Will the availability of C-sections give humans bigger brains?

Blogging on Peer-Reviewed Research

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.

Old scientists never clean out their refrigerators

Blogging on Peer-Reviewed Research

We all know the story of the Miller-Urey experiment. In 1953, a young graduate student named Stanley Miller ran an off-the-wall experiment: he ran water, methane, ammonia, and hydrogen in a sealed flask with a pair of electrodes to produce a spark, and from those simple building blocks discovered that more complex compounds, such as amino acids, were spontaneously produced. Stanley Miller died in 2007, and in going through his effects, the original apparatus was discovered, and in addition, several small sealed vials containing the sludge produced in the original experiment were also found.

This isn’t too surprising. I’ve gone through a few old scientists’ labs, and you’d be surprised at all the antiquities they preserved, all with notes documenting exactly what they are. It’s habit to keep this stuff.

Now the cool part, though: the scientists who unearthed the old samples ran them through modern analysis techniques, which are a bit more sensitive than the tools they had in the 1950s. In 1953, Miller reported the recovery of five amino acids from his experiment. The reanalysis found twenty two amino acids and five amines in the vials. He was more successful than he knew!

i-fd77777a341fb3ccad00b07dda6d2e80-miller.jpg
Moles (relative to glycine = 1) of the various amino acids
detected in the volcanic apparatus vials. Amino acids underlined have not been previously
reported in spark discharge experiments. Values for amines are minimum values because of loss due to their volatility during workup.

Yes, I know that Miller’s reducing atmosphere is no longer considered to be an accurate representation of the ancient earth’s atmosphere. However, the experiment still supported a key idea: that the synthesis of these organic compounds did not require any kind of guiding hand, but would naturally emerge from unassisted chemical reactions. Furthermore, the authors of this paper argue that while it was not a good model of the global atmosphere, it might still model local conditions in isolated areas.

Geoscientists today doubt that the primitive atmosphere had the highly reducing composition Miller used. However, the volcanic apparatus experiment suggests that, even if the overall atmosphere was not reducing, localized prebiotic synthesis could have been effective. Reduced gases and lightning associated with volcanic eruptions in hot spots or island arc-type systems could have been prevalent on the early Earth before extensive continents formed. In these volcanic plumes, HCN, aldehydes, and ketones may have been produced, which, after washing out of the atmosphere, could have become involved in the synthesis of organic molecules. Amino acids formed in volcanic island systems could have accumulated in tidal areas, where they could be polymerized by carbonyl sulfide, a simple volcanic gas that has been shown to form peptides under mild conditions.

So good work, Dr Miller!

Johnson AP, Cleaves HJ, Dworkin JP, Glavin DP, Lazcano A, Bada JL (2008) The Miller Volcanic Spark Discharge Experiment. Science 322(5900):404.

Why Evolution is True

I hope Jerry Coyne will forgive me that my frequent thought as I was reading his new book, Why Evolution Is True(amzn/b&n/abe/pwll) was, “Wow, this sure is easier to read than that other book.” That other book, of course, is Coyne and Orr’s comprehensive text on Speciation(amzn/b&n/abe/pwll), which is a technical and detailed survey of the subject in the title, and that I wouldn’t necessarily recommend to anyone who wasn’t at least a graduate student in biology. We all have our impressions colored by prior expectations, you know, and Jerry Coyne is that high-powered ecology and evolution guy at the University of Chicago whose papers I’ve read.

The new book is simple to summarize: just read the title. It’s aimed at a lay audience and answers the question of why biologists are so darned confident about the theory of evolution by going through a strong subset of the evidence. It begins with a discussion of what evolution is, then each subsequent chapter is organized around a class of evidence: fossils, embryology and historical accidents, biogeography, natural selection, sexual selection, speciation, and human evolution. If you want a straightforward primer in the experiments and observations that have made evolution the foundational principle of modern biology, this is the book for you.

Why Evolution is True makes an almost entirely positive case for evolution; it has an appropriate perspective on the current American conflict between science and religious fundamentalism that avoids dwelling on creationist nonsense, but still acknowledges where common misconceptions occur and where creationist PR, such as the Intelligent Design creationism fad, has raised stock objections. It’s a good strategy — the structure of this book is not dictated by creationist absurdities, but by good science, and creationism is simply noted where necessary and swatted down efficiently. It’s a more powerful tool for it, too — creationists can lie faster than anyone can rebut them, so the best strategy is to focus on the real evidence and force critics to address it directly.

You all really ought to pick up a copy of this book if you don’t already have a sound understanding of the basic lines of evidence for evolution (or, if you do, you could always get Speciation to get a little more depth). I recommend it unreservedly. Oh, except for one little reservation: it won’t be available until January. Go ahead and put it on your Amazon pre-order list, then.

Very peculiar

I’ve read Steve Jones’ books and enjoyed them — so I’m really baffled by this bizarre report of a talk he gave. It’s either a massive example of misreporting, or Jones has a solid grip on everyone’s ankles and he’s straining to pull our legs right off.

He claims human evolution is over. The reason? Because not enough fathers over 35 are having children. That’s bad because mutations are the source of evolutionary novelty, and older fathers are more likely to have accumulated errors in the replication of sperm, and therefore pass on more mutations.

This is because cell divisions in males increase with age. “Every time there is a cell division, there is a chance of a mistake, a mutation, an error,” he said. “For a 29-year old father [the mean age of reproduction in the West] there are around 300 divisions between the sperm that made him and the one he passes on – each one with an opportunity to make mistakes.

“For a 50-year-old father, the figure is well over a thousand. A drop in the number of older fathers will thus have a major effect on the rate of mutation.”

This is true, but it makes no sense. It’s not as if younger fathers produce no mutations — they generate plenty. It’s a difference in degree, nothing more, so we still have plenty of new mutations percolating into the population. And of course, over most of human history parents have been relatively young, since you couldn’t count on living to the age of 35.

And then there’s this odd argument.

Another factor is the weakening of natural selection. “In ancient times half our children would have died by the age of 20. Now, in the Western world, 98 per cent of them are surviving to 21.”

That makes even less sense. Natural selection is going to eliminate variants; by reducing its effects, we permit more mutations to persist in the population. One moment he’s complaining that fewer mutations are being produced, the next he’s complaining that the mutants are thriving. Which is it?

I’m thinking Jones must be making some colossal joke here, or maybe he’s testing his audience to see how much illogic and absurdity they will accept. That’s the only way I can explain these strange claims.

I see that Larry Moran has just thrown up his hands in exasperation at all the errors.

Watson/Wilson

There is good cause to be aggravated by some of James Watson’s recently expressed views, but he’s still an interesting fellow who made a significant contribution to our understanding of biology; and E.O. Wilson, of course, is always cool. So here they are together in an interview with Charlie Rose, discussing the significance of Charles Darwin:

(It was a little too gushy at the beginning — Darwin certainly did get some things very wrong! — but it’s still the kind of conversation it’s fun to hear.)

Anthropology of war

Last weekend’s bloggingheads was an interesting discussion between John Horgan and Brian Ferguson on the unfortunate misconception many people have about human evolution — the simplistic idea that evolution is always about selection for individuals who are better at killing their competitors. It doesn’t work that way! Ferguson discusses the interesting and obvious idea that the data does not back up the notion that being a great warrior is generally a good strategy, because being a great warrior also greatly increases the likelihood that you’ll end up dead.

Evolution is about whatever works, and often cooperation is a winning tactic.

We don’t need teleology — so why bother?

Tony Sidaway discusses a unifying property of theistic evolutionists: the desire or need for there to be some kind of universal plan for their existence. It’s not an attitude I understand very well; I don’t think it makes life better to believe that there is some ineffable teleological intent behind the events in your life, and no one ever bothers to explain why it would be preferable to be a pawn to a cosmic puppetmaster. Their reasoning also tends to be incredibly bad, as can be seen in the article by Mark Vernon that inspired Tony’s musings.

The work of Conway Morris, and now many others, is showing that evolution keeps coming up with the same solutions to natural problems. One of the better-known examples is that sabre-toothed cats. They evolved on at least three different occasions along independent Darwinian paths. And yet they look almost exactly the same. Dozens of examples of convergence have now been documented across a wide variety of biological phenomena, from animal and plant physiology to molecular biology.

Convergence raises the possibility of directionality in evolution. This is anathema to the old school. Strictly speaking, even to talk of adaptations being advantageous is to risk a false sense of teleology. The sense of “advantage” only comes because we have hindsight. As Stephen Jay Gould put it: according to this interpretation of evolution, if you re-ran the “tape of life”, life would look very different.

Convergence challenges this, because in a way, evolution has already re-run the tape of life several times, and it looks strikingly similar.

The argument from convergence is wrong and makes no sense, yet somehow it appeals to smart people like Simon Conway Morris and Ken Miller, who have both made it themes in their books. Convergence occurs, of course, but “dozens of examples” is not very impressive and does not imply that this is a dominant mode of evolution. The examples also exhibit the constraints of contingency; yes, several mammals have evolved saber teeth, which seem to be tools for a particular kind of predation that involves deep tearing to induce bleeding in prey. If we get away from mammals, though, it doesn’t appear very often, if at all. Raptors, for instance, probably used an overdeveloped claw in the same way. Convergence is often a consequence of limitations in anatomy and physiology that make a narrower range of solutions to common problems available.

Another good example is the eye. Eyes have independently evolved multiple times, and we do see examples of convergence — molluscs and vertebrates have simple camera eyes that are not related by ancestry. It’s not because of some master plan, however, but because using a lens to focus light on a sheet of photoreceptive cells is a simple, easily evolved strategy for putting an image on a neuronal array. This is a case where physics itself imposes some limitations on how a receptor organ can function. At the same time, though, life explores a wider set of solutions than we can imagine. Mollusc and vertebrate eyes differ in all the details of their development and anatomy, and obviously enough, other organisms, such as arthropods, have put together radically different solutions with compound eyes. Did a god have a plan that involved eyes forming as orbs with single lenses? Why? And does that make dragonflies satanic, for defying the plan?

Vernon is also completely wrong. The tape of life has not been replayed, except in a small scale and with historical limitations. You could argue, I suppose, that the Permian and Cretaceous extinctions represented a catastrophic rewinding of life’s tape for large terrestrial animals, but do note that each produced different solutions. Dinosaurs became ascendant (in a megafauna sense) after the Permian, but very different vertebrates took over after the Cretaceous.

It’s all very peculiar. This particular breed of teleologist seizes upon small functional similarities in organisms, tooth size or body shape or color pattern, and declares that because two species independently generate similar solutions to common problems, it must be because there is a guiding force producing these solutions. They want the guiding force to be a deity, but unfortunately, Darwin long ago identified the force as short-term local adaptation to environmental forces, nothing more, no grand planner, no deep purpose, and these instances of convergence provide no evidence otherwise.

There must be some psychological need in the teleologists that I lack. I don’t feel any a priori requirement that complexity and adaptation and similar solutions must be driven by any kind of master blueprint, and I find any kind of deterministic explanation for earth’s history to be personally horrifying (not that that is an obstacle to such explanations being true, but it does confuse me that some people think such an answer to be desirable).

We are each our own individual engines of purpose, operating in a hostile universe where randomness can shape our fates. There is no grand scheme behind our existence, other than the same function that all our ancestors had: to order our local environment to allow each to survive and to make the world a little better for our progeny. And that’s enough — that’s all that is needed to make a rich, diverse, living planet, and it’s all I need to live a satisfying life.

PZ’s Galápagos Adventure

Here follows a brief account of my sojourn in the Galápagos Islands, just to give you all a rough idea of what I was up to all this time. I’ve tossed in just a few pictures to illustrate what we experienced; I’m planning to dole out the rest a little bit at a time, each week. I took a lot of pictures, and I was a real piker compared to a few other people on the trip — I’m thinking that if I use mine and some of the other photographs people took, if I post one a week, I’ll be able to keep the blog going for about 3800 years.

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Fragments of a shipboard talk

Since it has been a long time since I contributed any content to Pharyngula…here’s something. I was asked to give a brief talk on the ship, so I’ve tossed my written draft below the fold. With these short talks I like to write the story first, but when I get up on the stage and actually perform it, I don’t bring notes or anything like that, so what is actually said follows the structure of what I wrote, and some of the wording comes through, but it tends to be rather different. Probably a lot different —I know I extemporized a fair bit on the last half. This is all you get until I’ve had a good night’s sleep, though.

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