Monday, August 27, 2012

That [obstetrical dilemma] really tied the [human evolution] together. Part 2.

Update (Aug 30, 6:41 am): Paper's up. Here.

Update (Aug 28, 3:32 pm):  Jeepers, if I'd have known readership (err, clickership) was going to jump way above normal with this post, or that writers would lift quotes from here, I'm sure I would have crafted it better. 

Note: We're past our embargo, so I'm posting this now even though it doesn't appear that the article is posted on-line yet. I'll update this post and link to it once it is.

Some colleagues and I have a paper this week in early view at PNAS (1). I already told a big part of the story here.

In our paper, we show how weak--given current evidence--the popular obstetrical dilemma hypothesis (OD) is for explaining human gestation length. And, we offer up an alternative hypothesis as well.

It gets a lot bigger than this, doesn't it moms!  

The EGG hypothesis
What limits fetal growth during pregnancy? The OD says it's the pelvis--implying it's a unique constraint due to bipedalism. But the EGG hypothesis suggests that the primary constraint on fetal growth and gestation length is maternal metabolism (energetics, growth, gestation). Mothers give birth when they do because they cannot possibly give anymore energy into gestation and fetal growth. And when you look at the data available on pregnancy and lactation metabolism in humans... it shows that right around 9 months of gestation, mothers reach the energetic throughput ceiling for most humans.

Here's Herman's Figure 3 showing the EGG for humans, plotted with real metabolic data. Circles are the offspring, squares are the mother. Notice how fetal energy demands increase exponentially as the end of a normal human gestation period approaches. To keep it in any longer, mother would have to burst through her normal metabolic ceiling. Instead, she gives birth and remains in a safe and possible (!) metabolic zone.

The starred dot is a human infant at the developmental equivalency of a newborn chimpanzee. This is the thought experiment that Stephen Jay Gould famously wrote about. That's the age you'd have to birth a human baby to be like a newborn chimp, since we're born born more helpless than chimps. Keeping a fetus in this long--that is, adding 7 or more months to our gestation--would be physiologically impossible because it would require a mother to exceed 2.1x the basal metabolic rate, bursting through the ceiling for most humans. We actually gestate as long or maybe a little longer than you'd expect for a primate or a mammal, not shorter! So our relative helplessness at birth is indicating how much more neurological growth we have to achieve during our lives, after birth, than chimps and other relatives.

The EGG is a more general incarnation and a broader application of Peter Ellison's "metabolic crossover hypothesis" for the timing of human birth. The EGG branches out beyond our species, considering humans to operate within the physiological confines of other primates and mammals. But comparable data for other species, for testing the EGG, are not yet available to our knowledge.

Why do we grow babies that seem too big to fit through our birth canals? A strong hypothesis is that it's our diets that have radically changed compared to most of our evolutionary history. Many humans have constant and easy access to high calorie foods while pregnant and they can grow bigger babies over longer pregnancies. Very much related to this idea, check out Herman's recent NYT article about how our energy intake affects our health: "Debunking the Hunter-Gatherer Workout."

We named the hypothesis for ease of communication, not because we're eggomaniacs. We were tempted to call it HAM (humans are mammals) but felt that EGG better described the idea and was also adorable considering how babies are made.

HAM and EGG, or EGG and HAM, to me, is the ideal name but try saying that without going all Dr. Seuss on a wumbus full of thneed-suited who-scientists.

And that goes for here too. Your pop culture references best remain R-rated if you're to retain an ounce of R-word. And because it's just so enlightening, we'll continue to employ the very mature and refined Lebowski theme from Part 1 in our discussion here.

Call me Maude.

“The [species] abides.”
Part of the trouble I and others have with the obstetrical dilemma is this: We do just fine in the face of the tight fit at birth. Just because there's a tight fit, just because childbirth is terrifying, just because it's not an easy or enjoyable experience, that's not necessarily a "bad" thing evolutionarily. Clearly it's the opposite. It's a good thing. We're here to think about it! It can't possibly be "bad" if we keep having babies despite the hellishness of childbirth. This perspective was one of the contributions of "The obstetrical dilemma revisited" (2): Our behaviors, our aiding of women during childbirth, have probably reduced selection pressures against the tightness of fit, or other contributors to childbirth difficulty and danger. The species abides.

When you look at childbirth not as a biological failure, or as God's plague on lascivious women invited by Eve, but when you see it instead as a raging success, the obstetrical dilemma hypothesis is much easier to doubt.

The picture of evolutionary success. (source)

"Say what you like about the tenets of [Natural Selection], Dude, at least it's an ethos."
The widespread popularity of the OD may well be rooted in its adaptationist appeal, where nonoptimality (e.g. human altriciality, or helplessness and relative underdeveloped-ness at birth) is explained as a contribution to the best possible design of the whole (e.g. big brain and efficient bipedalism). Gould and Lewontin (3) famously criticized the “adaptationist programme” by cautioning that “organisms must be analyzed as integrated wholes” that are “constrained by phyletic heritage, pathways of development, and general architecture” and that “the constraints themselves become more interesting and more important in delimiting pathways of change than the selective force that may mediate change when it occurs.” They faulted this approach for failing to “consider alternatives to adaptive stories” and for its “reliance on plausibility alone as a criterion for accepting speculative tales.”

From this perspective, it's inappropriate to root the evolution of human altriciality in a compromise between adaptations for big brains and adaptations for bipedalism when there are most likely more basic, conserved, phyletic constraints on pathways of development and general architecture (e.g. gestation, pregnancy and fetal growth) at play.

“Yeah, well, that’s just like, uh, your opinion man.” 
Over the last five years as I’ve been thinking about this, specifically, I've had a teeny tiny bit of resentment creep up now and again towards the field that coaxed me into buying this OD as dogma. But I have nobody to blame but myself! A hypothesis is just that and why I just swallowed it whole without doubt is partly because it's a cool idea! And partly because an alternative idea just wasn't as well-known yet! Our paper is not attacking anyone, despite the guilt we've induced in folks who have been treating the OD as fact and teaching it to hordes of students for the last 50 years. To those researchers and teachers who came before us, we’re grateful! And to anyone who thinks of our paper as gotcha or an attempt at it: Please remember how science works and how knowledge accumulation works. That's all this is. It's just a little more hyped because it's about humans, not sea squirts.

The OD is not dead. It's just put in a less omnipotent place. The heaviest burdens should always be on supporting hypotheses for human exceptionalism; we should never default to them. Humans are animals/mammals/primates/hominoids and when we fail at that default view, that's when we can claim human exceptionalism.

1. Dunsworth HM, Warrener A, Deacon T, Ellison P, and H Pontzer (2012) Metabolic hypothesis for human altriciality. PNAS on-line early view.
2. Rosenberg K, Trevathan WR (2002) Birth, obstetrics, and human evolution. BJOG 109(11): 1199–1206.
3. Gould, SJ and RC Lewontin (1979) The spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proceedings of the Royal Society of London, Series B 205(1161): 581-598.

Please do look to the PNAS paper to read about the EGG or to see how we've exposed the challenges to testing the traditional OD. I did not write this post to stand for anyone's sole source. If you cannot access the paper once it's posted on-line, then please email me and I'll happily send the paper to you.

Another Update (Aug. 30, 6:41 am)

No comments: