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SOCIAL EVOLUTION FORUM
A Student Expresses the Value of an Evolutionary Education
EvoS, a campus-wide evolutionary studies program, is attempting to solve a problem that pervades higher education worldwide: the restriction of evolutionary training to the biological sciences.
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An Evolutionary Approach to Sustainability Science
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"Childbirth" by cassy1723, via Flickr.

No one is saying that medicine isn't brilliant and hasn't saved lives. But it does intervene more than necessary when it comes to pregnancy and childbirth. Part of that unnecessary intervention is driven by lack of experience; it's difficult to decide when intervention is and isn't necessary, especially when things are heating up. Part of it stems from an economically-driven disrespect for time. (Give childbirth some motherlovin' time.)  But another part of the trouble actually lies in the evolutionary perspective. Unfortunately it's not all rainbows and unicorns when M.D.s embrace evolution. Instead, evolutionary thinking is biasing some medical professionals into believing that, for example, birth by surgical caesarean is an "evolutionary imperative." Here's one recent example in The American Journal of Obstetrics & Gynecology of how the evolutionary perspective is (mis)guiding arguments for increased medical intervention in childbirth. It's a paper called, "A large head circumference is more strongly associated with unplanned cesarean or instrumental delivery and neonatal complications than high birthweight." It's a fairly straight-forward study of over 22,000 birth records at a hospital in Jerusalem. The authors ask whether birth weight (BW) or head circumference (HC) is more of a driver of childbirth interventions (instrumental delivery and unplanned caesareans) than the other. Of course, the focus is on the biggest babies with the biggest heads causing all the trouble, so the authors narrow the data down to the 95th percentile for both. Presumably they're asking this question about BW and HC because both can be estimated with prenatal screening. So there's the hope of improving delivery outcomes here. And, of course, the reason they ask whether head size or body mass is more of a problem is because of evolution. They anticipate that they'll discover that heads are a bigger problem than bodies because of the well-known "obstetrical dilemma" (OD) hypothesis in anthropology. OD thinking goes like this: Big heads and small birth canals are adaptive for our species' cognition and locomotion, respectively, but the two traits cause a problem at birth, which is not only difficult but results in our species' peculiar brand of useless babies. (But see and see.) So, since we're on the OD train, it's no surprise when we read how the authors demonstrate and, thus, conclude that indeed HC (head circumference) is more strongly associated with childbirth interventions than BW (birth weight), at least when we're up in the 95th percentile of BW and HC. Okay. They use this finding to advocate for prenatal estimation of head size to prepare for any difficulties a mother and her fetus may be facing soon. Okay. Sounds good. Sounds really good if you support healthy moms and babies. But it also sounds really good if you already see these risks to childbirth through the lens of the "obstetrical dilemma" with that OD thinking helping you to support some sort of evolutionary imperative for the c-section. Okay. Too many "Okays" you're thinking? You're right. There's a catch. When you dig into the paper you see that "large HC" heads are usually about an inch (~ 2.5 cm) greater in circumference than "normal HC" ones. (Nevermind that we chopped up a continuum of quantitative variation to put heads in arbitrary categories for statistical analysis.) And when you calculate the head diameter based on the head circumference, there is less than 1 cm difference between "large" and "normal" neonatal heads in diameter. That doesn't seem like a whole lot considering how women's bony pelvic dimensions can vary more than that.  Still, these data suggest that the difference between a  relatively low risk of having a c-section and a relatively high risk of having a c-section amounts to less than a centimeter in fetal head diameter. And maybe it does. Nobody's saying that big heads aren't a major problem sometimes! But maybe there's something else to consider that the paper absolutely didn't. Neonatal heads get squeezed and molded into interesting shapes in the birth canal. The data say that normal HC babies get born vaginally more often than large HC ones. But this is based on the head measures of babies who are already born! If we're pitting head circumference (HC) of babies plucked from the uterus against the HC of babies who've been through hello! then of course the vaginally delivered ones could have smaller HCs. C-sected babies tend to have rounder heads than the ones squeezed by the birth canal. It's impossible to know but I'm fairly confident about this, at least for a subsample of a population: Birth the same baby from the same mother both ways, vaginally and surgically, and its head after c-section will have a larger HC than its squeezed conehead will after natural birth. When we're talking about roughly 2.5 cm difference in circumference or less than 1 cm difference in diameter, then I'd say it's possible that neonatal cranial plasticity is mucking up these data; we're sending c-sected babies over into the "large HC" part of the story just because they were c-sected in the first place. So without accounting for this phenomenon, the claim that large head circumference is more of a cause of birth intervention, of unplanned c-sections, than large body mass isn't as believable. If these thoughts about neonatal cranial molding are worthwhile, then here we have a seemingly useful and very high-profile professional study, grounded in the popular but deeply flawed obstetrical dilemma hypothesis, that is arguing for medical intervention in childbirth based solely on the difference in head size measures induced by those very medical interventions. The circle of life! [post_title] => When Evolutionary Medicine Gets It Wrong About Childbirth [post_excerpt] => Evolutionary thinking is biasing some medical professionals into believing that birth by surgical caesarean is an "evolutionary imperative." 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Human sacrifice as shown in the Codex Magliabechiano, Foundation for the Advancement of Mesoamerican Studies, via Wikipedia.

An article published this week by Nature is generating a lot of press. Using a sample of 93 Austronesian cultures Watts et al. explore the possible relationship between human sacrifice (HS) and the evolution of hierarchical societies. Specifically, they test the “social control” hypothesis, according to which human sacrifice legitimizes, and thus stabilizes political authority in stratified class societies. Their statistical analyses suggest that human sacrifice stabilizes mild (non-hereditary) forms of social stratification, and promotes a shift to strict (hereditary) forms of stratification. They conclude that “ritual killing helped humans transition from the small egalitarian groups of our ancestors to the large stratified societies we live in today.” In other words, while HS obviously creates winners (rulers and elites) and losers (sacrifice victims and, more generally, commoners), Watts et all argue that it is a functional feature—in the evolutionary sense of the word—at the level of whole societies, because it makes them more durable. There are two problems with this conclusion. First, Watts et al. do not test their hypothesis against an explicit theoretical alternative (which I will provide in a moment). Second, and more important, their data span a very narrow range of societies, omitting the great majority of complex societies—indeed all truly large-scale societies. Let’s take these two points in order. An alternative theory on the rise of human sacrifice and other extreme forms of structural inequality is explained in my recent book Ultrasociety. By “structural inequality” I mean more than just great differentials in income and wealth that characterize our modern, even democratic, large-scale societies. In addition to HS, these include ruler deification and more generally “despotism,” when there are no constraints on what rulers and elites can do to commoners; for example, kill them without any negative consequences for themselves. Briefly, my argument in Ultrasociety is that large and complex human societies evolved under the selection pressures of war. To win in military competition societies had to become large (so that they could bring a lot of warriors to battle) and to be organized hierarchically (because chains of command help to win battles). Unfortunately, hierarchical organization gave too much power to military leaders and their warrior retinues, who abused it (“power corrupts”). The result was that early centralized societies (chiefdoms and archaic states) were hugely unequal. As I say in Ultrasociety, alpha males set themselves up as god-kings. Human sacrifice was perhaps instrumental for the god-kings and the nobles in keeping the lower orders down, as Watts et al. (and social control hypothesis) argue. But I disagree with them that it was functional in making early centralized societies more stable and durable. In fact, any inequality is corrosive of cooperation, and its extreme forms doubly so. Lack of cooperation between the rulers and ruled made early archaic states highly unstable, and liable to collapse as a result of internal rebellion or conquest by external enemies. Thus, according to this “God-Kings hypothesis,” HS was a dysfunctional side-effect of the early phases of the evolution of hierarchical societies. As warfare continued to push societies to ever larger sizes, extreme forms of structural inequality became an ever greater liability and were selected out. Simply put, societies that evolved less inegalitarian social norms and institutions won over and replaced archaic despotisms. We now come to my second critique of the Watts et al. paper. I actually agree that Austronesia is a good laboratory for testing many (but not all) theories of social evolution. There is of course a serious deficiency in the dataset that Watts et al use: it’s static. The basis for their data are observations made in the “ethnographic present.” If you want to assign causality, however, you need to get at the dynamics—how things change with time (as causes typically precede effects, the ability to resolve socio-cultural trajectories in time is critical). Russell Gray’s group (AKA Watts et al) has come with a clever way to get around this problem by using linguistic trees. Some cultural evolutionists, like Joe Henrich, are quite critical of this approach. But even if it works, there is a much more important flaw in the Austronesian database. The most complex society in their sample is Hawaii, which is not complex at all when looked in the global context. I am, right now, analyzing the Seshat Databank for social complexity (finally, we have the data! I will be reporting on our progress soon, and manuscripts are being prepared for publication). And Hawaii is way down on the scale of social complexity. Just to give one measure (out of >50 that I am analyzing), polity population. The social scale of Hawaiian chiefdoms measures in the 10,000s of population, at most 100,000 (and that achieved after the arrival of the Europeans). In Afroeurasia (the Old World), you don’t count as a megaempire unless you have tens of millions of subjects—that’s three orders of magnitude larger than Hawaii! Why is this important? Because it is only by tracing the trajectories of societies that go beyond the social scale seen in Austronesia that we can test the social control hypothesis against the God-Kings theory. If HS helps to stabilize hierarchical societies, it should do so for societies of thousands, tens of thousands, hundreds of thousands, millions, tens of millions, and so on. So we should see it persist as societies grow in size. If HS is destabilizing, then once societies become too large, HS would be selected out. HS is a liability at any size, but the larger a society becomes, the harder it is to hold it together. In this sense HS is a greater liability for megaempires than for chiefdoms. Moreover, if you live on a huge continental mass (and not in an isolated archipelago), once somebody figures out how to make hierarchical societies less unequal (and more cooperative), these people will conquer their more despotic neighbors, spreading egalitarian institutions—eventually—through the whole Afroeurasia. Note that equality here is very relative—post-Axial megaempires (read Ultrasociety on the importance of the Axial Age) also had kings and nobles, and were quite unequal by today’s standards. But they were better than the archaic states. As time went on, they gradually accumulated various egalitarian institutions. After a few thousands of years this process resulted in pretty decent modern societies in which most of us live today. Even the worst ones, like North Korea, don’t practice human sacrifice. Cultural evolution is faster than genetic evolution, but it still needs time to cumulate useful, functional traits, such as the norms and institutions that promote equality, and therefore cooperation. So we need to go outside the Austronesian material to test the predictions of the social control hypothesis versus the God-Kings hypothesis. This is what we are doing in the Seshat project, and I estimate that we will complete data collection on extreme forms of inequality by the end of summer or early Fall. So stay tuned. But as a preview (which will not come as a surprise for those of you who read a lot of world history), preliminary results suggest that HS is very common (if not ubiquitous) at a certain stage of social development—and then it invariably fades away. HS must impose a pretty hefty price on social durability and ability to resist external enemies to generate such a clear-cut macrohistorical pattern. Also, our preliminary results in the Seshat Databank indicate that there are many other kinds of human sacrifice than the one Watts et al. discuss, which targeted people at the low end of the social hierarchy. In some societies, it is aliens, such as captured warriors, are sacrificed. In others we see sacrifice of high-status individuals, such as children of the elites, or their military followers. Things are much more complex—an interesting!—than one might conclude on reading their paper. A final thought, hearkening back to another paper Watts et al published last year, in which they argued that moralizing High Gods are not necessary for the evolution of political complexity. Same problem. Perhaps High Gods were not needed for complex chiefdoms in Hawaii and Samoa, but the jury is still out on whether megaempires cold survive without them. Again, stay tuned! [post_title] => Is Human Sacrifice Functional at the Societal Level? [post_excerpt] => There are problems with the conclusion that human sacrifice is linked with the evolution of hierarchical societies. 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H. floresiensis cranium from the North Carolina School of Science and Mathematics (NCSSM), via Flickr.

A new paper out last week in Nature by Sutikna et al. has generated a great deal of excitement, by reporting a revised date range for the diminutive hominin, H. floresiensis, nicknamed the “hobbit” because of its stature (just 3.5 feet for adults). The previous accepted date range for H. floresiensis (95,000-12,000 YBP) implied that humans and hobbits co-existed on Flores for some considerable period of time, prompting fun analogies to the setting of J.R.R Tolkein’s legendarium, Middle Earth, in which humans, hobbits, dwarves, and other races co-existed and interacted. (I actually love this comparison, because it really resonates with my students and makes teaching human evolution easier.) The original dates for H. floresiensis were based on the (then reasonable) assumption that the depositional sequence in one part of the cave was representative of other parts of the cave. But caves are very active geological systems, and it often turns out—as in this case—that stratigraphy isn’t uniform. When researchers excavated elsewhere in the Liang Bua cave, they found new stratigraphic details (erosion of older deposits followed by filling in of younger deposits) that prompted them to redate the sequence. These new data gave a range of 100-60,000 YBP for the skeletal remains of H. floresiensis and 190,000-50,000 for floresiensis-associated artifacts. The end of this date range—50,000 YBP—happens to be close to the estimated appearance time of modern H. sapiens on the island, which raises some intriguing questions.  For those of you interested in a detailed analysis of the errors of the original dates, and a discussion of some still-unresolved issues with the paper, I highly recommend John Hawks' post What the revised Liang Bua chronology leaves unanswered. [caption id="attachment_120004555" align="aligncenter" width="1024"]The cave at Liang Bua. By Rosino - [1], CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=4567792 The cave at Liang Bua. By Rosino - [1], CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=4567792[/caption]
Unfortunately, some in the media have gone beyond discussion of those questions and breathlessly reported that these new dates likely mean that our species wiped out the hobbits. While that hypothesis certainly remains a formal--and fascinating-- possibility, it’s only one of several. It’s based simply on the observation that under the current chronology, approximately the same time modern humans got to Flores, we stop seeing hobbit tools in the deposits at Liang Bua. But I want to point out that we have no direct evidence (as yet) showing any interaction between H. sapiens and H. floresiensis, as the authors noted in the closing sentence of their manuscript:
“Parts of southeast Asia may have been inhabited by Denisovans or other hominins during this period, and modern humans had reached Australia by 50 kyr ago. But whether H. floresiensis survived after this time, or encountered modern humans, Denisovans or other hominin species on Flores or elsewhere, remain open questions that future discoveries may help to answer.”
So as of right now, we actually don't know that humans killed off (either directly or indirectly) hobbits. But this is probably the interpretation that the interested public has walked away with after this week. Call me conservative, but I'm a bit uncomfortable that our speculation might have given people the impression of greater certainty than we actually have. There are two other aspects of this new finding that interest me. The first is the implication that these older dates have for the debate over whether H. floresiensis was a pathological modern H. sapiens or a separate species with distinctive morphology, likely caused by insular dwarfism. The older date range for these fossils suggests that H. floresiensis was indeed a separate species. Furthermore, as Kristina Kilgrove discusses, these new dates also undermine cryptozoological interpretations of the Indonesian legend of Ebu Gogo as deriving from sustained interactions between humans and H. floresiensis as recently as 12,000 years ago. The second aspect of these new dates that I find interesting is that although they mean that the hobbits were older than we initially thought, they still fall within the range of time in which it’s possible to obtain ancient DNA from skeletal remains. I have no idea whether there will be further attempts to extract aDNA from the hobbits (previous attempts were unsuccessful), but I continue to be hopeful that someday we will have hobbit DNA. If H. floresiensis is, as some suspect, a descendant of H. erectus, then their genomes could give us a glimpse of that species’ genetic diversity and help us better understand the evolutionary history of ourselves, Neanderthals, and Denisovans. [post_title] => No, We Don’t Actually Know That Modern Humans Killed Off The Hobbits [post_excerpt] => We have no direct evidence showing any interaction between Homo sapiens and Homo floresiensis, let alone that humans killed them off. But this is probably the interpretation that the interested public has walked away with after this week. [post_status] => publish [comment_status] => open [ping_status] => closed [post_password] => [post_name] => no-we-dont-actually-know-that-modern-humans-killed-off-the-hobbits [to_ping] => [pinged] => [post_modified] => 2016-04-04 13:32:04 [post_modified_gmt] => 2016-04-04 17:32:04 [post_content_filtered] => [post_parent] => 0 [guid] => https://evolution-institute.org/?post_type=blog&p=120004553 [menu_order] => 0 [post_type] => blog [post_mime_type] => [comment_count] => 0 [filter] => raw ) [3] => WP_Post Object ( [ID] => 120004544 [post_author] => 52 [post_date] => 2016-04-01 08:00:23 [post_date_gmt] => 2016-04-01 12:00:23 [post_content] =>
Galaxies... or budding yeast? Image by Mararie, via Flickr.

Sex isn’t quite what it seems – while superficially wasteful in an evolutionary sense (why pass on only one half of your genes, when you can inherit all of them asexually, or why waste resources in mating when you don’t need a mate asexually?), theory and empirical studies have argued for the evolutionary advantages of sex in the context of favoring recombination, efficacy of natural selection in purging or fixing new mutations, and thus evolving faster (see my review of literature and arguments for the evolution of recombination, and thus sex). My favorite summary on the subject by far has to come from Sarah Otto, in the article on “Sexual Reproduction and the Evolution of Sex”, which decimates some common misconceptions on the evolutionary advantages of sex. You think it’s pleasurable? As Otto puts it,
The first eukaryotes to engage in sex were single-celled protists that appeared approximately 2 billion years ago, over 1.3 billion years before development of the first animals with neurons capable of assessing pleasure…Surely, pleasure was not in a bacterium's realm of experience.
Otto also argues using a simple example how sex evolves adherent to recombination producing new combinations of genes, maintaining diversity within a population, whereas non-recombining, or asexual populations deleterious mutations in a “Muller’s ratchet”.
Birds do it, and bees do it. Indeed, researchers estimate that over 99.9% of eukaryotes reproduce sexually. What, then, are the true costs and benefits of sex?
However, there has been little empirical evidence that points directly towards the adaptive advantages conferred by sex. McDonald et al. (2016) sought genomic evidence of what we’ve known for a good part of population genetics since the modern synthesis – does sex speed adaptation? Using experimental evolution strains of S. cerevisae across 12 asexual (mitotic, budding) and 6 sexual (meiotic, sporulating) populations for ~1000 generations. Fitness assays of evolved populations were performed against ancestral strains. Genomes of every 90th generation were sequenced across four sexual, and one asexual population. [caption id="attachment_120004545" align="aligncenter" width="525"]The rate and molecular signatures of adaptation M J McDonald et al. Nature 1–4 (2016) doi:10.1038/nature17143 The rate and molecular signatures of adaptation
M J McDonald et al. Nature 1–4 (2016) doi:10.1038/nature17143[/caption] Key findings from this study include (1) the striking significant increases in fitness of sexually evolving populations, compared to the asexual populations (see above figure), (2) an average of 44 de novo mutations per population, with similar proportions of nonsynonymous, synonymous, and intergenic mutations between sexual and asexual populations (indicating that there was no net difference in the nature of mutations that were segregating or fixed in the asexual versus the sexual populations), (3) but importantly, a significant difference in rates of fixation of de novo mutations – fewer new mutations fix in sexual populations, indicating that sex improves the efficiency of natural selection, disallowing potentially harmful mutations to fix, and (4) the ubiquity of epistasis wherein a new mutation mayhaps be beneficial in one genomic background, but deleterious in another. This also makes it difficult to make an umbrella statement about which new mutations are "harmful" or "beneficial", since it is bound to depend on the genomic background in which the mutation arose.
Future studies are needed to fully understand the consequences of this interplay between sex and balancing selection, and to investigate how epistasis interacts with recombination to alter the dynamics of sequence evolution. By combining precise control of the sexual cycle with whole-population time-course sequencing, this experimental system offers the potential to understand how these factors affect the rate, molecular outcomes, and repeatability of adaptation.
References: McDonald, Michael J., Daniel P. Rice, and Michael M. Desai. "Sex speeds adaptation by altering the dynamics of molecular evolution." Nature 531.7593 (2016): 233-236. DOI: 10.1038/nature17143 Otto, S. "Sexual reproduction and the evolution of sex." Nature Education 1.1 (2008): 182. (A version of this article was previously published on The Molecular Ecologist here) [post_title] => The Fitness Benefits of Sex [post_excerpt] => Why pass on only one half of your genes, when you can inherit all of them asexually? Or why waste resources in mating when you don’t need a mate asexually? [post_status] => publish [comment_status] => open [ping_status] => closed [post_password] => [post_name] => the-fitness-benefits-of-sex [to_ping] => [pinged] => [post_modified] => 2016-03-31 18:35:42 [post_modified_gmt] => 2016-03-31 22:35:42 [post_content_filtered] => [post_parent] => 0 [guid] => https://evolution-institute.org/?post_type=blog&p=120004544 [menu_order] => 0 [post_type] => blog [post_mime_type] => [comment_count] => 0 [filter] => raw ) [4] => WP_Post Object ( [ID] => 120004237 [post_author] => 62 [post_date] => 2016-02-19 08:00:45 [post_date_gmt] => 2016-02-19 13:00:45 [post_content] =>
Red Crayfish (Procambarus clarkii) by Pam Corey, via Flickr CC BY-NC-ND 2.0.

It’s early on a Friday morning, and I’m standing in front of a crowd of eight-year-olds holding a fistful of dead crayfish. The mudbugs are a biological supply company purchase sorted into sealed sandwich bags – the subject and substrate for the science project I’ve volunteered to lead today. The activity I’ve planned is the most basic sort of comparative anatomy: we’re going to dissect crayfish and talk about how their bodies differ from our own. These kids won’t learn about evolution in school for years yet, but I’m hoping to lay some groundwork now that will make it a little less alien when they finally do get to it. Before we start, I ask if they can think of any other animals that are related to the crayfish. Hands shoot up. Lobsters, crabs, ants and other insects are named. I add a few other arthropods to the list, then drop the bombshell that makes them really pay attention: biologists think that every one of them is also a very very very distant cousin to those crayfish, the relative we shared probably looked a lot like a worm, and over the next hour I want them to think about how both the crayfish and vertebrates like us have solved the same problems while becoming less wormlike. They put on their gloves, I give them some quick instructions on how to use their dissecting scissors and probes, and at last, I hand out the specimens. There’s a chorus of ‘eeew’s as the kids dump crayfish onto their dissecting trays and the faint smell of preservative fluid wafts out of the bags. It’s followed by 40 minutes of barely controlled chaos. We count crayfish legs. We look at the eyes and the antennae. We look for the mouth, and talk about what all the little leg-like structures that surround it are for. We figure out whether each group has a boy crayfish or a girl crayfish. We cut open the thorax and look for gills, heart, and stomach. We open the stomach and poke the hard gastric teeth with our fingers. (The kids are delighted by the very idea of “stomach teeth.”) We find the nerve cord running along the belly and the intestine running along the back of the animal – the exact opposite of our own body plan. Some kids ask what’s inside the eyes. Others ask where the crayfish keeps its brain. I try to find something recognizable to point out inside the specimen that one group has poked to paste. At one table, a dark-haired girl has pushed her body as far away from the crayfish as possible, face twisted in disgust, but still cranes her neck around to see what her partners are doing. At another, a girl and a boy keep repeating to one another, “This is the best day ever!” After we clean up, the kids ask questions – but very few of them are actually about the dissection. Most of them, it turns out, are about me. What other animals have I dissected? What’s the worst thing I ever smelled? How long does it take to get a Ph.D.? What do I do at work all day? Do I have a dog? I get it, really. I’m probably the first scientist they’ve met in the flesh. All those questions are just proxies to ask: “If I wanted to, could I be a scientist when I grow up, too?” Hopefully, I was able to let them know that even though it takes a lot of hard work, the answer to that question could be yes. [post_title] => Evolution for Third-Graders [post_excerpt] => It’s early on a Friday morning, and I’m standing in front of a crowd of eight-year-olds holding a fistful of dead crayfish. [post_status] => publish [comment_status] => open [ping_status] => closed [post_password] => [post_name] => evolution-for-third-graders [to_ping] => [pinged] => [post_modified] => 2016-02-18 17:04:47 [post_modified_gmt] => 2016-02-18 22:04:47 [post_content_filtered] => [post_parent] => 0 [guid] => https://evolution-institute.org/?post_type=blog&p=120004237 [menu_order] => 0 [post_type] => blog [post_mime_type] => [comment_count] => 0 [filter] => raw ) ) [post_count] => 5 [current_post] => -1 [in_the_loop] => [post] => WP_Post Object ( [ID] => 120004599 [post_author] => 43 [post_date] => 2016-04-14 08:00:51 [post_date_gmt] => 2016-04-14 12:00:51 [post_content] =>
"Childbirth" by cassy1723, via Flickr.

No one is saying that medicine isn't brilliant and hasn't saved lives. But it does intervene more than necessary when it comes to pregnancy and childbirth. Part of that unnecessary intervention is driven by lack of experience; it's difficult to decide when intervention is and isn't necessary, especially when things are heating up. Part of it stems from an economically-driven disrespect for time. (Give childbirth some motherlovin' time.)  But another part of the trouble actually lies in the evolutionary perspective. Unfortunately it's not all rainbows and unicorns when M.D.s embrace evolution. Instead, evolutionary thinking is biasing some medical professionals into believing that, for example, birth by surgical caesarean is an "evolutionary imperative." Here's one recent example in The American Journal of Obstetrics & Gynecology of how the evolutionary perspective is (mis)guiding arguments for increased medical intervention in childbirth. It's a paper called, "A large head circumference is more strongly associated with unplanned cesarean or instrumental delivery and neonatal complications than high birthweight." It's a fairly straight-forward study of over 22,000 birth records at a hospital in Jerusalem. The authors ask whether birth weight (BW) or head circumference (HC) is more of a driver of childbirth interventions (instrumental delivery and unplanned caesareans) than the other. Of course, the focus is on the biggest babies with the biggest heads causing all the trouble, so the authors narrow the data down to the 95th percentile for both. Presumably they're asking this question about BW and HC because both can be estimated with prenatal screening. So there's the hope of improving delivery outcomes here. And, of course, the reason they ask whether head size or body mass is more of a problem is because of evolution. They anticipate that they'll discover that heads are a bigger problem than bodies because of the well-known "obstetrical dilemma" (OD) hypothesis in anthropology. OD thinking goes like this: Big heads and small birth canals are adaptive for our species' cognition and locomotion, respectively, but the two traits cause a problem at birth, which is not only difficult but results in our species' peculiar brand of useless babies. (But see and see.) So, since we're on the OD train, it's no surprise when we read how the authors demonstrate and, thus, conclude that indeed HC (head circumference) is more strongly associated with childbirth interventions than BW (birth weight), at least when we're up in the 95th percentile of BW and HC. Okay. They use this finding to advocate for prenatal estimation of head size to prepare for any difficulties a mother and her fetus may be facing soon. Okay. Sounds good. Sounds really good if you support healthy moms and babies. But it also sounds really good if you already see these risks to childbirth through the lens of the "obstetrical dilemma" with that OD thinking helping you to support some sort of evolutionary imperative for the c-section. Okay. Too many "Okays" you're thinking? You're right. There's a catch. When you dig into the paper you see that "large HC" heads are usually about an inch (~ 2.5 cm) greater in circumference than "normal HC" ones. (Nevermind that we chopped up a continuum of quantitative variation to put heads in arbitrary categories for statistical analysis.) And when you calculate the head diameter based on the head circumference, there is less than 1 cm difference between "large" and "normal" neonatal heads in diameter. That doesn't seem like a whole lot considering how women's bony pelvic dimensions can vary more than that.  Still, these data suggest that the difference between a  relatively low risk of having a c-section and a relatively high risk of having a c-section amounts to less than a centimeter in fetal head diameter. And maybe it does. Nobody's saying that big heads aren't a major problem sometimes! But maybe there's something else to consider that the paper absolutely didn't. Neonatal heads get squeezed and molded into interesting shapes in the birth canal. The data say that normal HC babies get born vaginally more often than large HC ones. But this is based on the head measures of babies who are already born! If we're pitting head circumference (HC) of babies plucked from the uterus against the HC of babies who've been through hello! then of course the vaginally delivered ones could have smaller HCs. C-sected babies tend to have rounder heads than the ones squeezed by the birth canal. It's impossible to know but I'm fairly confident about this, at least for a subsample of a population: Birth the same baby from the same mother both ways, vaginally and surgically, and its head after c-section will have a larger HC than its squeezed conehead will after natural birth. When we're talking about roughly 2.5 cm difference in circumference or less than 1 cm difference in diameter, then I'd say it's possible that neonatal cranial plasticity is mucking up these data; we're sending c-sected babies over into the "large HC" part of the story just because they were c-sected in the first place. So without accounting for this phenomenon, the claim that large head circumference is more of a cause of birth intervention, of unplanned c-sections, than large body mass isn't as believable. If these thoughts about neonatal cranial molding are worthwhile, then here we have a seemingly useful and very high-profile professional study, grounded in the popular but deeply flawed obstetrical dilemma hypothesis, that is arguing for medical intervention in childbirth based solely on the difference in head size measures induced by those very medical interventions. The circle of life! [post_title] => When Evolutionary Medicine Gets It Wrong About Childbirth [post_excerpt] => Evolutionary thinking is biasing some medical professionals into believing that birth by surgical caesarean is an "evolutionary imperative." 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