Sunday, October 9, 2011 - 1 comments

The Evolution of Human Birth : 6. Growing Bigger Brains

By : Karen R. Rosenberg and Wenda R. Trevathan

IF BIPEDALISM ALONE did not introduce into the process of childbirth enough difficulty for mothers to benefit from assistance, then the expanding size of the hominid brain certainly did. The most significant expansion in adult and infant brain size evolved subsequent to the australopithecines, particularly in the genus Homo. Fossil remains of the pelvis of early Homo are quite rare, and the best-preserved specimen, the 1.6-
million-year-old Nariokotome fossil from Kenya, is an adolescent often referred to as Turkana Boy. Researchers have estimated that the boy’s adult relatives probably had brains about twice as large as those of australopithecines but still only two thirds the size of modern human brains.
BABY BORN FACING FORWARD makes it possible
for a monkey mother to reach down and
carefully guide the infant out of the birth canal.
She can also wipe mucus from the baby’s face
to assist its breathing.

By reconstructing the shape of the boy’s pelvis from fragments, Christopher B. Ruff of Johns Hopkins University and Alan Walker of Pennsylvania State University have estimated what he would have looked like had he reached adulthood. Using predictable differences between male and female pelvises in more recent hominid species, they could also infer what a female of that species would have looked like and could estimate the shape of the birth canal. That shape turns out to be a flattened oval similar to that of the australopithecines. Based on these reconstructions, the researchers determined that Turkana Boy’s kin probably had a birth mechanism like that seen in australopithecines.

In recent years, scientists have been testing an important hypothesis that follows from Ruff and Walker’s assertion: the pelvic anatomy of early Homo may have limited the growth of the human brain until the  evolutionary point at which the birth canal expanded enough to allow a larger infant head to pass. This
assertion implies that bigger brains and roomier pelvises were linked from an evolutionary perspective. Individuals who displayed both characteristics were more successful at giving birth to offspring who survived to pass on the traits. These changes in pelvic anatomy, accompanied by assisted birth, may have allowed the
dramatic increase in human brain size that took place from two million to 100,000 years ago.

Fossils that span the past 300,000 years of human evolution support the connection between the expansion of
brain size and changes in pelvic anatomy. In the past 20 years, scientists have uncovered three pelvic fossils of archaic Homo sapiens: a male from Sima de los Huesos in Sierra Atapuerca, Spain (more than 200,000 years old); a female from Jinniushan, China (280,000 years old); and the male Kebara Neandertal—which
is also an archaic H. sapiens—from Israel (about 60,000 years old). These specimens all have the twisted pelvic openings characteristic of modern humans, which suggests that their large-brained babies would most likely have had to rotate the head and shoulders within the birth canal and would thus have emerged facing
away from the mother—a major challenge that human mothers face in delivering their babies safely.

The triple challenge of big-brained infants, a pelvis designed for walking upright, and a rotational delivery in which the baby emerges facing backward is not merely a contemporary circumstance. For this reason, we suggest that natural selection long ago favored the behavior of seeking assistance during birth because
such help compensated for these difficulties. Mothers probably did not seek assistance solely because they predicted the risk that childbirth poses, however. Pain, fear and anxiety more likely drove their desire for companionship and security.

Psychiatrists have argued that natural selection might have favored such emotions—also common during illness and injury—because they led individuals who experienced them to seek the protection of companions, which would have given them a better chance of surviving [see “Evolution and the Origins of Disease,” by Randolph M. Nesse and George C. Williams; Scientific American, November 1998]. The offspring of
the survivors would then also have an enhanced tendency to experience such emotions during times of pain or disease. Taking into consideration the evolutionary advantage that fear and anxiety impart, it is no surprise that women commonly experience these emotions during labor and delivery.

Modern women giving birth have a dual evolutionary legacy: the need for physical as well as emotional support. When Sophia Pedro gave birth in a tree surrounded by raging floodwaters, she may have had both kinds of assistance. In an interview several months after her helicopter rescue, she told reporters that her mother-in-law, who was also in the tree, helped her during delivery. Desire for this kind of support, it appears, may well be as ancient as humanity itself.

* * *

KAREN R. ROSENBERG and WENDA R. TREVATHAN bring different perspectives to the study of human birth. Rosenberg, a paleoanthropologist at the University of Delaware, specializes in pelvic morphology and has studied hominid fossils from Europe, Israel, China and South Africa. About 15 years ago she began studying the pelvis as a way to reconstruct the evolution of the birth process. That’s when she met Trevathan, a biological anthropologist at New Mexico State University, whose particular interests include childbirth, maternal behavior, sexuality, menopause and evolutionary medicine. Both authors have experienced birth firsthand: Rosenberg has two daughters, and Trevathan is trained as a midwife.


Linda January 3, 2012 at 8:31 AM

"a rotational delivery in which the baby emerges facing backward is not merely a contemporary circumstance. For this reason, we suggest that natural selection long ago favored the behavior of seeking assistance during birth because such help compensated for these difficulties."

While the head does (usually) come out facing the mother's back, it actually turns to the side before the body emerges. If the mother is in an upright position (for instance, kneeling) all she needs to do is position her hands under the baby to guide it forward. It is really not difficult at all. Also, childbirth assistance for most of the history of humankind has not been routine manual assistance (otherwise known as obstetrics.) There must have then been other compelling reasons for it -- emotional support, as you mention, and possibly also as a social ritual that symbolically tethered the mother and baby to the tribe. Michel Odent theorizes also that because so many birthing practices actually disrupt the normal hormonal process, that there must be an evolutionary advantage to that (or was.)

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