…and this is a 
Will the availability of C-sections give humans bigger brains?
Category: Evolution • Reproduction • Science
Posted on: October 17, 2008 12:23 PM, by PZ Myers
While Steve Jones might think human evolution has stopped, I have to say that that is impossible. If human technology removes a selective constraint, that doesn't stop evolution — it just opens up a new degree of freedom and allows change to carry us in a novel direction.
One interesting potential example is the availability of relatively safe Cesarean sections. Babies have very big heads that squeeze with only great difficulty through a relatively narrow pelvis, so the relationship in size between head diameter and the diameter of the pelvic opening has been a limitation on human evolution. We know this had to be a factor in our evolution: the average newborn mammal has a cranial capacity that is roughly 50% of the adult size, chimpanzee babies have heads about 40% of the adult size, but human babies have crania that are only 23% of what they will be in adults. While our brains have gotten larger over evolutionary time, they have not gotten proportionally larger in utero, because large-headed babies increase the difficulty of labor and cause increased mortality in childbirth. If childbirth could bypass the pelvic bottleneck, that would allow for fetal heads to grow larger without increasing the risk of killing mother and/or child.
And childbirth is a risky proposition for women; 529,000 die every year from this natural process (although only about 1% of those deaths occur in places where women have access to good, modern medical facilities — hooray for modern medicine). About 8% of those deaths occur from obstructed labor, where the fetus is unable to proceed through the birth canal for various reasons, and these are the kinds of birth problems that can be circumvented by C-sections. In practice, teaching health care workers how to carry out emergency C-sections has been tested in regions in Africa, where it has actually worked well at reducing maternal mortality.
This is the subject of an article by Joseph Walsh in the American Biology Teacher, which suggests that C-sections will have an effect on human evolution.
"Nothing in biology makes sense except in the light of evolution." This was the title of an essay by geneticist Theodosius Dobzhansky writing in 1973. Many causes have been given for the increased Cesarean section rate in developed countries, but biologic evolution has not been one of them. The C-section rate will continue to rise, because the ability to perform a safe C-section has liberated human childbirth from natural selection directed against too small a maternal pelvis and too large a fetal head. Babies will get bigger and pelves will get smaller because there is nothing to prevent it.
The evidence so far is entirely circumstantial, but Walsh makes an interesting case. There are several correlations that imply an effect, but I can't help but think there are alternative explanations that may swamp out any heritable, evolutionary effect. The kinds of evidence he describes are:
A known trend for increasing birth weight in the US, by about 40 g over 18 years in one study. It's there, all right, but these studies don't demonstrate a genetic component to increased size — it could be a consequence of better nutrition and medical care.
An increasing frequency of C-sections. Again, this isn't necessarily genetically based at all, but could be a consequence of fads in medicine, or social factors, such as an increase in the likelihood of medical malpractice suits making doctors more cautious.
Walsh describes a couple of studies that seem to show that cephalopelvic disproportion (small pelvis or large babies or both together) does have a genetic component. So at least it is likely that there are heritable variations in these parameters that could influence the likelihood of obstructed labor.
There is statistical variation in neo-natal mortality that varies with birth weight in a suggestive way. Low birth weight clearly puts infants at risk, and there is an optimum weight around 3600 grams for newborns that minimizes mortality. Death rates also rise with increasing birth weight above the optimum. There is some data that suggest that availablity of modern medical care and C-sections reduces infant mortality at larger birth weights.
That increasing availability of C-sections might lead to an evolutionary shift towards increasing cranial capacity at birth is a reasonable hypothesis, but I'm not convinced that it has been convincingly demonstrated yet. There are too many variables that effect brain size at birth to make a clean analysis possible; in addition, many of the measures are indirect. Often, we use birth weight as a proxy for cranial capacity, and that means the numbers and correlations are sloppier than they should be. Many of the measurements made are of factors that are readily influenced by the environment, which makes it difficult to imply that these are the product of genetics.
So the idea is weakly supported, but tantalizing. Even as a purely theoretical exercise, though, what it does say is that it is obvious that human culture cannot end human evolution…all it can do is shape the direction in which it can occur.
Walsh J (2008) Evolution & the Cesarean Section Rate. The American Biology Teacher 70(7):401-404.
Life Science
Comments
Posted by: Paul Burnett | October 17, 2008 12:36 PM
"...increasing birth weight...could be a consequence of better nutrition and medical care." - PZ Myers
Increased birth weight may also be tied to overeating / underexercising by the mother - which now (with C-sections) is not the risk it used to be. Increased weight of both mother and child is also tied to gestational diabetes which has other risks.
Posted by: I am so wisem | October 17, 2008 12:42 PM
The natural birth and C-section hating crowd will go bonkers over this. I bet this is a conspiracy by Big Obstetrics to con women into having sections
You think creationists are bad, see how natural birth advocates view evolutionary theory.
Posted by: Hairy Doctor Professor | October 17, 2008 12:42 PM
This is something I've wondered about for quite a while. Nice to know other people are thinking about it, too, particularly those with the chops to speak to the matter rationally. Of course, big-headed-super-evolved-humans-of-the-far-future have been a science fiction staple for years. (Why are they always bald?)
Posted by: Sean Peters | October 17, 2008 12:43 PM
This is all rather pie-in-the-sky in any case. We've been doing C-sections routinely for what, 50 years? Call me 10,000 years from now, after we've been doing this long enough for evolution to actually produce some noticeable change. I'm somewhat dubious that current trends in C-section rates will continue long enough to make any difference - given the length of time required, it seems likely that we'll either have reverted to savagery, or uploaded our brains into computers or something.
Very interesting to think about, though.
Posted by: MF | October 17, 2008 12:44 PM
While I appreciate the worth of larger brains and so on, it does depress me sometimes to think that our species is increasingly dependent on technology. A race of large-headed, narrow-hipped people would be ripe for extinction, although I guess that's the fate of all species eventually.
Posted by: MarkusR | October 17, 2008 12:47 PM
Sean Peters stole my comment. I have it on tape.
Posted by: JStein | October 17, 2008 12:48 PM
PZ, wonderful post.
I have had a lot of conversations (especially with fundamentalist Christians) who don't think that evolution is happening right now.
Whatever you believe about the ethics of eugenics there are certainly changes happening, slowly but surely. This is an interesting idea, and while I'm not enough of a scientific mind to tackle the plausability, anything is possible with time.
Posted by: Marlon | October 17, 2008 12:50 PM
A few years ago, my mom and dad brought me back a nice wool cap from one of their trips to Scotland. It was too small! One would think that of all people, one's own mother would have a good idea of a son's head size.
Posted by: Bachalon | October 17, 2008 12:52 PM
One thing I've wondered about is if c-sections are going to remove the ability of any woman to give live birth.
I mean, think about it: there are woman who would have died had they had more than one child without a c-section; they're having more children now, girls, too, who may also have the same disadvantage that their mothers do.
Toss in a bit of time and we might have a human race incapable of reproducing on it's own.
Posted by: Greg Laden | October 17, 2008 12:53 PM
I have not read the paper yet, but will. But, a couple of quick comments.
If you reduce constraining selection, you have to look at what is being constrained, not the sexy feature that resides among these constraints. The selective 'intersection' here is birth canal size which in turn is a function of a couple of well studied anatomoical features of the female-configured human pelvis. These are currently under selection, and that selection is reduced. So you would see a change in female pelvis shape. Since the brain is a very costly thing, and as time goes by we seem to use them less and less (brain size has been going down for several thousand years now in most populations) I would suggests that the pelvic shape is what would change.
On the baby end of it, there are two variables you have to wade through before you get to the sexy variable (of brain size). One is neonate head deformation, which is the single biggest factor. Babies do not have heads that are 10 cm in diameter, yet they squeeze through a 10 cm diameter hole.
Considering head size and not considering this squeezing act is like thinking of a gymnastics routine in terms of the landin/final pose only and totally ignoring everything that comes before.
Finally, the real variable here is probably head size, not brain size. It is concievable that human head size could grow without the brain getting bigger. Thicker skulls may have low cost but a great benefit (humans are killed by head injuries far more easily than any other mammal) and as stated above brain size is currently under selection (apparently) to go down.
yea, I'll have to think about this more and write it up..
Posted by: Glen Davidson | October 17, 2008 12:55 PM
Reductions in smoking should play a role, as well as the deliberate attempts to increase birth weight.
We are far from seeing any meaningful selective pressure for genetically increasing birth weight or cranial size at birth.
No apparent cause, no inferrable effect.
Glen D
http://tinyurl.com/2kxyc7
Posted by: Tim Fuller | October 17, 2008 1:03 PM
If having a larger frame (not just a larger brain) was the relevant factor in our likely continued success, we would already be extinct, or likely not have evolved in the first place. My reading of evolution is that we owe our success almost exclusively to our larger brains, which give us the ability to adapt to myriad environmental situations.
We'll still go extinct. For sure when the Sun expands if we haven't gotten sufficient distance between ourselves and the event by that time. I'm just hopeful we can make it through the next few generations, what with the greater selective dangers presented in films like Idiocracy.
Enjoy.
Posted by: Bostonian | October 17, 2008 1:05 PM
This is very interesting, and I agree it seems plausible. If the genes for large babies and small pelvises are more common in the wild, we should see C-Sections get ever more common, and even perhaps universally required vital at some future point.
On a somewhat related note, I've often wondered if the existence of corrective eyewear means that near- and far-sightedness are becoming more common over time, there being fewer limitations on the viability of individuals who see as well as I can ... without my contacts.
Posted by: Chris Davis | October 17, 2008 1:05 PM
The extra jawbone snakes have that allows them to swallow things twice as big as their head has, in mammals, wound up in the inner ear, I believe.
We need to winkle it out and insert it in the pelvis, where it belongs. If women are a little harder of hearing as a result, us boys'll just have to shout.
CD
Posted by: Dahan | October 17, 2008 1:12 PM
,blockquote> A race of large-headed, narrow-hipped people would be ripe for extinction, although I guess that's the fate of all species eventually.
This just goes to show how blind you are! Isn't that just like you "science" types, you can't see the obvious. Just look at how all the survivors of Alien abductions have described them. Big heads, tiny bodies, almost no hips... Obviously, what we've got here is just a small step in our realizing what our celestial brothers and sisters went through millennia ago. When will you all stop being accomplices to the cover-up?
OK, I'll toss in the ;) to avoid the Poe.
Posted by: Dahan | October 17, 2008 1:14 PM
Sorry about the blockquote screw-up.
Posted by: kermit | October 17, 2008 1:16 PM
If things were stable for a very long time... sexual selection might end up choosing narrow-hipped women and sensitive guys. But things won't be stable; Sean is right. Before a dozen generations go by we will be re-engineering our children.
First remove all those pesky defects like heart disease and diabetes and myopia, then enhance. Pity the last generation of non-enhanced children. So short, dwarfish and crooked. Heavy eyebrows. Not Hollywood/Bollywood beautiful. Then big-headed and bald, with lasers in their foreheads, or something. Not all enhancements will work. Will the first attempts to produce hyper-intelligent babies "take", or will they be insane or idiot savants? Eventually these changes will work. Natural, economic, military, and cultural selection will determine which traits get passed on to future generations. The pool of variability will no longer be random. The fashion industry will play a larger role than predator habits. (Or are they the same thing?)
Posted by: MF | October 17, 2008 1:22 PM
Oh, I'm not questioning the importance of increasing brain size in our evolution. What I meant is that once we pass the point at which baby heads cannot fit through female pelvises, we cannot survive in the wild.
Posted by: Joel Sammallahti | October 17, 2008 1:25 PM
I wonder how big an adaptive advantage forgetfulness is nowadays. I'd guess it's not an insignificant number of pregnancies that result directly from forgetting to take the pill... =)
Posted by: Ann | October 17, 2008 1:28 PM
I echo the good Doctor Professor's concern: Sure, they'll have bigger heads, but what about the baldness?
Posted by: Didac | October 17, 2008 1:33 PM
Only a few points:
1) How is the correlation between cranial capacity and intelligence in modern human populations?
2) Far more important, there is a significant correlation between cranial capacity and fitness (reproductibility)?
The first point is very probably true for all australopithecine/human species combined. However, using cranial capacity (aside from microcephaly cases) to predict intelligence sounds to me like phrenology or something.
The second point is harder to accept. I'm not of the Galtonian sense of accepting that a high IQ is detrimental to reproductive success. However, a higher IQ is correlated to later nuptiality ages and a reduced offspring: social position for intelligent people seem more valuable than high-rate breeding.
And, of course, in only two generations, the selective advantage of tiny C-section differenced should be enormous in order to be reflected in anthropometrical statistics.
Posted by: Cardinal Shrew | October 17, 2008 1:35 PM
How long have C-Sections been safe and easily available? It doesn't seem like enough generations to see a shift in a population yet.
As for human evolution, I can think of one trait right off that probably wasn't present in our ancestors. I for one would not want to be hunting or gathering or avoiding predators without my glasses. It would not be wise. I suspect that line of my genetic ancestry would have been selected out if we were still hunter gatherers.
Evolution will of course continue, where will it take us is the question.
Posted by: Russell | October 17, 2008 1:36 PM
I find this fascinating; please correct any misconceptions that I have in this. Plus I'm at work, so thoughts are a bit distracted.
As the human cranium enlarges, the bone structure of women should widen to accommodate, over time on an evolutionary basis. This can be affected by culture to a point, namely men's preference to a certain hip to waist ratio. If culturally, men prefer smaller hip to waist ratios, then women with smaller bone structures will have an advantage over those with wider hips, (despite evolutionary adaptations of wider hips to insure the accommodation of successful births). How then could this process affect men in any way?
I realize this has nothing to do with C-sections.
This opens a new train of thought for me. Thank you.
Posted by: Anthony | October 17, 2008 1:37 PM
Re #17:
If everyone is beautiful then nobody is beautiful. They'd all just be normal.
Posted by: IR | October 17, 2008 1:38 PM
Question: I see the observation about evolution taking a long time so we aren't going to know if this is the case for another ten thousand years, but I've read about the speed at which evolution can take place, surprising a great many people. The example that comes to mind is the couple who spent decades studying Darwin's Finches in the Galapagos and documenting the speed at which beak size could change.
All the other problems aside, is time necessarily a factor that discounts this possibility?
I'm asking because I really don't know, being an amateur and all. Any biologist's opinion on this would be welcome.
Posted by: Russell | October 17, 2008 1:39 PM
I should ask, how can the process I mentioned above affect mens preference for larger hip to waist ratios?
Posted by: Cuttlefish, OM | October 17, 2008 1:39 PM
You silly humans--
Thinking you can fool nature.
You'll get a big head!
Posted by: Tim Fuller | October 17, 2008 1:48 PM
For sure, but to be honest with you, we are already way past that point. Imagine the die-off in Western society if there were shortages of electricity. We already lose a pretty good chunk of old and poor people whenever the climate (in the cities no less) becomes extreme to either direction. There the breakdown is economic, but an odd plasma blob thrown our way by the Sun could conceivably damage the grid to an extent a Mad Max scenario could easily emerge. As a matter of principle it seems we passed the point of 'individual' survivability when we quit growing our own food, etc.
Enjoy.
Posted by: Patricia | October 17, 2008 1:50 PM
Where's w00+ ? We could use some boobies to go with our fat heads.
Posted by: Chris Schoen | October 17, 2008 2:03 PM
Hopefully these large-brained Omega-humans of the future will quickly realize how barbaric it is to turn childbirth into a major surgical procedure for the sake of bigger brain pans, and encourage that the practice be discouraged unless absolutely necessary.
At any rate, as you point out, PZ, small natal cranium size is a matter of comparative size to the mature skull (and presumably, brain) relative to other mammals. The birth canal constraint means that humans take much much longer to reach adulthood (in effect, we're all born prematurely). Having a bigger skull or brain at birth does not necessarily translate into having a bigger brain at maturity. The fact that humans have evolved to have bigger proportional brains despite the same proportional pelvic size to other mammals suggests that the "limitation" you refer to has already been surmounted, without the influence of human technology.
Posted by: jkessler | October 17, 2008 2:08 PM
I thought that cranial size affected more the time in utero, which is why human babies are so helpless after they are born; human birth is relatively earlier because the babies heads are bigger.
Posted by: raven | October 17, 2008 2:10 PM
The end of human evolution is obviously wrong. There are just huge new selection pressures on us.
1. The ability to live in what historically would be incredibly overcrowded conditions.
2. The ability to recognize different looking people as variants rather than scary strangers not of the tribe and to be killed first and questioned later.
3. The ability to use Hi Technology. This is a fast moving target. Fifty years ago it might have been driving machinery, today it is using computers and the internet.
4. The ability to plan ahead more than 6 months. While this hasn't quite happened yet, it would be useful.
5. Ability to metabolize and tolerate xenochemicals and pollution.
6. Cancer resistance. Cancer occurs as the 3rd or so power of age. We never used to have to worry about it because we never lived to 80.
7. The ability to not overindulge in cheap abundant food and a large variety of drugs and alcohol just because they are there.
8. A bunch of other environmental conditions incident to a modern, fast changing (ability to adapt to lightening fast change!), modern technological civilization.
9. Ability to recognize dumb superstitions and avoid them. Dead, cut up albinos are not magic gold bait. Faith healing kills. The Alzheimer old lady down the street is not a witch with supernatural powers.
There is even data that says human evolution has picked up in the last 10 kiloyears.
Intelligent Design my ass. Really just Bad Design obviously. Someone really needs to invent the artificial womb one of these days.
Posted by: gillt | October 17, 2008 2:11 PM
A few things I haven't heard mentioned yet:
1. The mother releases Oxycontin during natural childbirth--something to do with the pelvic muscles. Oxycontin, among other things, stimulates bonding in the child to its mother. We'll have to take this into consideration if we're promoting c-sections.
2. If the increased brain-size hypothesis holds true and catches on, as a species we'd be reliant upon c-sections, no long afforded the option of natural birth.
Posted by: Paul Burnett | October 17, 2008 2:12 PM
"One thing I've wondered about is if c-sections are going to remove the ability of any woman to give live birth." - Bachalon, #9
This has been explored at length in science fiction, possibly best (IMHO) by Lois McMaster Bujold in the Miles Vorkosigan stories - Miles was gestated in a uterine replicator. See http://en.wikipedia.org/wiki/Artificial_uterus for a general discussion.
Posted by: Prof MTH | October 17, 2008 2:12 PM
Will the availability of breast implants give women bigger (or smaller) breasts?
Will the availability of penile implants give men bigger (or smaller penises?
Will the availability of piercing gages give humans bigger ear lobes?
Posted by: jkessler | October 17, 2008 2:20 PM
Rats, Chris S beat me to it.
Posted by: Bill Dauphin | October 17, 2008 2:25 PM
Dang it, PZ, what's with all the sciency stuff lately? Isn't this supposed to be an angry rants blog?
;^)
Posted by: Glen Davidson | October 17, 2008 2:28 PM
Oh, I wouldn't go telling people that birthing women are a source of oxycontin. They'll try to get their favorite narcotic from them.
"Oxytocin" was probably what you meant to write.
Glen D
http://tinyurl.com/2kxyc7
Posted by: JohnFen | October 17, 2008 2:28 PM
@gillt, #33: I suspect that you mean oxytocin, not Oxycontin. Although the latter might make childbirth a bit more fun!
Posted by: JohnFen | October 17, 2008 2:32 PM
Jinx!
Posted by: DjtHeutii | October 17, 2008 2:33 PM
The obvious next step is to go the artificial womb route. If we can perfect technology to grow babies outside of the confines of the human body then we can remove that boundary of limitations on the development of humans. As well as free women from the burdon of bearing children altogether if they wish.
Posted by: MegK | October 17, 2008 2:34 PM
we should see C-Sections get ever more common, and even perhaps universally required vital at some future point.
Well, we've already selectively bred certain dog breeds (bulldogs, for one) to the point that somewhere around 90% or more require c-sections.
Posted by: Pierce R. Butler | October 17, 2008 2:37 PM
This brings up a question that's bugged me for years.
Consider two essentially equivalent female specimens of, say, Homo erectus. One, due to her own DNA &/or choice of mates, has a slightly larger-brained child; the other has a child of the same cranial capacity.
Cut away for a moment to the paleontological record, which (last I read - no reference immediately available) has it that "technology" (chipped flints) shows no signs of development during the evolutionarily-brief interval during which our ancestors' brains enlarged. Only after that growth spurt do improved designs emerge.
Back to our two mamas: the one with the larger-brained infant has incurred a slight but non-trivial disadvantage in natural selection, since her pelvis - not even fully adapted to a bipedal posture - is a bit too narrow for her offspring. Her neighbor has less of a problem surviving the delivery of her young'un. Multiply this scenario by thousands of generations, and the persistence of the megacephaloid mutation seems more than a little problematic.
Yet, somehow, the big-brained variant is the one that lasted. Apparently, even though there was no detectable advantage in exophenotypical* development, something over the protohumanoid millennia gave an advantage to those with a higher probability of maternal/infant death.
The usually-proposed scenarios aren't all that convincing - greater cooperation among pack members? Lots of smaller-brained critters do that quite well? Sexual selection? If the females charmed by more articulately-grunting males are more likely to be killed attempting to bear their brats, it would be the jock groupies who dominate the succeeding generations. Non-durable artifact creation? Then why would stonework be excepted? Etc, etc.
Yet, obviously, here we are, with (in the wild) an improbably high obstetric mortality rate compared to (sfaik) practically all other species. Just what type of selection pressures, repeated over hundreds of generations, could have resulted in the observed outcome?
*Making up new big words - clear evidence of crackpottery. This query may thus safely be disregarded by Serious People.
Posted by: Richard Harris | October 17, 2008 2:40 PM
Let's assume that catastrophic global warming, religious wars, water wars, resource depletion, & global epidemics of fatal diseases & crop failures don't end our civilization. Human evolution by natural selection will then become irrelevant.
Genetic engineering, when applied to ourselves, will take us places natural selection never could, & fast.
Posted by: Frank Anderson | October 17, 2008 2:43 PM
Thank you, Didac! I'm glad someone raised those issues.
The selective advantage of bigger heads -- even if we assume bigger head = bigger brain = more smarterer -- in modern humans has not been demonstrated, as far as I know. I could imagine that, over the next few hundred years, the tail of the newborn cranial capacity distribution could spread out in the direction of larger newborn heads as C-sections become common around the world (assuming we avoid a global apocalypse and we do not merge with artificially intelligent machines), but I just don't see a shift in average newborn head size happening in humans.
Why? For a shift toward larger average newborn head size in humans to occur, newborn head size 1) needs to be heritable to some degree (the more the better, as far as selection is concerned) and 2) size of the head at birth needs to be positively correlated with fitness. I would buy #1; #2 is possible, but it has not been demonstrated.
Even if newborn head size has increased on average over the past several decades (has it?), I can't believe it's due to C-sections and natural selection. C-sections were probably pretty rare over most of the past fifty years (and still are in many parts of the world), the selective advantage of a larger head at birth (if an advantage exists at all) is probably minimal, and human generation times are just too long.
Cool stuff to think about, though. No, we have definitely not stopped evolving.
Posted by: JohnFen | October 17, 2008 2:43 PM
Sean Peters @ #4 sez:
It might not take that long. As I understand it, there is no orderly pace that evolution proceeds at. Sometimes it can happen very quickly, sometimes hardly at all. The mean rate caries from species to species, too, largely as a function of how quickly the species creates new generations.
Here is a scenario that could cause a noticeable evolutionary change within two generations in humans. If an environmental disaster of some sort pollutes the entrie globe with a chemical that kills 90% of humans. The remaining 10% reproduce, passing on the trait that makes them immune. Perhaps a large percentage of their offspring die (maybe the trait is recessive?) but they won't reproduce. Viola! A brand new human genetic trait.
Posted by: Gregory Kusnick | October 17, 2008 2:44 PM
MF, #18:
Define "wild". As individuals we cannot survive naked in the snow, and yet indigenous cultures of the high Arctic have survived for millennia using portable, improvised technology developed under intense cultural selection pressure. In the scenario you imagine -- the collapse of a civilization dependent on C-section birth -- the selection pressure would be equally intense, and there would necessarily be a lot of willy-nilly experimentation with in-the-field C-section techniques. Most of those experiments would fail, of course, but some might succeed. It's not inconceivable that some sort of surgically reproducing tribal culture might emerge from the wreckage.
Posted by: Tim H | October 17, 2008 2:46 PM
I don't see any reason to believe C-sections will lead to bigger brains. In order to change gene frequencies in the pool, a change has to lead to a reproductive advantage.
1) Bigger heas don't necessarily mean bigger brains. It could mean bigger sinusses, or jaws. Those might(or might not) lead to a reproductive advantage.
2) Bigger brains don't necessarily mean higher IQ. Structure and early developement might be more important.
3) Higher IQ doen't necessarily lead to more reproductive success. Sometimes it hurts.
4) The true factor at birth isn't head size, but cross-sectional area. C-sections might lead to lessening the disadvantage for broad vs narrow skulls. If greater head size actually does help reproductive success, we should all be coneheads by now, or on our way.
Posted by: Cannabinaceae | October 17, 2008 2:49 PM
On the topic of brain evolution - some people think that the intuitive physics that our brains evolved to be able to handle is what makes it impossible to "understand" relativity or quantum mechanics. Essentially what we have is the ability to intuit what happens when throwing rocks or spearing fish, by simulating certain differential equations with our neural networks.
Now, imagining a future where manipulation of matter at quantum or relativistic scales is of increasing importance: I wonder if brains that benefit from a mutation allowing them to intuit better on those scales will make it easier for those individuals to be successful in that kind of world, increasing their fitness.
Posted by: Bill Dauphin | October 17, 2008 2:51 PM
Here's a perfect opportunity for some expert in evolutionary theory among you to correct my ignorance:
Would a distinction that's only apparent in post-childbearing adults really have any effect on selection? Jus' wondrin'...
Posted by: Fatpie42 | October 17, 2008 2:51 PM
I have to say that I was also surprised by Steve Jones' comments. However, when I discovered that he was the writer of "Almost Like A Whale" I realised that he probably had some good points.
Actually he has already conceded a few things in the debate. He is only referring to human beings and one of the factors is that there are less areas where we are 'naturally selected'. In many cases we have artificial methods by which we can keep people alive longer. Of course, this is a sign of how the human race has adapted that as a group we can aid the survival of our less well-adapted members. However, an exception to this is Aids in Africa where the epidemic is likely to mean that the population will, to an ever larger extent, be made up of those who contract the virus less easily.
Perhaps a more important point he makes, however, is that men are most often choosing to have children while they are younger and then stopping later on. Many mutations came from the children of older men and now that this does not happen the new developments caused by mutations are far rarer.
C-section is a nice example, so well done. But there's a limit to how big our heads can get. Without a nice and regular occurrence of new mutations any evolutionary processes are going to be limited. Aren't we always being told by ID-theorists that evolution isn't true because mutations don't happen? Well, while that's completely bogus it might have an element of truth when applied to modern western humans in the 21st Century. If don't mutate, evolution is going to be scaled down quite a bit....
Posted by: tceisele | October 17, 2008 2:51 PM
Gillt (#33)
I don't think this is seriously an issue. My wife had both of our daughters by C-section, and she bonded with them just fine. As in, if she was any more bonded to them, I would be afraid for her sanity.
And, anecdotally, our family is a case study for "evolving larger heads". The first daughter was born by C-section because her head was so big (99th percentile). If we'd had to depend on natural childbirth, she and my wife would most likely both have died (as it is, she was stuck pretty badly for a long time). The second daughter is very similar, so the two of them will be contributing to propagating the "big head" trait that would otherwise have been selected out.
Posted by: Kimbo Jones | October 17, 2008 2:54 PM
How do we know that big heads would be selected as opposed to small pelvises? The head to pelvis ratio doesn't necessarily tell us which is the "culprit" unless we have measurements for each. Whereas the mother would have died in childbirth, she can now give birth to daughters who also have small pelvises and also require c-sections, but this would have nothing to do with an increase in cranium size of the baby. Furthermore, to increase in the population, these people would have to be more fit than the average and not just lucky to have been born at all.
Also, not all c-sections are for the same reason, so it makes no sense to assume that an increase in c-sections will lead to any one evolutionary trend -- particularly given that in Western societies where childbirth has been heavily medicalized, c-sections are common whether medically necessary or not. It's entirely probable that an increase in c-sections would have no effect other than to diversify characteristics in the population (as opposed to selecting for particular traits such as large brains).
Furthermore an increase in cranium size would not necessarily be linked to an increase in brain size.
Basically, I think this is bullshit.
Posted by: mjfgates | October 17, 2008 2:59 PM
Humans evolving larger brains with the removal of the constraint that babies' heads have to fit through the birth canal? Does this explain why the zombie lobby is pushing so hard for the "Compulsory Universal C-Section Act of 2008?"
Posted by: itzac | October 17, 2008 3:00 PM
"If human technology removes a selective constraint, that doesn't stop evolution -- it just opens up a new degree of freedom and allows change to carry us in a novel direction."
Unless you replace the constraint with some other, wouldn't you instead get a heterogenizing, and eventually, homogenizing effect? That is, rather than a novel direction, wouldn't change be marginal and in a number of directions at once, constantly canceling itself out?
Posted by: ME | October 17, 2008 3:03 PM
What about fertility treatments? Aren't they also opening the door to more and more people being unable to reproduce naturally in the future? I can't see how this isn't inevitable.
Perhaps we should be more wary of doing things that could make our species incapable of reproduction naturally, including c sections and fertility treatments. Though, obviously I couldn't ethically argue for denying a woman a c-section who needs it...
Posted by: Gregory Kusnick | October 17, 2008 3:03 PM
#50:
Sure, why not? If your survival and behavior have any effect on the reproductive success of people who carry your genes (e.g. your grandchildren), then they're subject to natural selection.
Posted by: Pierce R. Butler | October 17, 2008 3:05 PM
Complete different tangential question: If humans have c-sections over hundreds of generations (an already dubious proposition), then presumably we'd have contraception also.
While the usual struggle between males and females over control of the latter's bodies is likely to continue, it so far seems to be that women gain a relative advantage in higher-tech cultures. So, assuming that "the sweet double-X's"* get to choose the males whose children they prefer to bear, what are the evolutionary implications?
The men of the future may not be big-domed baldies, but Paul Newmans, Robert Redfords, and Brad Pitts.
*Nicolas van Pallandt, Anvil
Posted by: ME | October 17, 2008 3:11 PM
"The men of the future may not be big-domed baldies, but Paul Newmans, Robert Redfords, and Brad Pitts."
But the availability of contraception makes that less likely because although Women choose to have sex with sexy guys, they don't necessarily choose to make a family with them. By creating the conditions for that choice, contraception may very-well lead to more family-men in the future, not sexy men. (not that the two are mutually exclusive, but studies have shown that better looking men get married later if at all, since they can get so much ass they don't want to settle down)
Posted by: ME | October 17, 2008 3:18 PM
" If don't mutate, evolution is going to be scaled down quite a bit...."
The fact is that there are already many varieties of many genes. You could remove mutation altogether at this point and you would still see the population change over time as existing genes compete within the population for "market share" for lack of a better term (can you tell I'm not a biologist?)
Posted by: Bill Dauphin | October 17, 2008 3:19 PM
But that was exactly my puzzlment: If I die of cancer at 80 and my neighbor, who has a heritable resistance to that same cancer, dies instead by being shot by a jealous husband at 100... does any of that really make my granchildren any less likely to reproduce than his? It's not immediately obvious to me that the survival and good health of already-elderly grandparents has any effect on the fecundity of their descendents.
But of course, the fact that it isn't obvious to me doesn't mean it's not so... which is why I asked.
Posted by: Paper Hand | October 17, 2008 3:32 PM
Interesting. I have wondered in the past what would've happened had intelligence evolved among marsupials. Birth would not have had the same restriction on brain size in that case. Indeed, their young would not have to be so undeveloped at the point where they leave the pouch (is there a word for that?) as our young are at birth.
However, I rather suspect that widespread C-sections will prove to be a short-term phenomenon. At some point, we will perfect an artificial womb, and at that point, it will likely become standard practice to remove the fetus at an early stage of development, and continue its development in this artificial womb, where conditions can be controlled far more delicately than in utero, which would likely reduce non-genetic birth defects.
Piercce @ 43:
I suspect that sexual selection and intratribal competition played a big role in larger brains. More intelligent individuals were able to use their brains to manipulate their companions, especially important in their opposite-sexed companions. That is, an intelligent woman would be better able to woo a good mate (or, alternately, to keep her infidelity hidden). Likewise, an intelligent man would be better able to attract, or at least seduce, a good mate. And in non-sexual matters, too, the ability to track other people's behaviors would allow one to form more reliable alliances with fellow tribe-members, to avoid fights or to have good allies if a fight does break out, or even to trick two rivals into wasting their time fighting each other rather than you.
I call it the Machiavelli Theory of Human Evolution. :-)
John Fen @46
It would have to be dominant for that. If it were a recessive gene, only those who were homozygous would survive in the first place, thus all their children would likewise be homozygous. If it were dominant, on the other hand, the survivors could be heterozygous. If there were no homozygous individuals for that trait, then 25% of their young would receive both copies of the normal, non-immunity-conferring, allele and would die. Over the course of several generations, the normal allele would (assuming that the toxin remained in the environment) be gradually removed as children homozygous for that allele would be born. Of course, if the toxin left the environment, then the gene frequency would not change (except through genetic drift, *or* if the immunity allele had some harmful side-effect)
Bill Dauphin @ 50
It can, especially with late childbirth. If you wait until your late 30's or into your 40's to have children, then dying at 50 would leave young children behind, without a parent. In addition, long-lived grandparents can be quite beneficial to their grandchildren.
Of course, these factors are less important in today's society. (And the likely future use of genetic engineering, or at the very least, artificial insemination combined with choice of sperm, will probably outweigh natural selection)
ME @ 56
Another good point. Some forms of infertility are due to genetic mutations that would normally not survive past the first generation (or, if recessive, survive only in very low numbers in the population, being constantly removed whenever a homozygous child is born). With fertility treatments, those individuals could have children, allowing the creation of children who inherit their parent's infertility - normally a contradiction in terms.
So, we could end up with a species of small-pelvised women, big-brained babies, and widespread infertility of both sexes ...
Posted by: ME | October 17, 2008 3:33 PM
"It's not immediately obvious to me that the survival and good health of already-elderly grandparents has any effect on the fecundity of their descendents."
It's not obvious to me either... I would even say that in some circumstances, it could be the opposite. Having to care for elderly parents could place a burden on thsoe who might otherwise have their own children. Having your parent's die when you are 20, for example, could also give you a reproductive advantage by inheriting wealth/insurance money that wouldn't otherwise be available for spending on women. loose women. :) I kid.
Posted by: Prof MTH | October 17, 2008 3:35 PM
Could an increase in prevalence rates in artificial penile implants lead to more discriminating female detection devices for naturally large penises?
Could an increase in prevalence rates in artificial breast implants lead to more discriminating male detection devices for naturally large breasts?
Why are natural breasts more "valued" than artificial ones? (same for penises) Artificial breasts can glow in the dark!
Posted by: Ichthyic | October 17, 2008 3:38 PM
There is nothing in our future except extinction
Hey, Charlie, if you're considering doing yourself in, would you please get on with it already?
thanks.
Posted by: Paper Hand | October 17, 2008 3:43 PM
Me @ #59
Even more relevant is this: since the development of sexual reproduction, evolution has favored genes that make for a strong sex drive, and, in species with parental investment, genes that drive behaviors likely to increase survival of offspring. Evolution has not influenced *desire for children*. Before contraception was available, the only choices were "have sex" or "not have sex". Those individuals capable of resisting their sex drives wouldn't leave children. Those individuals unable to control their sex drive - whether they wanted children or not - would end up with children.
However, now reproduction is increasingly under our control. We can use fertility treatments for those unable to conceive, and contraception to prevent conception for those who don't want children.
This means that, no matter what your sex drive or natural fertility, if you don't want children, you won't leave offspring. Likewise, if you do want children, you'll have children. The desire for children is likely to be, at least partially, genetic, probably some of the same genes that control parental behavior.
Over a number of generations - probably not that many, since it would be a rather strong selective pressure! - you'll find only individuals who want children leaving progeny (to some extent, sperm banks might reduce this pressure, but even then, if women are allowed to chose, they might prefer "family man" sperm, since that would be likely to give them grandchildren, and a person who wants children would be likely to want grandchildren, as well). Over time, the "desire-for-children" genes will become far more common, and everyone will want children. At that point, we'll probably have to have some kind of restrictions on the number of children people can legally have. (Indeed, we'd have to if everyone wanted big families!)
Another point I wonder about re: evolution - twins. Twins have been rare in the past because of the increased danger of their dying, and the increased strain on the mother. That issue is far less important now. Could twins become more common over time? It would be impossible to know whether it's having an effect now, thanks to fertility drugs creating more twins on their own, masking any genetic changes, but over time, it does seem reasonable to suppose that twins would be more common.
Posted by: Brian | October 17, 2008 3:48 PM
Re: Baldness
The only thing holding hair onto people's heads now is sexual selection. We don't need it to prevent heat loss or for protection from the sun, not anymore. So big-domed baldies are the wave of the future.
Posted by: Jim A | October 17, 2008 3:50 PM
Here's a link to an very sad WaPo article on maternal mortality in the absence of modern medicine
http://www.washingtonpost.com/wp-dyn/content/article/2008/10/11/AR2008101102165.html
Posted by: Umilik | October 17, 2008 3:50 PM
The heritabilty estimates for most reproductive traits incl. birth weight are relatively low - at least that's what we have learnd from studies in farm animals. That would suggest that birth weight itself, which would presumably include head weight, would be slow to change.
I am also not sure why head size should increase at all - the correlation between head size and intelligence seems tenuous, and even if it did exist, why should increased intelligence lead to better reproductive fitness ?
Posted by: Brian | October 17, 2008 3:52 PM
Also, Glen - the situation we have right now with neonatal head size is constraining selection - the head wants to be bigger (as it is in all other mammals), but can't. Remove the constraint and drift takes over.
Posted by: Phoenix Woman | October 17, 2008 3:53 PM
I don't think it'd take that long. The problem of narrow-hipped women giving birth to big-headed babies is by no means a purely academic one. This was and is a problem for women of small stature (whether due to genetics or nutrition) who get pregnant by men with DNA calling for large bodies, and/or who get pregnant whilst having access to better nutrition than they had while their bodies were still devel