perform to fit through the tight birth canal. While the baby's head is basically round, the birth canal has a varying width along its length, meaning that the baby must twist while traversing it. It starts out facing to the mother's side and then turns ninety degrees when it reaches the pelvic inlet in the mother's birth canal where the transverse diameter of the canal is at its narrowest. In most other mammals and primates the infant can stay facing the same direction throughout the process. Even after such maneuvering, the skulls of babies often appear squashed for a few days after delivery as if a testament to how hard the process was.

Humans almost always need assistance in the birth process and they probably always have. It is reasonable to theorize that this is the case because through the process of human birth described above, the infant emerges with its face away from the mother. Normally, other primates, who give birth the baby facing them, can reach down and help the baby out with their hands by pulling on it. If humans were to do this they could severely injure the baby's back and spinal cord because of the tension they would place on the baby's spinal cord by forcing it to bend backwards towards their body.

The assistance that mothers received the birth process most often came in the form of a midwife until recent times. It is interesting to notice then that in the early development of a human beings as the brain was still becoming larger there was the potential for a sort of feedback loop to have developed. As larger brains were evolving, it is likely that mothers would need more assistance from others during birth, which required more socialization and intelligence, which, in turn, may have required a larger, more complex brain.

The whole process would be much easier if the pelvis was wider, but the width of the pelvis is constrained by the physical properties of the hip and the pivot it makes on the hip joint. In bipedal walking, when one leg enters swing phase, the muscles near the hip of the other leg must balance the weight that would normally rest on the first when standing. Simplified slightly this results in a first class lever, with the hip joint acting as the fulcrum and the body weight and the force exerted by the abductor muscles in the hip balancing each other on opposite sides of it. Each of these forces, however, are linearly scaled by their distance from the fulcrum (hip joint), so the force from the hip muscles are scaled by the rather short distance to the attachment of the abductor muscles on the femur and the body weight, which is taken to act at the middle of the body, is scaled by the much larger distance to the middle of the pelvis. This means that a larger pelvis to make the birth process easier would require this muscle action to produce a much greater force to counteract the increased torque of the body weight. Furthermore, such an increase in pelvis width would introduce a waddle into the walking which would be very inefficient.

This problem has resulted in a partial solution of sorts. The female pelvis is extended as wide as possible without making bipedal locomotion too inefficient and infants are born earlier so they are smaller at birth. This compromise requires more of the development of the brain to take place after an infant is born, which results in the requirement that human infants be extensively cared for after they are born for a much longer period than other mammals. This can only go so far, however, as there is a point where keeping the infant alive outside of the mother is extremely difficult even with today's modern medical knowledge.

Even with these adaptations, the birthing process remains difficult and painful, as evident by the fact that almost 20% of births are delivered by Caesarean Section in industrialized countries and the fact that many women that do give birth vaginally choose to use powerful painkillers. During birth many complications can arise, and, before modern medical expertise, both the mother and the baby were at great risk. For example, the infant must follow a precise sequence of moves to traverse the birth canal, and, if these steps are altered in any way, such as when an infant attempts to be born feet first, the chances that either the mother or infact will be killed goes up dramatically. Extensive damage can also be caused to soft tissues, including the possibilty of a tear from the vagina to the anus that can easily get infected if an episiotomy is not performed.

Despite the difficulties and, until recently, dangers of giving birth, it is evident that the selective advantages of having as efficient bipedal locomotion as possible and large brain are important enough to have overcome the selective disadvantage of a difficult birth. The continued competition between the pressure to have a relatively wide and narrow pelvis and the uneasy compromise that currently exists between them result in unfavoratble features that are definitely a "scar" of human evolution.