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:
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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.
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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.
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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.
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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.
“…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.
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.
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?)
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.
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.
Sean Peters stole my comment. I have it on tape.
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.
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.
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.
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..
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
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.
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.
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
,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.
Sorry about the blockquote screw-up.
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?)
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.
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… =)
I echo the good Doctor Professor’s concern: Sure, they’ll have bigger heads, but what about the baldness?
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.
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.
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.
Re #17:
If everyone is beautiful then nobody is beautiful. They’d all just be normal.
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.
I should ask, how can the process I mentioned above affect mens preference for larger hip to waist ratios?
You silly humans–
Thinking you can fool nature.
You’ll get a big head!
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.
Where’s w00+ ? We could use some boobies to go with our fat heads.
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.
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.
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.
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.
“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.
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?
Rats, Chris S beat me to it.
Dang it, PZ, what’s with all the sciency stuff lately? Isn’t this supposed to be an angry rants blog?
;^)
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
@gillt, #33: I suspect that you mean oxytocin, not Oxycontin. Although the latter might make childbirth a bit more fun!
Jinx!
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.
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.
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.
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.
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.
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.
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.
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.
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.
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’…
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….
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.
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.
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?”
“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?
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…
#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.
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
“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)
” 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?)
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.
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 …
“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.
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!
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.
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.
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.
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
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 ?
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.
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 developing. Before survivable C-sections, these women, and usually their children, died after days if not weeks of unimaginable agony (and the kids, if they managed to survive, often suffered brain damage). With routinely survivable Cs, these women and their kids both thrive.
Around the turn of the last century in Irish enclaves like Boston, first- and second-generation Irish women who grew up impoverished would get knocked up by much beefier men and suffer this very fate; it was common enough to be used as a plot device in the novel The Cardinal, in which the protagonist, a Catholic priest, lets his beloved sister die hideously and slowly rather than abort her unborn fetus — “God’s will”, don’tcha know. (The book, by the way, depicts this all quite approvingly.)
I took the original point to which I was responding to be about resistance to cancers whose incidence is greatest in the elderly, but I take your point. Would this be a fair summary:
1. Genetic resistance to cancers that typically emerge in early adulthood (or earlier) contributes directly to the survival of the genetic line, in that individuals lacking that resistance are less likely to survive long enough to reproduce at all.
2. Genetic resistance to cancers that typically emerge in the years when adults are actively engaged in childrearing (say, early twenties through mid fifties) have a secondary effect, in that the children of individuals lacking that resistance may face economic obstacles to producing further offspring in the line.
3. Genetic resistance to cancers that emerge in later adulthood has a significantly reduced effect on the survival of the genetic line, and that effect diminishes for particular cancers as age of typical onset increases.
Does that make sense, or am I totally off the rails here?
To be sure… but in ways that affect the grandchildren’s likelihood of reproducing? I’m not sure.
Then why do later humans have smaller brains than Cro-magnons and Neanderthals?
To say that “the head wants to be bigger” is a meaningless statement in evolutionary theory, about equivalent to “god wants the human head to be bigger.”
Glen D
http://tinyurl.com/2kxyc7
Shorter Charlie: Modern humans differ from neanderthals. Therefore, evolution has ceased.
Bigger brains or just smaller vaginas?
In my experience, with the proper application of amaretto sours, you’d be surprised what you can squeeze into (and presumably out of) a pelvis.
I, for one, welcome our new giant-headed, bigger-brained overlords.
Ok, well, the head wants to be bigger relative to the birth canal. Not bigger as adults. The sort of human births that take place now are a balance between allowing the infants to develop as much as they can in utero and not having birth kill the mother. Human infants are useless for quite a while – another unique thing about humans. It would be better for the infants to develop longer in utero.
ME @ # 59: Women choose to have sex with sexy guys, they don’t necessarily choose to make a family with them.
You mean… all those data points of women saying “I want to have his baby!” are invalid and must be discarded?
Damn, evolutionary science is HARD!
@33 – It’s oxytocin, and it’s childbirth that makes the mother produce more of it, not natural childbirth. Here in Brazil we have a widespread culture of c-sections among the middle and upper classes (i.e. people who can afford insurance or private medical care). Women and doctors tend to prefer it. And I think doctors here have perfected the procedure, because it’s not viewed as “oh noes major high-risk surgery”, like American women think of it (those maternity shows on the Discovery channel are really a culture shock experience for me and my friends). And all the mother-child bonding is doing fine, by the way.
And sorry for going OT, but I dunno, include Brazil in any study of population change due to c-sections, because this has been going on at least since I was born (in the 70’s), and I’m not sure about the exact rate, but I would guess somewhere between 80 and 90%.
(Having experienced both, I personally don’t understand why c-sections aren’t more popular. I would *never* subject myself to natural childbirth if I can give birth in 30 minutes with virtually zero pain. It’s that simple.)
And God does want the human head to be bigger. He just couldn’t think of a way to do it without making us look like Twilight Zone rejects.
#71:
It’s not about adult brain size or intelligence. It’s about neonatal helplessness. Human infants, unlike virtually all other mammals (except marsupials), must be carried around for a year after birth before they can walk. This is a direct consequence of the narrow pelvic canal; babies must be born before their brain gets too big. Without that constraint, babies could stay in the womb longer and/or grow their brains more rapidly prenatally, increasing their neonatal competence and survivability. That’s where the selective pressure (if any) will come from, not from the putative value of higher IQ as an adult.
The increased availabilty of C-sections has already led to an increase in the size of Saint Bernard heads. Of course, breeders were selecting for that, but it is still proof of concept.
Jim @ 70 — Thank you for linking to that. The story was horrible, heartrending, and pretty close to what women in America risked routinely as recently as six generations ago. Go into burying grounds that are at least a hundred years old and note how many of them feature a husband’s headstone surrounded by two or three (or more) smaller headstones belonging to his wives. (Ironically, “loose women” were and are better off, at least against pre-eclampsia and eclampsia; the more times a woman has sex (yes, even fellatio) with the same male partner before she gets pregnant, the less likely she is to get either disease. It’s the women who get knocked up the very first time they had sex that are most at risk.)
Thanks everyone for reminding me of the most painful 2 days of my life – just thanks. Sheesh – they say smart women never forget you know. I was well on my way. *sigh*
Bill @ #74:
Now that you’ve qualified it, then sure, obviously there’s a point of diminishing returns. But your original question at #50 seemed (to me) to be asking whether post-childbearing mortality could in principle have any selective effect at all. When I was in school, back before the publication of The Selfish Gene, the answer I was taught was that it cannot. We now know that answer to be wrong.
Forgive me if this has been said already, I don’t have time right now to scroll though all the responses.
Of course human evolution is still going on, we just don’t notice the hour hand moving on the clock. It may not be entirely “natural selection” anymore, depending on how you look at it. As we have developed tools and technology, we have influenced the course of our evolution. Our specialized civilization means that you no longer have to be physically strong to survive and pass on your genes. People that 1000 years ago would not be able to survive to pass on their genes can do so today, thanks to modern medicine.
I suppose one could argue that our development of civilization and technology is part of our natural evolution. We have become so evolved/specialized that we might not be able adapt fast enough to any major, cataclysmic change in our environment, just like the dinosaurs, despite all our technology. In the world of evolution, BEWARE OVER SPECIALIZATION, it’s not a recipe for truly long tern success.
Harder than you know (and in more senses of that word than you likely intended): Even if contraceptive technologies and social changes have tended to decouple women’s sexual choices from their reproductive choices, we’ve just gotten stunning new evidence that men’s sexual choices are linked to women’s fertility.
I’d say the availability of C-sections is necessary but insufficient for larger-head-development. As it’s been pointed out, this availability means that large-baby-head is not being selected AGAINST nearly as much. You still run into, say, the fact that having had open-uterus surgery makes future pregnancies more likely to cause uterine rupture, not to mention the general associated problems with major surgery to begin with, of course.
On the other hand, there’s no selection FOR large-baby-head, that is, no selective pressure against small-baby-head. A strong correlation between brain size and intelligence might, just might, have the potential of such selection, if stupid people didn’t breed as much…. but I haven’t seen any evidence of that correlation, and in the US at least, it seems stupid people breed *more*. So, yeah. Interesting thought experiment, but I dunno how useful it is.
“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.”
Both my grandmothers babysit their grandchildren/great-grandchildren. That provides a better environment than random daycare, and allows the parents to dedicate more resources to the kids.
Whether or not that’s significant enough to lead to a noticeable evolutionary affect, I don’t know. But it shows at least one possibility.
“but I haven’t seen any evidence of that correlation, and in the US at least, it seems stupid people breed *more*”
This, I am really tired of seeing. Really, just for a minute, think about what a narrow range of intelligence comprises “stupid” and “smart” people in modern society. Stupid people can drive a car. Stupid people can *read*, even if they don’t do it very often (think about the level of abstract thinking is required to do this, compared to all other species on this planet). More importantly, stupid people are just as competent, if not more so, than “smart” people at dealing with social relationships. Which is kind of why we have these big brains in the first place. The real thing that separates humanity from other animals is our sociability – being able to do puzzles very quickly is not going to save your pink, hairless ass from a tiger.
The problem is, there’s no selective pressure against smaller brain size, is there?
Hmmm, severe myopia (as in the kind that prevents you from functioning without glasses) is largely a product of modern civilization. It’s like Type II diabetes. The theory is that staring at close, flat work (computer screen, book, cross-stitch, etc.) reduces the strength of the muscles that focus the eyes, which results in the development of myopia. Current understanding is that there is a genetic predisposition for myopia, but that it takes this environmental effect to trigger the disease.
You can have a vaginal delivery without undergoing natural childbirth. Several of my friends believe the epidural is the best invention ever.
Just because the actual c-section isn’t painful doesn’t mean you have a pain-free recovery. Anytime your abdominal wall is cut open there’s considerable recovery time (and it is major surgery with attendant risks of infection, etc.). The two women I know (yea, yea data is not the plural of anecdote) who have had vaginal and c-section deliveries agree that the vaginal birth was preferable.
Can we tell the direction of nature’s selection–
Will our heads be enlarging or shrinking?–
Speculation is cute, but it’s all rather moot,
Cos the little head does all the thinking.
The genes with the most variability in humans are the placental genes and the genes related to immune system control of placentation. Getting the head through the pelvis is already a huge problem, and anything that removes the constraint will have significant downstream effects not just on babies, but on things like the rate of heart disease in the human population. See here and here and here for starters.
Oh how I love the Grandmother Hypothesis. The grandparental investment is interpreted as a selection factor for menopause. It makes as much sense as anything…
I’ve got to wonder if taking children away from daycare where they interact with other people is actually a good thing. The quality of care may not be better, but surely at that stage it’s important for children to learn how to interact socially with their peers.
Interestingly, we already have two *reasonably well-matched* cohorts for a study on this since our European friends have a significantly higher rate of natural childbirth vs. C-section than does the U.S. Wait, this may explain a few things…
Role of Near Work in Myopia: Findings in a Sample of Australian School Children.
J. M. Ip, S.-M. Saw, K. A. Rose, I. G. Morgan, A. Kifley, J. J. Wang, and P. Mitchell (2008)
Invest. Ophthalmol. Vis. Sci. 49, 2903-2910
Here’s a recent article on myopia and association with near work. The authors found an association with reading, but not other near work.
@ 96
quote:
“Just because the actual c-section isn’t painful doesn’t mean you have a pain-free recovery. Anytime your abdominal wall is cut open there’s considerable recovery time (and it is major surgery with attendant risks of infection, etc.). The two women I know (yea, yea data is not the plural of anecdote) who have had vaginal and c-section deliveries agree that the vaginal birth was preferable.”
That’s why I was saying it seems to me (yeah, I know, it’s just an opinion based on my limited personal observations, but let’s call it a hypothesis) that all those obstetricians working in private hospitals and performing c-sections on a routine basis for the last 30 years…it doesn’t seem far-fetched to think that they would improve the technique. Like I said, the numbers are overwhelming. I don’t know a single person who didn’t give birth by c-section. And while my aunts complain that the recovery was rather painful, my friends, cousins, etc. did not have that experience, and neither did I. Actually, the recovery from vaginal childbirth was much more uncomfortable (the only reason I had to go through that is because I was very young, had no money, and didn’t want to ask my family for help, so I ended up in a public hospital thinking “it’s just childbirth”…bad idea).
Yes, data is not the plural of anecdote, but the “c-section culture” in Brazil is a real phenomenon, and every time I discuss this with my American friends, they say “but it *is* major surgery!”, and all I have to say is, “erm, no, it’s not…” so perhaps I should stop being lazy and do some actual research. Well, at the very least it’s an interesting (to me) difference in mentality.
Kel – 20,000 years ago grandma *was daycare. She and grandpa were also the library, the repository of knowledge. “Hasn’t rained in weeks. When I was a kid, this happened, and we walked down to the lowlands and dug around the roots under a green tree. There was water down there, not too far.”
Also, I’m reaching an age where I wouldn’t be as much good on a hunt as 30 years ago, but I could stay in the camp and make things. Once technology started requiring some real finesse in the manufacturing, a full time smith-librarian-baby sitter came in handy. And enhanced the grandkids reproductive chances.
For more scenarios in which the circumstances of Grandma’s death can affect the fortunes of her descendents, consult Hercule Poirot and Jane Marple.
Which brings up another point: Exceptionally long-lived grandparents might actually be an economic detriment to reproduction among their grandchildren, since they may not pass on their accumulated wealth until after the childbearing years are past.
Unless, of course, the grandchildren hatch the sort of nefarious plans your examples hint at!
Mariana,
While I don’t know whether there’s an official definition for “major surgery,” that term is used by laypeople for a wide variety of “serious” procedures, definitely including one that involves cutting into the abdomen and then into the uterus. Your information about c-sections being common and safe wouldn’t take them out of the category of major surgery. When your American friends say that it’s major surgery, they’re right. Modern c-sections use much smaller incisions than the procedures of a generation ago, but still, the incisions must be made!
I’d be interested in knowing whether recovery and infection rates in American and Brazilian populations are really different.
Your friends are quite possibly reacting to the perceived vanity of elective c-sections, which doctors here are often reluctant to do; but they might not want to say that, for fear of sounding judgmental!
Of course, this blatantly ignores that the more cesareans we do, the fewer children we have, the fewer to carry on any characteristics, and the higher the death rate is of both mothers and infants in the short and long-term. Population control, sure. Forced evolution dependent on technology, doubtful.
Not to mention, most cesareans are not for size and size has NOT changed considerably in the last 150 years or so, though it has changed drastically since the 1940’s-1960’s when many mothers were encouraged to smoke to lose weight during pregnancy and have low birth weight babies as well as many of the mothers themselves suffering from malnutrition. . Most cesareans are done without medical cause to avoid malpractice lawsuits after failed inductions with malpresentations. Most indications for cesareans would never re-occur. Though, it’s always amusing to watch people try to justify that somehow we are getting fatter and therefore, we’re needing more cesareans. Way to fall for ACOG’s line on why cesareans are happening, though. Amazing. Ignorance abounds and follows assumptions like Alice after a rabbit.
Maybe women should just have bigger hips?
It’s an interesting hypothesis, and I can’t see any reason why it would be necessarily incorrect, but I can’t help but think it’s largely… irrelevant. Long before cesarean sections have any substantial influence on the human gene pool, we’ll have much better options like ectogenesis, via artificial uterine substitutes and so on.
Might not be evolutionarily very far fetched.
A few years ago, someone dug up a Neanderthal skeleton. It was male and the pelvis was relatively intact. One of the anthropologists made the comment that one could drop a babies head through the pelvic opening without any trouble.
And this was a male. One can imagine what the females pelvis looked like.
A lot we don’t know about things that haven’t happened yet. Who knows, maybe all the really intelligent species are like marsupials or lay eggs. Or our decendants will have very long narrow skulls of roughly the same width.
# 99 Scott E: bingo!
Comment on PZ’s original post:
Obviously since evolution is the change of percentage of alleles in a population, more C-sections will result in change of that percentage and so evolution at those loci which result in larger heads.
But significance of this to human welfare: is a larger head size going to result in greater intelligence, though? As I have read of it, pruning (apoptosis) in the number of neurons occurs during childhood & adolescence, with the result of having fewer neurons at the onset of adulthood than at birth.
Again, as what I understand it, is that what determines adult intelligence in humans has much less to do with the number of neurons than the dendritic connections one has grown due to environmental exposure in the neurons that remain.
So, yeah, of course we are evolving a larger brain size as a species because of C-section tech, but whether that is a benefit, deficit, or neutral: show me the money!
“We should see C-Sections get ever more common, and even perhaps universally required vital at some future point.”
Maybe we could engeneer an organic-compatible zipper to install in the lower abdomens of women for quick easy access.
Poor humanity. I see a sad darker future with big-headed persons and women with hips of 10 years old girls…
Posted by: Bill Dauphin @ 61
” ” “Would a distinction that’s only apparent in post-childbearing adults really have any effect on selection?”
Sure, why not? …”
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? …”
I would think not. As long as it doesn’t affect your prime child-bearing years, it wouldn’t matter if you dropped dead at 50, evolutionarily speaking. For a long time few people made it past 30, according to stories I’ve heard. (not sure how true that is, I’ve heard a few versions.)
There would be some added advantage to your descendants if grandparents helped bring up/protect the grandkids, but by the time you’re 80, your grandkids are probably adults.
Paper Hand @ # 62: 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. :-)
Some of those factors could operate just as dynamically in favor of the smaller-brained, as I addressed in # 43; as for the others…
It may be possible for someone better-educated than myself to put a rough number on the increased maternal/infant mortality incurred with larger brains (and insufficient gestation, and prolonged dependency). A tangent about that gestation: most other primates are birthed at a level of development humans reach about 3 months after birth. Ask any mother her thoughts about delivering a 3-month-old; ask any evo-devo researcher what it might cost a species if forced to forgo 25% of gestation. Bigger brains carry a premium price tag.
Someone even sharper may be able to make some estimates as to the survival advantages of increased Machiavellianism. But to make it plausible that the second number should be larger than the first, it seems to me, requires rather extraordinary circumstances prolonged over hundreds of generations – or we’d see evidence of it, at least historically, in multiple species.
I still like women with “breeder’s hips”. Gets me going every time :)
Increased brain size might be an evolutionary -effect- of existing intelligence and not necessarily a direct cause. Because of age-related neuronal attrition, a brain with a greater supply of brain cells will last longer (neuronal redundancy), thus the selection bigger brains to maximize experience and intelligence resulting in older experienced leaders, or at least the wisdom and knowledge of elders as a greater repository of culture and technology.
IOW, a higher average longevity of a population group will enable it to contain and maintain more man/years of experience than that of a shorter-lived population group. (or if not greater life expectancy, at least greater potential maximum life span of individuals) Of course, being long-lived does no good if all the elders are senile. So evolution selects for brain durability (i.e. larger brains) in order to enhance the use of preexisting intelligence.
With the arrival of civilization, with its softer lifestyle and record keeping (and the invention of writing rendering the knowledge repository role of elders somewhat less important), the selection pressure of big durable brains has been reduced or perhaps even reversed. That is, if the above reasoning is correct.
Ann @ 105:
“I’d be interested in knowing whether recovery and infection rates in American and Brazilian populations are really different.”
Yes, that’s exactly the kind of data that I’ve been thinking of looking up. A comparison between the recovery processes after natural birth and c-sections would be one aspect, and the other would be infection rates. I somehow doubt the latter are much different in these two countries – I could be wrong, of course – but I wonder if they warrant a perception of c-sections as high-risk surgery in any case.
And, yeah, I’ve caught on to the “perceived vanity” thing. That’s just part of the way in which this is a major cultural difference. Nobody is judged as vain for having elective c-sections here, of course, as it is the rule rather than the exception, but it’s also not seen as a source of any kind of fear or anxiety, as you’d expect to see in anyone about to undergo major or high-risk surgery.
So, I see your point that it’s not the patient’s perception that defines the magnitude of the procedure, but I can’t really imagine such a culture would have developed here if the infection/complications rates were significant.
At the rate we are learning how genes control development, in utereo and beyond, we will soon be able to direct our own evolution and perhaps reroute the vagina to exit in the lower abdomen thus avoiding the pelvic restriction. It will play hell with thongs but sacrifices will be made.
Human evolution will only end if we are replaced by the machines and, with us as their progenitors, their evolution might be considered an extension of ours.
I remember reading– possibly on Orac’s blog?– that babies who are going to be autistic have measurably larger heads than average.
Could it be that the autism “epidemic” comes partly from improved medicine making it possible for the autistic-leaning babies to be safely delivered?
Brian, I’m not really sure what you’re tired of seeing, as the rest of your comment was kinda related to my overall point. There’s not much selective pressure for intelligence in modern humans, as even the dumbest humans can get by and reproduce in a developed society. So, even IF bigger brains meant more smarts, there wouldn’t be much pressure against smaller brains.
I hope this doesn’t happen, as any Dark Age would kill off most everybody. And I think humanity will last much longer than our current civilization.
wow, we happened by this topic in my PRIM501 class two weeks ago…
Posted by: Karl Withakay | October 17, 2008 4:35 PM
In the world of evolution, BEWARE OVER SPECIALIZATION, it’s not a recipe for truly long term success.
Specialization is definitely not a recipe for long-term success. However, a short-term it is, and so more or less specialized species dominate any landscape. But generalistic species tend to survive in the form of evolutionary radiations. Think of the last common ancestor of all living mammals: probably, it was a very modest species in terms of biomass or ecological relevance. The contradiction between long-term success and short-term success is solved in a way not so much different that the contradiction between cell specialization in a pluricellular organism (somatic cells) and the retention of a few totipotent cells (germ cells).
In the same way, modern technology is actually decreasing the average breast endowment of women. Consider: With plastic surgery becoming much safer, and the incidence of breast augmentations rising, the cultural desire for large breasted women is being fostered. Since artifically enhanced breasts are (or appear to be) more aesthetically pleasing, this raises the reproductive opportunities of women who have had this surgery. Since women who are normally large breasted will not have augmentation (and enhancement), it is the flat-chested ones who will be reproducing at a (statistically significant) higher rate, thus ‘selecting’ for smaller breasts.
I was somewhat surprised to see c-sections labeled as “vanity” – the “aesthetic” argument I usually hear is that c-sections are bad, because the scars don’t look pretty when you’re in a bikini!
Looking it up, about 17% of births was by c-section in Sweden in 2004, the trend being rising.
“Call me 10,000 years from now, after we’ve been doing this long enough for evolution to actually produce some noticeable change.”
Define “noticeable.” If there is a genetic component under selection here, allele frequencies in the population are certainly already changing dramatically!
“but I haven’t seen any evidence of that correlation, and in the US at least, it seems stupid people breed *more*”
Ignorant (i.e. uneducated) people MIGHT breed more, but that doesn’t mean there’s a significant genetic component of INTELLIGENCE under selection here. The main issues here are poverty and culture, and I highly doubt that socioeconomic status has a direct genetic component. How many uneducated creationists understand a motor better than anyone posting on this blog? Will the human race eventually consist entirely of mechanics? You’re espousing a grossly oversimplified model of genetic variability in the human species. Uneducated populations probably have roughly the same “intelligence genes” in roughly the same proportion in their population as educated populations. We are, after all, one interbreeding species!
“The ignorant are out-breeding us! What will we do?” is just eugenics. A blog like this doesn’t have room for outdated pseudoscience.
C-sections, on the other hand, allow what would otherwise be LETHAL genotypes to become viable (and possibly even favored!) YOU are talking about a system where hundreds, maybe thousands of alleles may have a slight tangential effect the number of offspring. PZ is talking about a handful of changes to developmental genes and their regulatory networks that would have had zero fitness for tens of thousands of years that have suddenly become viable.
“Of course, this blatantly ignores that the more cesareans we do, the fewer children we have, the fewer to carry on any characteristics, and the higher the death rate is of both mothers and infants in the short and long-term.”
Do you have evidence backing this up by any chance?
Yes. Cesareans increase the likelihood of secondary infertility both by choice and by physically being unable to conceive, carry and have live infants and it increases the death of infants in the first year. If you add in that physicians themselves are telling women they “can’t” have future children beyond 2-3 and are encouraging (this is the nicest way I can put it on a public forum) women to be sterilized after 2 kids to keep from having repeated cesareans and refuse to do vbacs…well, you have a potentiality for lower and lower birth rates as our cesarean rate climbs higher and higher. Cesareans also increase the death rate over vaginal births in mothers, fewer women to breed.
I can’t remember if someone said this up thread somewhere, but the female pelvic girdle is under incredibly tight natural selection. Ideally it would be very narrow with a small inlet and outlet as a consequence of bipedalism. However, females have to give birth. The pelvic inlet and outlet have to be large enough to allow a baby and its rather large head to pass through. If the pelvis is too narrow, the baby gets stuck. If it’s too wide, you start having problems with locomotion and such wonderful things as slipped organs. Not fun.
raven,
I was wondering which Neandertal specimen you’re talking about…do you have a reference (or even just a specimen number?) Were the pelvic inlet and outlet large or was the os coxa simply robust? I find it highly unlikely that a pelvis large enough for a baby to pass through would be classified as male. That’s essentially the definition of a female.
Query,
I am young, naive, woman just working on college so I am willing to read anything and think it over. This however is preposterous. Not only do C-sections increase infant mortality rates (hello U.S.A. is like 24th in the world with 1 out of 3 women getting c-sections)but there are ceratin biological issues here. After all just because your mohter has plastic surgery to get a new nose does not mean that you will have her new nose. Major abdominal surgery will not contribute to larger heads becuase you are not affecting the actual person, merely their outward appearance. Besides in order to test this thoery would cost thousands of lives of both mothers and their babies. Lives that cannot be afforded. Maybe this issue needs to be looked at by the populace and not by the evolutionists.
Oh? Haven’t seen those research findings. Care to share?
Safety of medical precedure by popular vote? Sounds very creationist! Perhaps merely praying for non-complicated safe births is the answer!
If the hypothesis is valid, then a high percentage of children delivered through caesarean should have parents who were products of the procedure.
PZ, Steve Jones isn’t exactly wrong… he’s basically right for the wrong reason. We are on the verge of the gene being completely dominated by the meme. Within a century or so, we will have the ability to control biological processes at the molecular level. Science fiction notions of growing wings or gills, making our brains smarter (mind bogglingly smarter), regeneration of all tissues, youth treatments, are inevitable within a handful of generations.
The selective pressure of C-sections on brain size will only be slightly extant for some tens of generations, after which it will become entirely irrelevant. Anything resembling ordinary biological evolution (i.e. the replication with variation of genes within organisms) will occur essentially at the whim of our genetically-self-engineered descendants — in other words, it won’t be “natural selection” except in the vacuous sense that everything is “natural”.
Perhaps some people resembling circa-2000 humans and beasties will be kept around for nostalgic reasons and they of course will continue to evolve in whatever environment is provided for them. Maybe planet earth will be abandoned by our “advanced” descendants, but preserved as a sort of anachronistic park. Maybe this has already happened… hmmmmmm…. if we get too advanced will our ultra-advanced cousins come back to welcome us to the galactic civilization? …or will they cull us, wipe out evidence of our modern civilization, and thereby “reset” us back to the stone age?
This reminds me of my thoughts when I first heard of intracytoplasmic sperm injection as a ‘treatment’ for male infertility:
“So Doc, you mean I’m totally sterile and I inherited it from good old Dad?
“Excuse me while I call a lawyer…”
My wife and my three-year-old son are alive today only because, probably, of the availability of a reasonably safe Caesarian section procedure performed by a trained surgeon and her staff in a well-equipped hospital.
From my perspective, as you might imagine, cephalopelvic disproportion is the final, ineluctable proof that Intelligent Design is bullshit. I’ve had occasion to ask a couple of ID believers how they explain the “intelligence” behind the manifestation of CD, and I’ve always got back the same response: the Lord moves in mysterious ways.
Yeah. It’s a real mystery.
I’m waiting for the wingnuts to start complaining the Caesarian sections are a blasphemous interference with God’s plan for humanity. They never seem to get there– at least, not while I’m watching them.
PZ, Steve Jones isn’t exactly wrong… he’s basically right for the wrong reason. We are on the verge of the gene being completely dominated by the meme. Within a century or so, we will have the ability to control biological processes at the molecular level. Science fiction notions of growing wings or gills, making our brains smarter (mind bogglingly smarter), regeneration of all tissues, youth treatments, are inevitable within a handful of generations.
The selective pressure of C-sections on brain size will only be slightly extant for some tens of generations, after which it will become entirely irrelevant. Anything resembling ordinary biological evolution (i.e. the replication with variation of genes within organisms) will occur essentially at the whim of our genetically-self-engineered descendants — in other words, it won’t be “natural selection” except in the vacuous sense that everything is “natural”.
Perhaps some people resembling circa-2000 humans and beasties will be kept around for nostalgic reasons and they of course will continue to evolve in whatever environment is provided for them. Maybe planet earth will be abandoned by our “advanced” descendants, but preserved as a sort of anachronistic park. Maybe this has already happened… hmmmmmm…. if we get too advanced will our ultra-advanced cousins come back to welcome us to the galactic civilization? …or will they cull us, wipe out evidence of our modern civilization, and thereby “reset” us back to the stone age?
To JH
Most cases of CPD are misdiagnosis after failed induction or lack of care provider training on turning a malpresentation. A failing of our OB-GYN community. From 70-85% of all CPD mothers (given the chance at a vaginal birth by an OB-GYN) would have a normal vaginal birth after a previous CPD diagnosis. This number goes up further (85-95%) when giving birth with a well-trained midwife.
apologies for the duped comment.
@Pygmy: I wasn’t asking the question seriously. I just don’t like unnecessary C-sections.
SM,
Our case was not a misdiagnosis. Leo’s is an unusually large child, and something like less than 1% of infants are born with a larger head without it being a deformity. His growth has been consistently off the charts. If it wasn’t to be his head (actually probably), then it almost certainly would have been his shoulders.
I stand by my statement that cephalopelvic disproportion is proof that Intelligent Design is a crock of shit.
JH,
You are always entitled to your opinion and I certainly don’t have to agree with it, but simply from a scientific standpoint alone, without theology involved, CPD is a much-overused diagnosis especially when infants are malpresenting and “large”. In our society, 15inch heads with 10+lb bodies are still in the normal bell curve. CPD cannot be determined without a mother going into spontaneous labor (no induction) which does not complete in a vaginal birth given adequate time, which almost never happens in a US hospital. A “proactive” cesarean for CPD without labor does not exist.
Also, there is always the point that CPD would be a design flaw in the mother’s pelvis being intractable and unmovable, not the baby’s head size, in most cases. Smaller pelvises without flexibility, not bigger heads.
And without arguing whether or not Intelligent Design is probable or not, strictly scientifically, not all models succeed. There is always the possibility that a Designer would not want all models to succeed as well, for a variety of reasons, or that there is a model that fails in production.
Why bigger brains? Why not tighter vaginas?
Sorry. I have a terrible sense of humor.
Joanna,
I’m sorry. My sarcasm meter is impaired right now :)
“. . . such as an increase in the likelihood of medical malpractice suits making doctors more cautious.
Yes, thanks to Sen. Edwards and probably his faux-Appalachian accent.
First. Yes the head size is a factor in labour pains for women but also the journey during birth. This is why only women have pain at birth and not animals. The bible says women were cursed with this unique condition and even evolution admits it and says its because people walk upright.
Oh brother. C-sections are allowing a bigger brain future?
First brains size is unrelated to intelligence. Surely the computer age has endeed this concept. Small is brillient.
This is a good reason why evolution has on the way out as even reasonable in its conclusions.