On a recent episode of the Dear Hank and John podcast (in which two brothers give “dubious advice”, I recommend it!) a listener sent in a question about why human infants were so useless. And even though both of them have children of their own, they didn’t really know what the answer was so they gave it their best shot: (paraphrasing here) because of bipedalism, women’s hips are too narrow and the baby needs to come out prematurely otherwise they won’t fit.
If you’ve read the earlier parts in this series debunking the Obstetrical Dilemma, you can imagine my epic head shaking and eye-rolling. I had already had this post waiting in the wings but I decided to rework the intro and title when I realized that lay people who have never heard the phrase “obstetrical dilemma” will forward it without even being aware of it. So, why are newborn humans useless?
In the myth of the Obstetrical Dilemma, female humans, with a choice between bipedalism or easy childbirth ended up with the worst of both worlds: they are inefficient walkers and nearly incapable of childbirth without lots of assistance and, even then, they have to give birth to developmentally premature babies so the head will fit. IF ONLY, the story goes, women had wider pelvises our offspring could spend more time in the womb and come out better developed. It was and is a sexist hypothesis (with surprisingly racist roots).
Part two of this series, The Myth of Childbearing Hips, I took on the foundations of the Obstetrical Dilemma. Research shows that the pelvic width has no effect on bipedal efficiency (Warrener, et al. 2015) and it isn’t a constraint on fetal head size (Epstein, 1973; Dunsworth, 2012). The role of the pelvis is to support the load of the torso, and in bipedal females, the fetal load during pregnancies, thus female bodies have wider pelvises than male bodies (Whitcome, 2007). It’s nothing to do with giving birth. Furthermore, natural selection has favored a narrowing of pelvic width in bipeds since the emergence of bipedalism and proportionally large newborns some four-million-years-ago– the combination is either beneficial or neutral in terms of survival. If it made giving birth a death sentence then the trait would have died out and pelvic width would have remained wider. And finally, newborn heads deform during birth because they can, not because they must. The bones of the skull are not fused until early adulthood long after the brain has reached full size. It’s not an adaptation just for birth but rather for rapid brain growth for the years after birth. Even proponents of the OD will admit that where heads squish, shoulders are rigid (Trevathan and Rosenberg, 2000) so the entire rationale of our babies being born prematurely because their brains didn’t grow enough in order to fit through a too-narrow maternal pelvis is just ludicrous.
Basically (tl;dr) the Obstetrical Dilemma is bullsh!t from start to finish. And that’s putting it sweetly… with a cherry on top even. Maybe even sprinkles.
… mmmm, nonpareils.
But I digress: (eh-hem) Useless newborns.
Precocity vs. Altriciality
The first thing to discuss is the developmental spectrum found amongst mammals, those vertebrate animals who give live birth and lactate. There are the ready-out-of-the-box babies, smugly galloping around, umbilical cord swinging, which are known as precocious. They are typically born with brains 33 – 59% of their adult size (DeSilva, 2016). Of course, I am exaggerating about the galloping (but not the smugness), they are a bit shaky and bewildered for a while after birth but rest assured their mothers are firmly nudging them to get up and start walking asap– they have to for survival. On the other end of the spectrum are animals like kittens or pandas, often born without hair or fur, their eyes and ears underdeveloped and still sealed shut, with brains anywhere from 2-28% of adult size.
Humans are considered precocial on some measures but altricial on others, which has been referred to as secondary altricity (Portman, 1990), as precocial development is considered the ancestral state of primates– our evolutionary ancestors were precocial– and the altricial elements were added on. Compared with chimpanzees who are born with 40% of their adult brain size (also known as an advancement factor), humans are born with closer to 28% of their adult brain size (Desilva, 2016). Humans win the Darwin Award for having the “lowest advancement factor of all the modern primates and the fifth lowest of any precocial mammal” outdone only by the wild boar, sambar deer, the nine-banded armadillo, and the coypu– assuming the measurements are accurate (ibid). Now, if that isn’t the stuff of erudite cocktail party banter, then I don’t know what– why yes… I am single, how could you tell?
However small our brains are at birth, human newborns are certainly not in need of extra gestation, in fact, compared with other more precocious (my spellcheck really wants me to say “precious” here) apes, humans gestate longer and give birth to proportionally larger newborns. Dunsworth’s research showed that humans are born when their mother’s metabolism just can’t keep up anymore (adorable lil’ parasites). Another theory as to why infants are born when they are is the limit of oxygen that can be carried to the ravenous fetal brain through the placenta. Only so much blood oxygen can supply both the mother’s body and the rapidly growing fetus. And in terms of triggering labor, mature fetal lung proteins have been labeled a potential candidate. When the fetus’s lungs are ready to breathe on their own, a signal is sent to the placenta to get the eviction underway (Gao, et al., 2015).
Based on regression modeling and fossil specimens, it is estimated that the Australopiths, which included A. Afarensis (the best known, and possibly first fully bipedal species) had brains 37.7% of adult size at birth. This is just under 10% more developed than modern humans newborns but it also marked a decrease from previous species of Ardipithecine whose infants were born with brains 40.6% of adult size– the same as modern non-human primates (DeSilva, 2011). This reduction in the advancement factor of the australopiths comes with the emergence of proportionally larger infants– the increase in size (of the infant-mother: mass ratio) from 2-3% to 5-6% may account for their offspring being born with less brain growth because the mother’s metabolism just couldn’t keep up.
So there are constraints to fetal size and development at birth but the pelvis has nothing to do with it. Instead of looking at fetal constraints and helplessness at birth as a negative thing, we should consider (and appreciate) the benefits.
Often, the comparison is between human newborns and our nearest relation, chimpanzees. For our offspring to be born as precocious as a newborn chimp, they would need to remain in the womb 16-18 months. Consider for a moment missing nine months of your infant’s life because they were in the womb, more importantly, consider what they would miss experiencing. Hint: a lot of learning…
Specifically, social learning. Physically we’re a pretty weak species, we need technology to survive and technology comes from culture, which is made up of other people. Being born with a wickedly clever mind and sharp senses but totally physically helpless ensures that human newborns are surrounded by people to help care for them, people they can learn about their culture and environment from. What Portman called an extrauterine spring (as in butterflies and sneezing until you have whiplash, not boing-boing). As a bonus, not being physically precocial helps human newborns focus energy, literally in terms of calories, on brain growth (unless your folks are into CIO, then you’ll spend your calories on existential terror).
To sum it all up: There is nothing wrong with the female body, or human gestation or birth, or with infancy. As a species, we gestate as long as we need to, labor and give birth in such a way as to draw caring people close to us, and our infants are born with a perfect mix of intelligence and helplessness to ensure round the clock care for social learning and rapid brain growth.
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DeSilva, Jeremy M. 2016. “Brains, Birth, Bipedalism, and the Mosaic Evolution of the Helpless Infant” in Costly and Cute: Helpless Infants and Human Evolution. Edited by Wenda R. Trevathan and Karen R. Rosenberg. New Mexico: University of New Mexico Press. 67-86.
DeSilva, Jeremy M. 2011. “A Shift toward Birthing Relatively Large Infants Early in Human Evolution.” Ed. C. Owen Lovejoy. Proceedings of the National Academy of Sciences of the United States of America 108.3, 1022-027. PNAS.
Dunsworth, Holly M., Anna G. Warrener, Terrence Deacon, Peter T. Ellison, and Herman Pontzer. 2012. “Metabolic hypothesis for human altriciality.” PNAS 109:38, 15212-15216; DOI:10.1073/pnas.1205282109
Dunsworth, Holly M. 2016. “The Obstetrical Dilemma Unraveled” in Costly and Cute: Helpless Infants and Human Evolution. Edited by Wenda R. Trevathan and Karen R. Rosenberg. New Mexico: University of New Mexico Press. 29-50.
Epstein, Herman T. 1973. “Possible metabolic constraints on human brain weight at birth.” The American Journal of Physical Anthropology 39:1, 135-136. https://doi.org/10.1002/ajpa.1330390114
Lu Gao, Elizabeth H. Rabbitt, Jennifer C. Condon, Nora E. Renthal, John M. Johnston, Matthew A. Mitsche, Pierre Chambon, Jianming Xu, Bert W. O’Malley, Carole R. Mendelson. 2015. “Steroid receptor coactivators 1 and 2 mediate fetal-to-maternal signaling that initiates parturition.” Journal of Clinical Investigation. DOI: 10.1172/JCI78544
Portman, Adolf. 1990. A Zoologist Looks at Humankind. Translated by Judith Schaefer. Chicago: Columbia University Press.
Trevathan, Wenda and Karen Rosenberg. 2000. “The shoulders follow the head: postcranial constraints on human childbirth.” Journal of Human Evolution 39, 583-586.
Warrener, Anna G., Kristi L. Lewton, Herman Pontzer, and Daniel E. Lieberman. 2015. “A Wider Pelvis Does Not Increase Locomotor Cost in Humans, with Implications for the Evolution of Childbirth.” PLoS ONE 10:3, e0118903.
Whitcome, Katherine K., Liza J. Shapiro, and Daniel E. Liberman. 2007. “Fetal Load and the Evolution of Lumbar Lordosis in Bipedal Hominins.” Nature 450, 1075-1080.