The Evolution of Human Birth

Glenn R. Morton (
Thu, 25 Jun 1998 22:21:12 -0500

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 anteropposterior 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. 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). This would represent a level
of care and concern rarely seen in other apes and monkeys although
occasional reports of animal midwifery ave 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
availabe, 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. During

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 someplaces, 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 Homo erectus 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.

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.


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