View Full Version : 'Human Birth: An Evolutionary Perspective' by Wenda R. Trevathan

Thursday, May 19th, 2005, 12:16 PM
The Evolution of Human Birth

By Glenn R. Morton

I just finished a wonderful book Human Birth: An Evolutionary Perspective by Wenda R. Trevathan. She is an anthropologist at the University of New Mexico. This post is somewhat of a book review and will report the latest thinking on the evolution of human birth patterns. I want to state up front that Prof. Trevathan is a woman and thus is much more knowledgeable than I in human birthing. I state this because, as a man, I can be accused of not knowing what I am talking about when it comes to human birth. The last human birth I witnessed was 19 years ago and being in a state of excitement, my observations were not very scientific. Thus I will quote extensively from Trevathan's books so that she can be the advocate of her views. The interest in this topic to the Christian apologist concerns the earliest advent of human care and concern as well as the advent of pain in child birth. Females helping other females birth their offspring, is a human type of care that is only rarely seen in other primates whereas, until the advent of modern times with the appearance of male obstetricians, it was nearly a universal human trait. The pain in child birth is caused by two conflicting evolutionary needs. Bipedalism requires that the legs be close enough together so that the person can walk without a waddle. But this causes the pelvic opening to be small. Intelligence requires large brains and thus large cranial sizes. These two conflicting features lead to the tight fit of the infant through the birth canal. Trevathan's thesis is that problems relating to human deliveries produced selective pressures which led to the nearly universal human practice of midwifery. This is not to say that midwifery is an absolute necessity. Women have for millennia delivered babies all by themselves with absolutely no assistance. But as Trevathan points out,

"There are also mechanical or physical reasons that having assistance at birth has been favored in our species. These include: the close correspondence between fetal head size and maternal pelvic size brought about by bipedalism and encephalization; the risks of injury to and infection of the perineal area resulting from too rapid delivery of the head and shoulders; the tendency for the fetus to present the occiput in a position anterior to the mother's pubic symphysis, risking injury and paralysis in the infant if it is brought forward too rapidly by the mother herself; and the helplessness of the neonate and associated greater difficulties in establishing respiration, nursing, and thermoregulation. Despite all of these difficulties, women can and have given birth unassisted for millennia, but mortality increases significantly in those cases." ~ Wenda R. Trevathan, Human Birth, (New York: Aldine de Gruyter, 1987), p. 109

The increase in mortality is what drove humans to use midwives in all cultures. For instance, Trevathon says that breech deliveries occur in about 3-4% of all modern human births and that unassisted most of them would result in infantile death. There are numerous problems delivering breech babies. Trevathan notes:

"There are numerous problems associated with breech presentations in humans, many of which are probably common in nonhuman primates as well. The buttocks, feet, or knees are poor dilators of the cervix, and, thus, labor and delivery are often prolonged. A common problem is that women feel the urge to push before the cervix has dilated completely. This could result in the fetal head being held too long at the cervix. "Early rupture of the membranes is also common in breeches, and since the fetus does not fit fully in the pelvic brim, prolapse of the umbilical cord may result. Probably the greatest danger is that the head cannot be easily compressed in a breech delivery, and it may be held too long by the pelvic outlet, resulting in hypoxia in the fetus. When the fetus experiences the rapid cooling of its lower body upon emergence, attempts at breathing may begin; asphyxia and aspiration of amniotic fluid are likely outcomes. This is apparently what often happens in breech deliveries in nonhuman primates." ~ Wenda R. Trevathan, Human Birth, (New York: Aldine de Gruyter, 1987), p. 92

Given the above, she concludes:

"Given the skill required in delivery and the maneuverings that are necessary for successful birth of a breech, it seems highly unlikely that a woman could deliver a breech entirely alone. I would suggest that mortality has been close to 100% for unassisted breech deliveries. This in itself would argue for selection favoring assistance at childbirth and, if there is a genetic component to breech presentations, powerful selection against that tendency." ~ Wenda R. Trevathan, Human Birth, (New York: Aldine de Gruyter, 1987), p. 94

Other problems that can arise in unassisted births is Shoulder dystocia, where the shoulders of very large infants become stuck in the birth canal. (p. 23) In such cases, if the shoulders are not quickly freed after the head emerges, death is the likely outcome. And even if the baby is freed, it can result in broken bones or paralysis of the baby's limbs. Shoulder dystocia occurs with a frequency of around 1%.

With these problems, how did humans evolve to the point where such care and concern were required to ensure safe entry into the world? Apes do not have the problems humans have. They don't walk bipedally very often and their pelvic openings are relatively larger. She says,

"At the hominid-pongid divergence, two different adaptive strategies developed that had an effect on parturition. The pongids embarked on a strategy that emphasized increased adult body size, although the selective pressures operating ont hat did not simultaneously favor increases in neonatal size. The result was a large pelvis in a large body, a neonate that was thus relatively small, and easy parturition." ~ Wenda R. Trevathan, Human Birth, (New York: Aldine de Gruyter, 1987), p. 22

Because of this, apes still give birth with babies facing the mother.

"In nonhuman primates, the fetus usually emerges with its face toward that of its mother. She may then reach down and pull it up toward her along the normal flexion of its body. In other cases, the infant may pull itself out of the birth canal by climbing up along the mother's abdomen. If the occiput emerges in an anterior position, with the face away from the mother, she will tend to pull the infant backward, risking injury to it in the process. All other things being equal, it is therefore advantageous for an infant to emerge facing its mother if she is likely to use her hands in pulling the body out. "In humans, however, all other things are not equal: The close equivalence of cephalopelvic dimensions has resulted in the usual process of an infant being born facing away from its mother. In this position, the use of her own hands to assist delivery before the shoulders have emerged could result in pulling the infant against the normal flexion of its body, again with the risk of injury particularly to the nerves of the neck." ~ Wenda R. Trevathan, Human Birth, (New York: Aldine de Gruyter, 1987), p. 89-92

When do we see the 'human' pattern of birth presentation? With the Australopithecines, we at least see a transitional pattern. Some authorities believe that the Australopithecines gave birth with the fetus facing the side, others think they had a human birth pattern.

"One proposal is that the australopithecine fetal head entered the pelvic basin in a transverse position and rotated to an anteroposterior position to exit, as it does in normal human birth today. Another extremely narrow transverse dimensions of both the inlet and outlet, remained in that position for emergence. Regardless of which pattern was actually followed, the series of rotations or twisting of the neck that would have been necessary for the shoulders to emerge would place additional stress on the fetal neck and spine and may have led to higher mortality. Only with increased attention and assistance on the part of the mother could infants be delivered without injury." ~ Wenda R. Trevathan, Human Birth, (New York: Aldine de Gruyter, 1987), p. 222

Thus, the first selective pressure for mid-wifery would have occurred in Australopithecus. Of course we will never know if they helped each other give birth, but they could have used the help. But when it comes to Homo erectus, Trevathan argues that they, like us, engaged in obligate midwifery. She says.

"This remodeling [of the pelvis-grm] likely reflected further modifications for efficiency in bipedal locomotion and pressure to alter the birth canal for delivering neonates that had larger brains than those of their predecessors. I will argue later that it was at this point that assistance at childbirth made a critical difference in mortality and morbidity for Homo mothers and infants. Not only was parturition more difficult, but the genus became encumbered with a unique need of obligate midwifery. This need was further intensified with encephalization in Homo erectus and Homo sapiens." ~ Wenda R. Trevathan, Human Birth, (New York: Aldine de Gruyter, 1987), p. 29

Walker and Shipman (The Wisdom of the Bones, p.223-228) notes that, like modern humans, Homo erectus tripled its brain size from birth to adulthood. The size of the erectus pelvis relatively would have required as much help in birthing as is required by modern humans.

And what is amazing, data that has become available since Trevathan published her book may imply the same thing for Homo rudolfensis, the first member of the genus Homo. Homo rudolfensis is first found at 2.4 myr ago in strata from Malawi. Steven Stanley used skeletal data from rudolfensis to estimate the rate of brain growth in this species. The male of this species that was used in the analysis was 5 foot 8 inches tall and weighed around 132 pounds. The height is about average for a modern American male even if the weight puts him on the skinny side. Stanley wrote:

"To estimate the birth weight of a male Homo rudolfensis, we must follow a more devious path. We first turn the owner of the fossil pelvis into a female by adding 3 percent to the breadth of the pelvic inlet (the average difference between modern men and women). After his sex change, in our analysis the owner can stand in for the woman who bore him. Even this liberally augmented pelvis is much narrower than that of an average modern woman, however, and from the degree of difference we can estimate that an average male baby of the extinct species would have tipped the scales at only 2.1 kilograms (4.6 pounds) compared to the 3.46 kilograms (7 pounds) for a Caucasian male in present-day America. Thus the pelvis provides estimates of the weight of the male Homo rudolfensis both in adulthood and at birth. Less directly, with application of the 10 percent rule for newborn primates, it also gives the animal's brain weight at birth." Steven M. Stanley, Children of the Ice Age, (New York: W. H. Freeman, 1998), p. 161-162

The 10 percent rule is the observation that among primates the brain weighs about 10 percent of the total birth weight. This means that rudolfensis was born with a brain around 210 cc. The adult H. rudolfensis had a cranial capacity of around 760-900 cc so obviously he more than tripled his brain size from birth to adulthood as did Homo erectus. Thus, it appears that relatively speaking rudolfensis would have the same difficulty in giving birth as do modern humans and H. erectus.

While Trevathan wrote prior to the discovery of this information, she did anticipate this data via other means. She believes that midwifing has taken place for more than a million years in the human lineage (p. 110). And given the information about H. rudolfensis, I would suggest that if midwifery was needed for H. erectus, then it was quite probable that it was needed for H. rudolfensis. In such a case, midwifery would go back at least to 2.4 million years. This would represent a level of care and concern rarely seen in other apes and monkeys although occasional reports of animal midwifery have been received. But in no animal species is midwifery as widespread as it is in humans.

But there is another aspect of human birthing which sets us apart. That is the relation between mother and infant. The origin of bipedalism altered the way infants were cared for. Chimpanzees have hands and feet which are capable of grasping objects. The opposability of the chimpanzee big toe is great enough that the chimpanzee infant can hold onto the fur of its mother by means of four attachment points-both hands and both feet. One of the photos in Jane van Lawick-Goodall, (In the Shadow of Man, following p.106) shows an infant chimpanzee holding on to an older sibling in just this fashion. The carrier of the infant chimp has no need to use her own hands and can walk around in a normal chimpanzee fashion. But this may have been different with Australopithecus. Australopithecus possessed a foot which was evolved for bipedal locomotion. While it was not exactly identical with modern human feet, it did not have an opposable big toe. This means that if an Australopithecine infant was carried around as a chimp is, by clinging to mother's fur, the australopithecine child was disadvantaged by only having two attachment points-both hands. The bipedal foot probably made it necessary for the Australopithecine mother to carry her infant rather than have the infant attached itself to her by the clinging reflex. Trevathan relates,

"Because of their bipedal foot, infants had even more difficulty clinging to their mothers. But, despite the fact that it made clinging virtually impossible, bipedalism was advantageous for carrying infants, because the hands were not necessary for locomotion." "In summary, the major challenges to parturition faced by the first hominids were those imposed by the evolution of bipedalism. This made the birth process even more difficult than for most primates and required greater skills on the part of the mother in assisting the delivery of the infant and in caring for it in the first days of its life. These initial challenges were obviously met successfully by some female hominids, whose descendants survived to meet the greater challenges that bipedalism imposed on hominid evolution in placing limits on the size of the neonatal brain." ~ Wenda R. Trevathan, Human Birth, (New York: Aldine de Gruyter, 1987), p. 223

The need to carry the infant around altered the nature of breast feeding. This eventually led the females to the invention of a baby-friendly technology. Trevathan tells us,

"A characteristic that evolved in our ancestors, who delivered precocial young, is a milk content appropriate for infants who are in more or less constant contact with their mothers and can nurse whenever they want. This places upon hominids yet another phylogenetic constraint: not only does the milk composition fit the needs of an infant for whom the breast is always available, but the protein and fat content is so low that the breast must always be available for proper growth and development of the infant." "Thus, early Homo mothers could not leave their altricial infants behind in a nest while they foraged during the day. No longer could infants cling at all to their mothers, and yet the composition of milk remained such that infants had to be in continuous contact with their mothers. Selection favored those mothers who devised ways to keep their infants with them and yet were able to continue to exploit food resources efficiently. Eventually, hominid females developed the sling for carrying their young, enabling them to gather food with both hands in order to meet their own needs and the needs of those dependent on them." ~ Wenda R. Trevathan, Human Birth, (New York: Aldine de Gruyter, 1987), p. 223-224

But by carrying the infant, the maternal-infant relationship was deepened in a way that did not occur in the primate birthing pattern.

The development of the two patterns of human birth, midwifery and constant nursing eventually gave rise to a population increase. Throughout most of the Paleolithic, human populations grew at no more than .001 to .003% per year (p. 228). Women only gave birth once every four years. Given a life expectancy of around 25 for women and sexual maturity at 16, this would mean 2-3 offspring per lifetime. And this pattern remained until the time of the agricultural revolution when nutrition and societal structure changed.

"Increased consumption of carbohydrate-rich foods, decreased mobility, and nursing at infrequent intervals all interact to make this possible, enabling women to conceive within 10-15 months of the last birth. Weaning earlier is made possible by the availability of appropriate infant foods in the form of cereal grains and, in some places, milk from domesticated animals. Ultimately the birth interval is reduced to approximately 2 years resulting in population increase." ~ Wenda R. Trevathan, Human Birth, (New York: Aldine de Gruyter, 1987), p.230

Such a change enabled women to have 5-6 or more offspring per life time creating a population explosion in agricultural times.

What conclusions can we draw from the above? First, this is one more deductive chain of reasoning showing that early Homo displayed human-like behavioral traits. If midwifery were common among Homo erectus as Trevathan believes, it places an entirely new perspective upon the care and concern given by erectus to others of their species. It also implies that 'pain in childbirth' is not unique to Homo sapiens but was shared by erectus and quite possibly, rudolfensis.

Secondly, the probably need for Australopithecus to carry its infants around, is equally pregnant with implications. Did they also require midwifery to lower mortality during birth? Did they carry their infants in a human-like pattern? Did they care for members of their species in a way that apes didn't? We can't say given today's evidence, but the possibility is there.