Monday, July 30, 2012

That [obstetrical dilemma] really tied the [human evolution] together. Part 1.

Some impressive colleagues and I* are about to have a paper published that pulls the rug out from under a classic paleoanthropological hypothesis/theory.

Scratch that. If we're gonna do this Lebowski theme right, I just have to say that our paper will have “micturated upon” an old beloved rug, I mean story.

Trust me. Our research did not come from a “shut the [bleep] up, Donny” dismissive sort of place nor did it come from a “the bums will always lose” holier-than-thou sort of place. Nothing like that.

It came from honest to goodness seeking to understand, man.

Here's the old beloved story:

The obstetrical dilemma (OD) hypothesis = Simultaneous selection for big-brained (or simply big) babies and bipedal locomotion caused a dilemma because while babies must be large, birth canals must remain small. The consequences of this dilemma, which are often called “solutions” and “tradeoffs,” include (1) difficult and dangerous childbirth with universal assistance due to the tight fit, (2) relatively underdeveloped, helpless, often termed “secondarily altricial” neonates compared to all other primates which are precocial, and (3) compromised or sub-optimal female locomotion, since (4) selection has favored sexual dimorphism in the human pelvis with females having not just relatively wider but absolutely wider dimensions of the birth canal.

Notice how--like the way that a nice oriental rug spiffs up a dumpy Los Angeles living room--the OD skillfully ties together many unique or fascinating phenomena in human evolution, such as human bipedalism, human encephalization, hellish human childbirth, helpless (i.e. hellish?) human babies, male-biased human athletic ability, and broad ladies' hips.

And we haven't proven this story wrong. But we have thrown some serious doubt on it, demonstrating how little of it holds up to current evidence.

“This is our concern, Dude.”

Way back in grad school, my wise advisor Alan Walker gave me a copy of Adolf Portmann's A Zoologist Looks at Humankind in which he argues against a pelvic constraint on human gestation and fetal growth (the OD). So that primed me to carry some doubt in this OD world.

Then later, in 2007, I was post-doc-ing with Nina Jablonski and immersed in the mammalian life history, energetic, and encephalization literature. It occurred to me that, Oh goodie! I’ll find out how selection could have shortened our gestation as we became encephalized but as selection also maintained our small birth canals for bipedalism.

And not only did I strike out all around. But, I mean, the mammalian life history literature looks as if there’s absolutely nothing constrained about human gestation length or the timing of birth. If anything it looks like we’re weirder in the other extreme… having slightly longer gestations than other primates and having relatively big babies. Leading up to birth we’re actually suped-up primates, not limited ones. That we're not particularly different in these terms, and definitely not limited, has all been known for decades. I was late to the party.

When you look at Bob Martin's work, and others' like it (below), you see that the size of the mammalian mother predicts the length of gestation and the size of the offspring.



Manger, PR. 2006. An examination of cetacean brain structure with a novel hypothesis correlating thermogensis to the evolution of a big brain. Biol. Rev. 81: 293–338. "Fig. 17. Allometric plot of the relationship between neonatal (Mbirth) and adult body mass (Mb) in three orders of eutherian mammals. The data used in this plot are derived from that given in Nowack (1999)."


These predictions hold even when you look across mammals that have single births or litters and note how this includes encephalized mammals, like whales, that don't even have bony birth canals!




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....Using an equation that takes into account neonatal brain weight, litter size, and “brain size advancement” (neonatal brain weight ∕adult brain weight),they predicted the gestation length for species of animals with known gestation length. These few variables, which exclude any pelvic dimensions, were successful at predicting gestation length in the vast majority of species in their study, including humans and the cetaceans which lack constricting bony birth canals.


It seems so obvious that there's an energetic limit to what a pregnant mammal mother can do. And it seems so not obvious that humans are exceptional as the OD would have it. And this perspective was strengthened after I read Peter Ellison's book On Fertile Ground: A Natural History of Human Reproduction in my spare time in the field one summer, fitting in a few pages after each hot day of Proconsul hunting, after retiring each night in my canvas tent on Rusinga Island.


“This [hypothesis] will not stand, man.”

So with all this research out there showing how birth seems to be limited primarily by maternal metabolism, why this notion that we’re compromised by our pelves? Why this notion that we could or should keep babies in our wombs longer if it wasn't for bipedalism keeping our birth canals too small for gestating any longer, for growing bigger babies?

After all, there was Anna Warrener (now at Harvard) presenting her dissertation research at our annual conference showing how wide hips aren't so bad for locomotion. And she cited other papers with similar results!

So why hadn't the OD been reevaluated yet given all this stuff. I wasn’t sure. And to be honest, I thought it was so obvious, this disconnect, this weakness of the obstetrical dilemma hypothesis that after I completed my first draft of the manuscript in 2008, I decided not to do anything with it. It was so obvious, to me, that it became so absolutely ridiculously pointless to write about it. But once I mustered up the gumption to send the manuscript out to several close friends (like Ken and Anne!) for a read and none of those clever folks said it was ridiculous, that’s when I felt some encouragement. It wasn’t ridiculous, it made sense. I just happened to be the first person that my friends had known to put it together.

Or so I thought.

Here I was ready to put this idea out there and then one of my readers, Jeff Kurland, a brilliant and beloved professor from my grad school days at Penn State mentioned, “I'm pretty sure Terry Deacon presented this same thing at the AAPAs back in the 1980s or ‘90s.”

This is when your heart sinks because you just went to all that trouble only to find out that someone already beat you to it. Again, I went back to thinking that my ideas must be so obvious to everyone in the field now. But no searching came up with any Deacon pubs on the topic so I wrote to him directly and he confirmed that he hadn't published it, but he shared the manuscript that he'd stopped working on long ago. It fit with mine in many ways--this doubt of the certainty that the bipedal pelvis is limiting further gestation length and fetal growth--and since he'd presented it publicly, I asked if he'd like to be on the paper and he did. This is when your heart soars because someone so clever shares your thinking. This idea is not ridiculous! (Plus, even if he had published it already, my paper could have been a much updated contribution and still not pointless. Hopefully I would have understood that, or at least someone would have shaken some sense into me.)

“You’re out of your element, [Holly].”

Right around the same time as I was getting this nice feedback from colleagues and friends I spoke to another close colleague, Herman Pontzer, about it. He's known as the "energetics guy" among other things so I figured he was the perfect litmus. And it seemed fairly straight forward to him. Again, it's not ridiculous. Hooray! I wasn't crazy! Plus now I had this fully capable human being on board, ready to replace my stolen figures from other pubs, to make similar points but with updated data. And, even better, he could test this hypothesis about maternal energetics by plotting out the data from various data sources. All was falling into place.

"This is what happens when you [birth] a [baby] in the [pelvis]!" ... “A world of pain.”



As we were putting our story together, I joked to a brand new mother, "You know, there's no obstetrical dilemma." And got a sharp-tongued, Oh yes there is, honey. I just went through labor. Hell yes there is!

In future such discussions, I was always sure to add, "...it's all energetics. A mom gives birth when she does because she can't possibly give any more energy into growing that fetus." And some moms who hear that are like, Duh! I could have told you that! 

You cannot win. At least I could not. But it was still encouraging.

By this point, we (Anna, Herman, Terry and I) had joined forces with Peter Ellison and we submitted our paper to a major journal for review. And I'll be back with a little digest of that paper and some thoughts on it a bit later...

Update, Sept. 1, 2012: Here's the next post. 

****

*Feynman's "half-advanced and half-retarded potentials" describes us nicely, with me as the latter.

19 comments:

Joachim D. said...

"...it's all energetics. A mom gives birth when she does because she can't possibly give any more energy into growing that fetus."

[jokeon]Sounds like semelparity.[jokeoff]

Earnestly, what's the pain for? Do ape mothers suffer the same? Is the birth pain necessary to make the mother muster the last Joules she has to throw that lodger out?

Holly Dunsworth said...

yes it does (to your joke :))

I don't know if apes have the same pain but I doubt it considering how much roomier their birth canal is relative to the size of the neonate.

And, earnestly, why does the pain have to be for something? Why not it just be something we must endure but do so well enough and are here to think and type about it, as proof :).

Joachim D. said...

It's just the inuitive reasoning of a layman. However, it might also have been a (tacit) part among the establishment. It's as follows:

Pain is a sign that the body is hurt, or else has the function to protect the body from being hurt. Therefore, birth pain is a sign that the mother's body is hurt or in danger of it. Therefore, the limitation is probably morphological or anatomical.

Now you made a case that the limitation is energetic, and I promply wondered what the pain is then for. I also wondered what the symptoms of energetic limitation should be - pain, exhaustion, or something else.

Anyway, good luck with publishing and we'll see whether the pain issue will come up in responses.

Ken Weiss said...

If pain existed long before these issues regarding humans specifically arose, as it certainly did (since, for example, fish feel pain), then there is no need whatever for a special, much less an evolutionary, explanation for this particular pain.

It's a neural mechanism for response to tissue stress, and in this case there is tissue stress so the mechanism is triggered.

So I think Holly's comment above yours is an answer. There doesn't have to be a selective reason 'for' everything that exists in life.

Holly Dunsworth said...

Here's an example: Does popping a zit hurt because humans have evolved to feel pain when they pop zits?

Holly Dunsworth said...

I'll go ahead and reveal my answer: No. And it's incredibly difficult for me to see how it could be yes.

Ken Weiss said...

If, as we poppers were as teenagers, the idea was that if you were too zitted you didn't get a mate, then there should have been great pleasure--approaching that of the sex drive itself--in popping, not pain!

So there must be some other reason. Maybe it's that if you were staring into a mirror (this, ancestrally, means like Narcissus, concentrating on your reflection in a pond or stream), you got caught by the lion (and hence no mating!). So the pain was an anti-predator adaptation.

Joachim D. said...

Sorry, I might have been unclear. I did not mean that the birth pain is an evolved trait (an adaptation of human birth), but that it intuitively sugegsts a physical (morphological/anatomical) limitation no tan energetic one.

The pain of popping zits just tells me that there is a physical limitation to what your skin tissue can endure - tissues stress - as Ken put it.

If you wish to ridicule an adaptationistic explanation for that, how about: you probably pop your zits, when you cannot possibly feed them any bigger, cause it would suck too much energy?-)

Ken Weiss said...

Yes, and let's throw in Malthusian pressure. If you feed your zits, you can't compete with others. That selects for (a) people with dry and zit-free skin, and (b) poppers.

Holly Dunsworth said...

Ah, your "birth pain" is referring to the tight fit at birth. Okay, yes. That's the traditional idea: that the pelvis is the limit to gestation length. The evidence for this is weak and it's a more challenging idea to test than the popularity of the notion would indicate! More on that later when our paper's out and I can write about it...

Joachim D. said...

Excuse me for asking one last stupid question. You wrote:

"That's the traditional idea: that the pelvis is the limit to gestation length. The evidence for this is weak and it's a more challenging idea to test than the popularity of the notion would indicate!"

I would have thought it should be easy to measure the sizes of pelves of mothers and skulls of newborns in different species and thus get an estimate of the tightness. Hasn't that been done?

Holly Dunsworth said...

It's not a stupid question. It's right in line with the traditional argument about the human pelvis limiting gestation length. The question is what limits further fetal growth: the pelvis (OD) or something else? And a tight fit is not enough evidence that the pelvis actually limits further growth. The pelvis limits fetal size in all species that give birth through a bony birth canal if you think about it. So are humans any different just because we have a tight fit? That is, are humans experiencing a unique pelvic influence on gestation length? There's little evidence for this.

Holly Dunsworth said...

Forgive me if I don't reply any more. I'll be out of regular contact with the blog and hopefully will anticipate and address further questions with follow up posts when the paper's out.

Melinda Gonzales said...

Hi! I know that this is kind of an old post but I was just reading this and wondering how you can contribute the short gestation length in humans to a metabolic limitation when in fact mothers are much more limited after bith during breastfeeding if Im not mistaken?

Holly Dunsworth said...

Mothers are limited after birth too, yes but look at the model we put in our paper. To see it, either keep reading the links that follow (at bottom of post) to more mermaid's tale posts, read the paper in PNAS (if you can get access or write to me for the paper), and/or check this out: http://blogs.scientificamerican.com/guest-blog/2012/09/05/how-to-apply-an-evolutionary-hypothesis-about-gestation-to-your-pregnancy/

Anonymous said...

Also a late question, but I'm slightly confused about the statistics you use. When you calculated energy requirement/NPNL BMR, why did you use the nonpregnant nonlactating BMR, and not the BMR at the measured gestation week? Does the fact that BMR changes over pregnancy have an effect on the ability to maintain such a rate?

Alex said...

What data did you use for maternal energy expenditure in Figure 3? At first, I though you used Energy req./NPNL BMR from Table S2, but those values don't match up.

Holly Dunsworth said...

It matches. What I think you're seeing is higher resolution reporting on the graph because we made that with the raw calculations, but to put those raw calculations in the table we had to round the numbers after the decimal points.

Aaron P. said...

How come the protein and fat costs need to be added to TEE? Wouldn't those numbers be a part of the TEE of a pregnant woman already?