Tuesday, August 11, 2020

Human babies are not born early, humans are not altricial, and human pregnancy is not shortened or truncated. Therefore, there is no obstetrical dilemma.

Hi Everybody,

Want to hear something funny?

Three years ago, when I wrote “There is no obstetrical dilemma” (OD) I thought that was the last time I’d have to write about the obstetrical dilemma. I relished writing that piece, but I truly believed I was writing the OD out of existence, for me. Killing it off, from my research program. Leaving it to others to carry on debating, testing. Freeing up my brain for researching other things.  After all, I’d been intensively working on the OD since 2007.

But who was I kidding? Me! Myself! Just me! It’s only me here! I work in an undergraduate program. I have no graduate students or post-docs to carry the torch.  I’m stuck holding it.

So, despite writing my final article on the OD in 2018, I’ve since written another one for a very cool anthropology volume coming out next year. (In it, I really go to town on this whole “early birth” misconception and can’t wait for that to be published.) And despite the OD being dead to me, I somehow piloted a fun and expensive study of marmoset pregnancy and lactation energetics, as a test of non-OD ideas, but which sadly failed to produce usable data. And, of course, because this is how it works, I’ve continued to receive OD manuscripts to review, many of which don’t even cite the damn 2018 paper, and none of which took up my argument, or arrived there independently, about how if there is no solution, then there is no dilemma in the first place.

The obstetrical dilemma is a beloved hypothesis which (I don’t believe) explains difficult childbirth and helpless babies. The OD says that as hominins evolved larger and larger brains, selection for bipedalism constrained the bony birth canal, which is a dilemma that was solved by birthing babies early to escape while they were small enough, but just barely. (“Obstetrical dilemma” as a term is often used as a synonym for the tight fit between a human birther’s pelvis and neonate, and for the difficult birth that ensues, but even when the term is applied in those ways, the OD hypothesis is almost always assumed. That is, when researchers apply that term to childbirth difficulty, underneath that is the notion that humans are born early.)

[the following grafs are from Dunsworth 2018]

To appreciate how far OD-thinking has spread beyond the academy, we can read the popular science literature where, for example, Meredith Small wrote in Our Babies Ourselves (1999) that “women couldn’t walk” if the birth canal were widened to accommodate a more developed neonate. In Paleofantasy (2013) Marlene Zuk penned, “You can’t give birth to large-brained infants and also walk on two legs trouble-free…” And there is the vast influence of Harvey Karp’s “Happiest Baby” enterprise, where he advises parents to treat their newborns like fetuses, asserting that human babies are ‘evicted’ early. To further demonstrate the reach of the OD, we can listen to the opening verse of the title track to Father John Misty’s 2017 Grammy-nominated album “PureComedy”:

The comedy of man starts like this:
Our brains are way too big for our mother’s hips
So, nature, she devised this alternative:
We emerge half-formed and hope whoever greets us on the other end
Is kind enough
To fill us in
And, babies, that’s pretty much how it’s been ever since

OD thinking is everywhere, and I helped with that. As a graduate student at Penn State I taught this narrative to my students while I was also enshrining it in a small reference volume, Human Origins 101 (2007), which I was writing alongside my dissertation. About our narrow-hipped direct ancestors of the species Homo erectus, I wrote how they “may have shortened gestation (i.e. the period of fetal development in the uterus) [in order] to be physically capable of giving birth to large brained babies through [their] relatively small birth canal. An earlier birth results in a more helpless, less developed, altricial infant. “ (p. 139) After that passage I listed all the significant aspects of being human that may have evolved as consequence of the evolution of this hips-induced earlier birth: Paternal investment, food-sharing, home bases, loving adult relationships, free time, elaborate language, singing, music, wit, dancing. Wow. 

[end of material from Dunsworth 2018]

As OD thinking goes, there’s a dilemma (big brains versus bipedalism) and it’s been solved (early birth). But without that solution, then where’s the dilemma? I think it's nowhere.

We have big brains.

We are bipedal.

We give birth to big babies whom we care for intensively.

Childbirth typically sucks, despite it being the most common “best day of my life." Oh, wow. Yes it was!

And the evolution of big brains, big babies, and bipedalism must have something to do with that.


We did not shorten pregnancy/gestation.

We are not born early.

[the following grafs are from Dunsworth 2018]

[When I began investigating the OD,] I wanted to see for myself was how short our pregnancies are compared to other primates. I found very easily that they are not. Our pregnancies are as dreadfully long as chimpanzees’, bonobos’, gorillas’, and orangutans’ and even a bit longer. Of all the primates, the great apes have the longest pregnancies (ranging across species from roughly 30-39 weeks; Animal Diversity Web), and among them humans lie at the long end, with maybe a few weeks more. This long, not truncated, human pregnancy defied my OD expectations and sparked my doubt of the entire story. 

When you read the scientific literature that covers birth-related variables across primates, humans don’t stick out as strange save for four traits. First, there is that detail about us having the longest pregnancies. Second, we are the fattest baby primate (Kuzawa 1998) and this contributes to our absolutely largest neonatal size. Third, we are born with the absolute largest brains of all baby primates. And, fourth, for a baby primate, we are relatively small-brained at birth (DeSilva and Lesnik 2006; for all four traits see also Dunsworth et al. 2012). Notice how a fifth trait is not the tight fit at birth. There are monkey species that have tight fits at birth.

Despite having absolutely long gestations, large bodies, and big brains at birth, the relative view of neonatal brain size is what convinces people that we are born early and seduces them over to the OD. By “relative” brain we are talking about newborn brain size divided by adult brain size—that is, the proportion of the adult brain that a baby has at birth and, by extension, how much of a brain the baby will need to grow.  Born with the smallest relative brain size, roughly only 30% of our adult brain size, it necessarily follows that humans have the most postnatal brain growth to accomplish of all the primates. But why should the perinatal proportion of our adult brain size be evidence that we’re born early?

One answer dominates the thinking but it is not a perspective I share any longer. It is this: Chimpanzees are born with 40% of adult brain size, eclipsing our mere 30%.  This difference requires an explanation. And so, the thinking goes, humans should be born with just as much growth accomplished towards their adult-sized brain as those apes, but that we are not means that something must be preventing it. Tradition assumes that something is the bipedal pelvis.

On his website for The Happiest Baby, Harvey Karp explains how, “I always tell my patients that babies are born too soon” and how, “your baby’s brain was so big that you had to ‘evict’ her after 9 months, even though she was still smushy, mushy and very immature. As a result, she isn’t quite ready for the big, bad outside world. So, for the first months, it can help to think of her like a fetus…outside the womb.” This is the basis for the “fourth trimester” concept he uses to advise parents in how to care for their newborns. This is very much out of the academic tradition that emphasizes, despite the absolute large, “so big” size of the newborn human brain, how relatively small it is—a focus that has been strongly influenced by classic works of Adolph Schultz (1949), Adolf Portmann (1969), and Stephen Jay Gould (1977), whose book Ontogeny and Phylogeny was read beyond anthropological and human evolutionary biology circles. [In his collection of essays, Gould even described human babies as embryos.] In the tradition of their great influence folks continue to assume that humans really should be gestating our fetuses longer.

Portmann even described humans as “secondarily altricial,” a term that has long populated lists of uniquely human traits. Primates as an order are precocial. For an example of a typical, and extreme, precocial mammal, “consider the horse,” as Mr. Foster in Huxley’s Brave New World said. ‘Precocial’ describes how horse foals and primate infants are far more developed at birth than species on the other side of the spectrum dubbed “altricial,” like most carnivore and rodent species—which are extremely helpless as newborn pups and cubs, usually furless, blind, parked in a den or nest, and incapable of clinging to their mothers except to suckle. Deeming humans “secondarily altricial” suggests we share significant traits in common with wolves and rats to hold us apart from the rest of the primates and that the Homo lineage has reverted back to a deeply ancestral altricial condition after a precocial phase in our more recent primate ancestry. And, what was powerful enough to cause this major, unique evolutionary shift in human evolution towards altriciality? A pelvic constraint due to bipedalism, so the OD thinking goes.

But, by having the largest adult brain of all the primates, doesn’t it just make sense that we would be born with the smallest relative brain size, regardless of the pelvis? Maybe it does not now, but it will after a closer look at other primates.

Chimpanzees and bonobos (closely related apes of the genus Pan) have the largest adult brains and the smallest relative brains at birth out of all the nonhuman primates. Born with roughly 40% of their adult brains, as mentioned above, chimpanzees have the most postnatal brain growth to accomplish of all the nonhuman primates. What is the explanation? Not the OD. Chimpanzees do not have a tight fight between bony birth canal and neonate and they are not habitually bipedal. But, of all the primates except for humans chimpanzees are also the most helpless as infants (they are intensely coddled by their mother because they cannot strongly cling to her, cannot walk independently, and are only active for a small portion of the day). With only 40% of their brain growth achieved at birth, they have the longest period of postnatal brain development of all the nonhuman primates. Those circumstances are significant “solutions” to the OD for humans, but there is no special explanation for them in our ape relatives. No one to my knowledge is suggesting chimpanzees are born “early.”  No one is suggesting that, given the roomy birth canals, they should be born later when they’re more developed and easier to care for.  No one is suggesting that they should be born with more brains, or that they should be born with 50% of adult brain size like capuchin monkeys are. No one is offering up an elegant hypothesis for chimpanzee gestation length and infant helplessness that is unique to their lineage’s evolution, and that conveniently links up to bony anatomy that fossilizes so the hypothesis can extend back to scientific interpretations of relics from their ancient past.

For humans to mimic chimpanzees and birth our babies with 40% of their adult brain size, we would need to lengthen gestation seven more months to a pregnancy of 16 months. At seven months of age, we have 40% of our adult brain size. [Past estimates by Portmann, and then echoed by Gould put our pregnancy at 21 months! But based on updated knowledge of neonatal brain growth in chimps and humans, 16 months is a better number (DeSilva and Lesnik 2006).] Could our pelves accommodate this slightly larger infant head, with its 3-4 cm increase in diameter? It is difficult to say with certainty. However, women already vary by this magnitude in dimensions of the bony birth canal and no one has correlated this to meaningful variation in their walking or running ability. Further, no one has demonstrated that increasing the present average in bony birth canal dimensions by 3-4 cm would ruin bipedalism. While many reactions I’ve received to this thought experiment highlight the very real trouble our broad neonatal shoulders and large neonatal body size can cause, the point is to shine light on the weak assumptions of the OD.

The simple act of searching what is known and what is unknown about the very simple, seemingly straightforward assumptions and assertions in the obstetrical dilemma hypothesis convinced me that it is flawed. If OD thinking sees the shortest kid in class as a unique biological circumstance, then I now saw her as being short for basically the same reasons as the next shortest kid in class. Human gestation is much more like other primates’ and other mammals’ than OD thinking had led me to believe—a realization which led me to doubt our pelvis was a unique influence on its duration.

As I pored over published charts of primate and placental mammal pregnancy length I learned how it scales nicely with maternal body mass. The larger the mother’s body, the longer the pregnancy, which explains why the great apes have the longest pregnancies of all the primates. Body mass is often a proxy for metabolic rate, which factors greatly into both enabling and constraining a species’ average gestation length and fetal growth. I was delighted to see that maternal mass was just as fundamental to pregnancy in whales and dolphins which lack bony birth canals (Sacher and Staffeldt 1974).

What jumped out to me was that maternal-fetal physiology is the primary constraint on placental mammal gestation and fetal growth, including the construction of costly brains. That constraint in humans is not reached until we grow our fetuses right up to the size of the bony birth canal, which is usually just big enough.  In other words, the tight fit at birth makes it seem like we are stuck in this uniquely human obstetrical dilemma, when really, we are just basically doing what placental mammals do—albeit with an often terribly laborious labor at the end of it.

With our relatively small brains at birth human newborns are given an “early” introduction to the world. But with our absolutely large brains and bodies at birth and our absolutely long pregnancies, surely our gestation was not cut short. [If humans are exceptional then perhaps it is our souped up metabolisms compared to our closest relatives (Pontzer et al., 2016).] And, surely our pregnancies were ending due to the fundamental metabolic constraints and energetic costs of growing a fetus shared across species, not because of a uniquely human premature evacuation of the fetus.  It was difficult from this point on for me to imagine what, if anything, the bony birth canal could have to do with the evolution of human gestation length and fetal size.

To be clear, we have only superficialities in common with actual “altricial” mammals like rat pups and wolf cubs. Our helplessness at birth is largely determined by our small relative brain size (constrained by the EGG hypothesis) and its relationship to motor-neuronal development. We also lost our grasping feet by 3.6 million years ago [see the Laetoli footprints in Tanzania that lack grasping big toes], which changed how we carry babies and how they cling (or not) to us. As toddlers, we develop bipedalism when it is expected for a mammal, based on brain mass, which is a good predictor of the time it takes to develop the brain (Garwicz et al. 2009). And unlike actual altricial animals that are born prior to peak brain growth rate, we are born after that peak like precocial mammals (Halley 2017). Humans do have a long developmental period during which we grow our enormous brains and during which we wire them up in wonderful ways, like for music and wit and other wows of humanity. But it does not deserve a uniquely human explanation. All big-brained primates take longer to develop than their smaller-brained relatives, and while they do so, they learn complex behaviors, just not as complex as ours.

Fans of Brave New World have long been aware of the consequences of our species’ stretched out life history:  “...at thirteen a man is not yet sexually mature; and is only full-grown at twenty. Hence, of course, that fruit of delayed development, the human intelligence.” In this vein, Portmann (1969) and others have argued that we are born early, not because of the hips, but because of selection for additional extrauterine stimulation and its intellectual fruit (see Dunsworth 2016b). But like the OD, that idea is also misguided because of its unfounded assumption of our early birth. So far it is strongest to assume that we are living proof of a birth canal that is large enough to accommodate what mother’s metabolism can grow in utero. Neither bipedalism nor selection for a longer postnatal learning period are significant determinants of gestation length or fetal growth—at least, we have no such evidence at this time.

I learned over the years that academic arguments can get personal. But the OD is not a person and it surely is not God, so I hope to offend no one when I repurpose Enlightenment lore here: We have no need of that hypothesis.

[end of material from Dunsworth 2018]

Bipedalism and the evolution of large-bodied, big-brained neonates do seem to have contributed to childbirth difficulty. Acknowledging that and researching that can occur outside the OD framework.  Here’s an example: https://digitalcommons.uri.edu/cgi/viewcontent.cgi?article=1036&context=soc_facpubs

There is much about the OD that is unsupported, weak, and questionable in terms of its logic and what has counted as evidence for it. I am extremely comfortable saying “there is no obstetrical dilemma” until/unless the evidence changes. 

No one has demonstrated that human babies are born early. If someone does, then that would be the first step towards supporting the OD, but it would still not seal the deal. Good luck demonstrating that (a) humans are indeed born early and *also* that (b) the cause of that early birth is the bony birth canal and not something else.  No, seriously, good luck. I’d go back in a DeLorean and re-do my dissertation if I could. Demonstration or failure, either way, would be equally awesome. No offense to Proconsul feet, because they led me to some great field seasons and they led me to here, but, wow… what a humdinger of a dissertation that would be!

Love & Evolution,




Animal Diversity Web. (retrieved Feb. 22, 2018) https://animaldiversity.org/

DeSilva, J, and J Lesnik. 2006. “Chimpanzee neonatal brain size: Implications for brain growth in Homo erectus.J Hum Evol 51: 207-12.

Dunsworth, HM and L Eccleston. 2015. “The evolution of difficult childbirth and helpless hominin infants.” Annual Review of Anthropology 44: 55-69.

Dunsworth, HM, Warrene,r A, Deacon, T, Ellison, P, and H Pontzer. 2012. “Metabolic hypothesis for human altriciality.” 2012. PNAS USA 109(38): 15212-15216.

Dunsworth, HM. 2016a. “Chapter 2: The ‘obstetrical dilemma’ unraveled.” In Trevathan W and K Rosenberg, editors: Costly and Cute: Helpless infants and human evolution.  Santa Fe: School for Advanced Research.

Dunsworth, HM. 2016b. “Thank your intelligent mother for your big brain.” PNAS USA 113(25): 6816–6818.

Dunsworth, H.M. 2018. “There is no ‘obstetrical dilemma’: Towards a braver medicine with fewer childbirth interventions.” Perspectives in Biology and Medicine 61(2): 249-263. https://pubmed.ncbi.nlm.nih.gov/30146522/. OA preprint: https://digitalcommons.uri.edu/cgi/viewcontent.cgi?article=1018&context=soc_facpubs

Dunsworth, HM. 2007. Human Origins 101. Westport, CT: Greenwood Press. 

Halley, AC. 2017. “Minimal variation in eutherian brain growth rates during fetal neurogenesis.”  Proc R Soc B 284: 20170219.

Huxley, A. 1932. Brave New World. New York: Harper and Row.

Garwicz, M, Christensson, M,and E Psouni. 2009.” A unifying model for timing of walking onset in humans and other mammals.” PNAS USA 106(51): 21889-93.

Gould, SJ. 1977. Ontogeny and Phylogeny. Cambridge, Mass.: Harvard University Press.

Karp, H.  “What is the fourth trimester?” (retrieved Feb. 22, 2018) https://www.happiestbaby.com/blogs/blog/fourth-trimester

Kuzawa, C. W. 1998. “Adipose Tissue in Human Infancy and Childhood: An Evolutionary

Perspective” Am J Phys Anthropol 41: 177–209.

Misty, FJ. 2017. Pure Comedy [Official Music Video]. YouTube  (retrieved Feb. 22, 2018) https://www.youtube.com/watch?v=wKrSYgirAhc

Portmann, A.1969. A Zoologist Looks at Humankind. Schwabe: Basel. Translated in 1990 German text by Schaefer J. New York, NY: Columbia University Press.

Sacher, GA, Staffeldt, EF. 1974. “Relation of gestation time to brain weight for placental mammals: implications for the theory of vertebrate growth.” Am Nat 18(963): 593-615.

Small, M. 1999. Our Babies, Ourselves: How biology and culture shape the way we parent. New York, NY: Anchor.

Trevathan, W. and K. Rosenberg. 2016. “Human evolution and the helpless infant.” In Costly and Cute: Helpless infants and human evolution, edited by Trevathan, W. and K. Rosenberg, 1-28. Santa Fe: SAR Press.

Zuk, M. 2013. Paleofantasy. New York: W.W. Norton.



Monday, August 10, 2020

A Sunshine Sonnet

Here's a song for solar, sitting in sun on the roof

Saving climate-warming smoke from smogging up the air

There to soak up sunlight--and not at all to seem aloof--

Helping make our climate cooler and, if cloudier, more fair!

To leave our vital orb in peace, up there at work alone

And be benign to living creatures here and yet to come

So even at our story's end, when we ourselves are done

Our followers find fallow earth abed in fertile loam

Still able to bear fruit on which those who come will live

A ready harvest for their livelihood, for them and for their foals,

Of man and beast, of flow'r and crop, ability to thrive

An ample source of nutrients alike for all their souls:

This is a basic wish, if as well a fervent plea

To give the future what we had--a decent destiny.