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Why Humans Give Birth to Helpless Babies

The views expressed are those of the author and are not necessarily those of Scientific American.

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Newborn baby

Image: Jlhopgood, via Flickr

Human babies enter the world utterly dependent on caregivers to tend to their every need. Although newborns of other primate species rely on caregivers, too, human infants are especially helpless because their brains are comparatively underdeveloped. Indeed, by one estimation a human fetus would have to undergo a gestation period of 18 to 21 months instead of the usual nine to be born at a neurological and cognitive development stage comparable to that of a chimpanzee newborn. Anthropologists have long thought that the size of the pelvis has limited human gestation length. New research may challenge that view.

The traditional explanation for our nine-month gestation period and helpless newborns is that natural selection favored childbirth at an earlier stage of fetal development to accommodate selection for both large brain size and upright locomotion—defining characteristics of the human lineage. In this view, adaptations to bipedalism restricted the width of the birth canal and, hence, the size of the baby that can pass through it. Human babies are thus born when their brains are less than 30 percent of adult brain size so that they can fit through the narrow passageway. They then continue development outside of the womb, with brain size nearly doubling in the first year.

But when Holly M. Dunsworth of the University of Rhode Island and her colleagues tested this so-called obstetrical dilemma hypothesis, their findings did not match its predictions. For example, the hypothesis predicts that because the female pelvis is broader than the male pelvis, walking and running should be more energetically demanding for women than for men. Yet most studies of the energetics and mechanics of locomotion in women and men found no such penalties for having a wider pelvis, the researchers report.

Furthermore, the team asserts, to accommodate an infant at a chimplike stage of brain development—that is, a brain that is 40 percent of adult brain size, or 640 cubic centimeters—the pelvic inlet (the top of the birth canal, which is the narrowest part) would only have to expand by three centimeters on average. Some women today have pelvic inlets that wide, and those larger dimensions have no measurable effect on locomotor cost. The researchers argue that instead of fetal brain expansion being constrained by the dimensions of the pelvis, the dimensions of the human pelvis have evolved to accommodate babies, and some other factor has kept newborn size in check.

That other factor, they contend, is mom’s metabolic rate. “Gestation places a heavy metabolic burden (measured in calories consumed) on the mother,” Dunsworth and her co-authors explain. Data from a wide range of mammals suggest that there is a limit to how large and energetically expensive a fetus can grow before it has to check out of the womb. Once outside of the womb, the baby’s growth slows down to a more sustainable rate for the mother. Building on an idea previously put forth by study co-author Peter T. Ellison of Harvard University known as the metabolic crossover hypothesis, the team proposes that “energetic constraints of both mother and fetus are the primary determinants of gestation length and fetal growth in humans and across mammals.” By nine months or so, the metabolic demands of a human fetus threaten to exceed the mother’s ability to meet both the baby’s energy requirements and her own, so she delivers the baby.

In their report, to be published online this week in the Proceedings of the National Academy of Sciences USA, Dunsworth and her collaborators conclude that “if the human reproductive system poses a dilemma between competing needs, then fetal energy needs and maternal energy supply are the competitors, rather than [brain expansion] and bipedalism.”

When I asked paleoanthropologist Karen Rosenberg of the University of Delaware, an expert on the evolution of human birth, what she thought about the new work, she called it “important and interesting.” But “just because there’s a metabolic moment when it becomes reasonable to have a baby doesn’t mean it isn’t also true that the pelvis is a tradeoff between giving birth and walking on two legs,” she contends.

Given how difficult human birth is, one would think that if the pelvis could get bigger without compromising locomotion then it would–but it doesn’t, Rosenberg observes. “I think it’s still the case that the pelvis is adapted to functions that select in opposite directions,” she says.

Rosenberg additionally noted—and I found this especially fascinating—that the authors mention the possibility that the timing of birth actually optimizes cognitive and motor neuronal development. That idea, first proposed by Swiss zoologist Adolf Portman in the 1960s, is worth pursuing, she says. “Maybe human newborns are adapted to soaking up all this cultural stuff and maybe being born earlier lets you do this,” she muses. “Maybe being born earlier is better if you’re a cultural animal.” Food for thought.




Kate Wong About the Author: Kate Wong is an editor and writer at Scientific American covering paleontology, archaeology and life sciences. Follow on Twitter @katewong.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. kclancy 1:22 pm 08/28/2012

    I just wanted to say this is fabulous coverage of this new manuscript. Kudos to Dr. Dunsworth and colleagues for their great paper, and I’m glad Dr. Rosenberg was also interviewed for added perspective.

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  2. 2. WRQ9 1:29 pm 08/28/2012

    It is present in bird species that vanity and appearance can have as much effect on selection as ability and ease of function. It could be possible that inherent vanity led to the selection as in trends presently notable.
    If this is the case it may have been simple happenstance that our brains evolved differently, and thus, led to a more “cultural” animal. Following this reasoning the human species has consciously made assumptions based on these differences that may not hold water. This path would explain a lot.
    We do reside on a precipice, figuratively, and consciousness will complicate our circumstance. A juggernaut indeed!

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  3. 3. Ikpex 5:07 pm 08/28/2012

    Humans must have evolved one DNA sequence at a time; those sequences that wotked are still with us, mostly.

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  4. 4. RiskArb 11:47 am 08/29/2012

    I have a much different contention – that gestation time is an equilibrium condition that’s a function of:

    i) Physical size of the adult species (clearly, larger takes longer) (+)
    ii) Life expectancy (+)
    iii) Number of offspring per pregnancy (-)
    iv) Incremental risk (e.g., of predation) to the mother as a result of extended pregnancy (-)
    v) Minimum physiological dev of the foetus to ensure its own survival (partly a function of caregiving/social conditions surrounding the mother)
    vi) Minimum neurological development threshold for foetal survival (+)

    One could quite easily compile the data for these variables across species (points iv – vi would need to be scored qualitatively) and develop a polynomial algorithm with decent predictive capability.

    What’s equally interesting is that as humans are delivered earlier [ and survive earlier, notably via social and technological means], there is a minute survivorship bias towards those with more rapid foetal development…. leading to, all other factors being equal, shorter gestation for humans in the generations to come. The assertion that Neanderthals had 12 months of gestation (a result of iv and v) would appear to be consistent with this.

    [ Prime facie, I don’t understand how physical birthing factors e..g., pelvis size, are the principal constraints here…. If body form follows function [evolutionary/natural law], then pelvis sizes would accordingly change as a result of changes in gestation periods, not the other way around ]

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  5. 5. ysamet 1:47 am 08/30/2012

    Focusing on gestational differences is nearsighted in ignoring the enormous post-birth differences in the developmental timelines between humans and other species. Why does it take 10 or 20 times as long for humans to become fully-functioning adults relative to some other species?

    This question was pondered by ancients. Some proposed that longer periods of childhood and adolescence allow for more sophisticated psychological and spiritual developent.

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  6. 6. leovaz59 11:36 am 08/30/2012

    It seems as though Dr Rosenberg’s comments are dead on, because the head of the fetus is growing at a great pace, the pelvis won’t accommodate later, as far as the helpless part, the chimp’s brain development after birth is instinctive just like all other mammals, at first, but human babes overtake them in no time, cognitively speaking or should I say based on memory.
    one point on energy consumption by the fetus it has been observed that in starving, rail thin African mothers babes are born fat and healthy, my heart goes out to the sacrifices made by this mothers.

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  7. 7. Icsaemm 3:51 pm 08/30/2012

    If brain size and baby delivery system are co-evolving then why wouldn’t a torpedo shaped head be expected as a possibilty?

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  8. 8. fabrizio 5:23 pm 08/30/2012

    To me, the last paragraph of the article contains indeed the most fascinating and illuminating statement. According to most recent studies, the development of intellective capacities depends critically on the amount of stimuli received in the first year of life. If we spent it in a womb, we won’t be stimulated enough…

    Another point that comes to my mind is that this could be the process of evolution of our species:

    -slightly larger brain (from random mutation) supports the ability to stand up (successful maybe for allowing to get high-hanging fruits?)
    -natural selection leads to slightly narrower birth channel (to outrun competitors, for example when chased by a lion)
    -slightly narrower birth channels favours earlier delivery (or better, makes it a necessary condition for birth)
    -earlier delivery allows increased exposition to external stimulation in the first months of life
    -external stimulation increases intellective capacities and adaptation, so these actions cycle to shorten delivery time until the present situation.

    Does this imply that, at a certain point, developing a narrower femal pelvis was not motivated by standing erect but by the opportunity to shorten gestation time?

    And where will future evolution lead us? to ever more skinny supermodels? :)

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  9. 9. RoedyGr 1:56 am 08/31/2012

    Why would it not be possible to keep the fetus internal, and just slow the growth to a sustainable pace?

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  10. 10. jawshoeaw 10:40 pm 09/9/2012

    This sounds like anthropology undergrads talking late at night. 25 years ago at NC State, I remember discussing the competition betweenmetabolismansocial development with fellow 19 year holds. I ts not really science of course. One thing our group questioned was not energy demands of running but wear on the knee joint.

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  11. 11. lookchimchimcandy 12:09 pm 06/27/2013

    So help me understand. Humans evolved from chimpanzees. Chimps have brains at birth that are roughly 10% closer to adult size than humans, and both have 8-9 month gestation.

    Chimp babies can walk at birth and survive on their own much earlier. So a 10% difference in brain size at birth equates to such disparity in physical ability and survivability?

    Why would any creature evolve so that’s young are more helpless and take so long to survive on their own? What happened to survival of the fittest? Why would such helplessness and slow brain and physical development be passed along as beneficial? As with bipedalism why would being slower, less agile and unable to climb be passed along as beneficial?

    Just another evidence that man is different and unique.

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