Darwin and the vermiform appendix

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Last night, I asked for a copy of an article (I have plenty now, thanks!) that was getting a lot of press. The reason I was looking for it is two-fold: the PR looked awful, expressing some annoying cliches about evolution, but the data looked interesting, good stuff that I was glad to see done. Awful and interesting — I’m a sucker for those jarring combinations. My favorite pizza is jalapeno and pineapple, too.

I’m going to split my discussion of this article in two, just to simplify dealing with it. This is the awful part. I’ll do the interesting part a little later.

The paper is about the appendix, that tiny little organ in your gut that doesn’t have a whole lot of obvious function. The point of the work is to try and show that yes, it does something — which is fine and interesting, although I will quibble a bit with their interpretation. Where they go awry, though, is in trying to pick a fight with a dead man, and making that the focus of their public relations.

Now, some of those same researchers are back, reporting on the first-ever study of the appendix through the ages. Writing in the Journal of Evolutionary Biology, Duke scientists and collaborators from the University of Arizona and Arizona State University conclude that Charles Darwin was wrong: The appendix is a whole lot more than an evolutionary remnant. Not only does it appear in nature much more frequently than previously acknowledged, but it has been around much longer than anyone had suspected.

“Maybe it’s time to correct the textbooks,” says William Parker, Ph.D., assistant professor of surgical sciences at Duke and the senior author of the study. “Many biology texts today still refer to the appendix as a ‘vestigial organ.'”

Charles Darwin is dead. Your research can’t be very cogent if your approach to drum up interest is to dig up a 120-year-old corpse and kick it around; is there anyone alive who disagrees with you who can put up a more informative and entertaining struggle? What this does is pick this one fellow as a symbol of the whole edifice of evolutionary theory, which has the advantage of making one’s work seem very, very important (even if one is stacking the deck to do it), but has the disadvantage of giving every creationist on the planet something to masturbate over, and they’re icky enough without your help.

It’s also annoying. Charles Darwin was wrong about many things — I’ll even give an example at the end of this article — and it’s part of the nature of science that everyone’s work will be revised and refined over time, and some of us will even be shown to be completely wrong. It’s rather unseemly to collect a lot of data that Darwin did not have, run it through PAUP 4.0 on a fast computer, map the data onto a molecular consensus phylogeny, and cackle gleefully over discovering something Darwin did not know. Really, it doesn’t make you a better scientist than Darwin.

To make it even worse, people who do this can’t even make the corpse-fight a fair fight — they have to stuff the pathetic dead body with straw. In this case, they’re padding Darwin’s investment in the appendix a fair amount. They cite one work by Darwin, The Descent of Man, which mentions this issue. He wrote one whole paragraph on the topic, and here it is, in its entirety; it was presented briefly as part of a long list of human rudimentary structures, such as wisdom teeth, muscles of the ear, and the semilunar fold of the eye.

With respect to the alimentary canal, I have met with an account of only a
single rudiment, namely the vermiform appendage of the caecum. The caecum
is a branch or diverticulum of the intestine, ending in a cul-de-sac, and
is extremely long in many of the lower vegetable-feeding mammals. In the
marsupial koala it is actually more than thrice as long as the whole body.
(46. Owen, ‘Anatomy of Vertebrates,’ vol. iii. pp. 416, 434, 441.) It is
sometimes produced into a long gradually-tapering point, and is sometimes
constricted in parts. It appears as if, in consequence of changed diet or
habits, the caecum had become much shortened in various animals, the
vermiform appendage being left as a rudiment of the shortened part. That
this appendage is a rudiment, we may infer from its small size, and from
the evidence which Prof. Canestrini (47. ‘Annuario della Soc. d. Nat.’
Modena, 1867, p. 94.) has collected of its variability in man. It is
occasionally quite absent, or again is largely developed. The passage is
sometimes completely closed for half or two-thirds of its length, with the
terminal part consisting of a flattened solid expansion. In the orang this
appendage is long and convoluted: in man it arises from the end of the
short caecum, and is commonly from four to five inches in length, being
only about the third of an inch in diameter. Not only is it useless, but
it is sometimes the cause of death, of which fact I have lately heard two
instances: this is due to small hard bodies, such as seeds, entering the
passage, and causing inflammation. (48. M. C. Martins (“De l’Unite
Organique,” in ‘Revue des Deux Mondes,’ June 15, 1862, p. 16) and Haeckel
(‘Generelle Morphologie,’ B. ii. s. 278), have both remarked on the
singular fact of this rudiment sometimes causing death.)

Note why Darwin classed this appendage as vestigial: because it is greatly reduced compared to the homologous organs in non-human relatives, and because it currently exhibits a great range of variation, which is apparently non-functional. These are criteria which the paper in question does not refute at all. Darwin does say that the appendix is “useless”, and the paper will show some evidence that that is wrong. It’s also irrelevant.

The reason why it is irrelevant is that the presence of some function is not part of the definition of a vestigial or rudimentary organ — Darwin obligingly concedes that evolution will salvage some utility out of organs with little retention of their original function, but which are present as a consequence of contingency. He discusses this at greater length in On the Origin of Species, and here is a significant chunk of the relevant writing.

Organs or parts in this strange condition, bearing the plain stamp
of inutility, are extremely common, or even general, throughout
nature. It would be impossible to name one of the higher animals in
which some part or other is not in a rudimentary condition. In the
mammalia, for instance, the males possess rudimentary mammae; in
snakes one lobe of the lungs is rudimentary; in birds the
“bastardwing” may safely be considered as a rudimentary digit, and
in some species the whole wing is so far rudimentary that it cannot be
used for flight. What can be more curious than the presence of teeth
in foetal whales, which when grown up have not a tooth in their heads;
or the teeth, which never cut through the gums, in the upper jaws of
unborn calves?

Rudimentary organs plainly declare their origin and meaning in
various ways. There are beetles belonging to closely allied species,
or even to the same identical species, which have either full-sized
and perfect wings, or mere rudiments of membrane, which not rarely lie
under wing-covers firmly soldered together; and in these cases it is
impossible to doubt, that the rudiments represent wings. Rudimentary
organs sometimes retain their potentiality: this occasionally occurs
with the mammae of male mammals, which have been known to become
well developed and to secrete milk. So again in the udders in the
genus Bos, there are normally four developed and two rudimentary
teats; but the latter in our domestic cows sometimes become well
developed and yield milk. In regard to plants the petals are sometimes
rudimentary, and sometimes well-developed in the individuals of the
same species. In certain plants having separated sexes Kolreuter found
that by crossing a species, in which the male flowers included a
rudiment of a pistil, with an hermaphrodite species, having of
course a well-developed pistil, the rudiment in the hybrid offspring
was much increased in size; and this clearly shows that the
rudimentary and perfect pistils are essentially alike in nature. An
animal may possess various parts in a perfect state, and yet they
may in one sense be rudimentary, for they are useless: thus the
tadpole of the common salamander or water-newt, as Mr. G. H. Lewes
remarks, “has gills, and passes its existence in the water; but the
Salamandra atra, which lives high up among the mountains, brings forth
its young full-formed. This animal never lives in the water. Yet if we
open a gravid female, we find tadpoles inside her with exquisitely
feathered gills; and when placed in water they swim about like the
tadpoles of the water-newt. Obviously this aquatic organisation has no
reference to the future life of the animal, nor has it any
adaptation to its embryonic condition; it has solely reference to
ancestral adaptations, it repeats a phase in the development of its
progenitors.”

An organ, serving for two purposes, may become rudimentary or
utterly aborted for one, even the more important purpose, and remain
perfectly efficient for the other. Thus in plants, the office of the
pistil is to allow the pollen-tubes to reach the ovules within the
ovarium. The pistil consists of a stigma supported on a style; but
in some Compositae, the male florets, which of course cannot be
fecundated, have a rudimentary pistil, for it is not crowned with a
stigma; but the style remains well developed and is clothed in the
usual manner with hairs, which serve to brush the pollen out of the
surrounding and conjoined anthers. Again, an organ may become
rudimentary for its proper purpose, and be used for a distinct one: in
certain fishes the swimbladder seems to be rudimentary for its
proper function of giving buoyancy, but has become converted into a
nascent breathing organ or lung. Many similar instances could be
given.

I’ve highlighted the part most important for this discussion. Darwin did not discuss the appendix or caecum at all in the Origin, but this description does apply. If a portion of the gut, a digestive organ, is diminished in size such that it no longer contributes to the primary function of the organ, but does retain a secondary function, such as assisting in immunity, or as the authors of the recent paper will argue, in acting as a reservoir of bacteria for recolonizing the gut, then it is still a vestigial organ. It has lost much of its ancestral function.

I do not understand why this is so hard for so many people to comprehend. Biology is plastic and opportunistic. Accidents of history will always still be incorporated into the whole of the organism; we make do, or we die. Just because something is does not mean that the entirety of its nature is the product of selection.

I mentioned that I’d point out errors in Darwin’s understanding. They’re there, but note that seeing them now 150 years after he wrote his big book does not make me smarter than Darwin, nor does it invalidate the overall picture of his theory. You can see one ‘error’ in the quote above: we are now pretty certain that the original function of the swimbladder in fish was respiratory. It evolved first as a supplement to the gills, providing access to the rich oxygen content of the atmosphere, and was secondarily adapted to function for bouyancy. Hah, silly Darwin, that he did not know a detail of paleontology and phylogeny that would be worked out a century after his death!

He also made a more substantial error. He wondered how organs became smaller over time, and his answer was, unfortunately, a bit Lamarckian and also a bit muddled.

It appears probable that disuse has been the main agent in rendering
organs rudimentary. It would at first lead by slow steps to the more
and more complete reduction of a part, until at last it became
rudimentary,- as in the case of the eyes of animals inhabiting dark
caverns, and of the wings of birds inhabiting oceanic islands, which
have seldom been forced by beasts of prey to take flight, and have
ultimately lost the power of flying. Again, an organ, useful under
certain conditions, might become injurious under others, as with the
wings of beetles living on small and exposed islands; and in this
case natural selection will have aided in reducing the organ, until it
was rendered harmless and rudimentary.

“Disuse” is the magic word there: if a cavefish lived in the dark and never used its eyes, the idea was that its progeny would then have smaller eyes. This is not correct, but it was a central part of Darwin’s invalid theory of heredity. This is a much more substantial failing of Darwin’s work, but again, I can’t claim credit for figuring this out; it took the work of Mendel to get the core of genetics puzzled out, and then it took a whole generation of scientists to work out how genetics and evolution fit together. We can say “DARWIN WAS WRONG!” about that, but we can’t really say that about his treatment of vestigial organs in general, which seems to hold up fairly well…perhaps because Darwin himself was not so fervently committed to the absolute adaptedness of every single feature of every single organism as some of his later followers.

That said, I’ll move along to the substance of the paper next, which really does have some good stuff in it. Most of my complaints here are with the abysmal presentation of the ideas in it by the popular press, aided and abetted by the scientists themselves. Just keep in mind that whenever these press releases that declare “Darwin was wrong” appear, it’s usually an example of grandstanding and the regrettable tendency of competitive scientists to think the way to impress people with the importance of their work is to get into a penis-measuring contest with poor dead Chuck.

Smith HF, Fisher RE, Everett ML, Thomas AD, Randal Bollinger R, Parker W (2009) Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix. J Evol Biol. 2009 Aug 12. [Epub ahead of print]

Yes, millions of years!

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That Answers in Genesis crackpot, Terry Mortenson, is speaking on “Millions of Years” at the Creation “Museum”. Those of us who visited that circus of charlatanry know that this is one of their obsessions — the idea that the earth is more than 6000 years old is one of the wrecking balls atheists use to destroy faith.

He’s right, of course. It’s a very useful tool. When fundamentalists tie their faith absolutely to a claim that is easily refuted, that contradicts the evidence, and that requires them to constantly escalate their denial and delusions in order to sustain their belief, it makes it really easy for atheists to demolish their religion. We don’t even need to attack religion in the classroom at all — we just calmly lay out the facts, let the students work out the conclusions, and sometimes…it’s epiphany time! They realize their pastor lied to them, or was just really ignorant, and suddenly their respect for Christian authority begins to crumble away.

It’s not the atheist’s fault, though. The lesson should be, “Don’t lie to your kids,” not “Silence the people who would reveal that you lied to your kids,” or worse, “Lie harder.”

This is not a lesson that Mortenson has learned. He is apparently planning to babble about revisionist history in his talk, claiming that the evidence for the age of the earth is the product of an atheist conspiracy among geologists.

To really understand what is wrong with belief in millions of years, we need to go back to the early 19th century and study the origin of this idea. This unique and interesting lecture, based on Dr. Mortenson’s PhD research, will clearly show that the idea was not the result of just letting the rocks and fossils “speak for themselves” but rather comes from anti-Biblical worldviews (or philosophical assumptions) being imposed on the geological evidence. The talk explains the key men who helped develop the idea of millions of years, one of the geologically competent Christians who opposed those theories, and the subsequent consequences of the church’s compromise with millions of years. Even non-Christians would find this lecture thought-provoking.

Hah! The only thought it would provoke in me is to wonder where they kept the straitjackets. Looney-tunes revisionist history is not thought-provoking in a good sense.

I actually spend a fair amount of lecture time on the early history of geology in my introductory biology course. One reason is that, if you talk to most people, you will discover this fallacious belief that evolution leapt fully-formed from the brain of Charles Darwin, and there’s an anachronistic idea that ideas about the age of the earth, which are built on independent evidence from geology and astronomy, are somehow rooted in biology. It’s not so! Darwin’s antecedents had already laid the foundations in working out that the earth was old, that life had undergone many transitions, and that maybe species were mutable. Evolution was an inevitable conclusion of the evidence; Darwin and Wallace were just the clever fellows who managed to pull the whole story together.

I find it very useful to give students a quick overview of 18th and 19th century geology before we talk about Darwin, since the creationists in the classroom usually have this image of Darwin as Satan who foisted a false belief on the world because he hated god (hey, sounds like Terry Mortenson!). It’s very useful to be able to show how views of the world evolved, not by ideology, but by the growth of a body of evidence.

Let’s begin with Robert Hooke (1635-1703). He dabbled brilliantly in many things, but one subject of particular interest was the origin of these curious fossils that people kept digging up, which were thought to be either creatures turned to stone by some miraculous process, or were the expression of an intrinsic nature of stone to mimic life. Hooke examined the details of fossils microscopically, and determined that they had once been alive, and also worked out how the transformation had occurred — by the perfusion of minerals into buried or immersed dead organisms. He also examined the distribution of fossils; finding fossilized clams on mountaintops, for instance, says something about the prior state of that environment.

Most of those Inland Places. . . are, or have been heretofore under the Water. . . the Waters have been forc’d away from the Parts formerly cover’d, and many of those surfaces are now raised above the level of the Water’s Surface many scores of Fathoms. It seems not improbable, that the tops of the highest and most considerable Mountains in the World have been under Water, and that they themselves most probably seem to have been the Effects of some very great Earthquake.

These conclusions were evidence-driven. Almost no one in the late 17th century would have been interested in opposing religion, so you can’t pin that heresy on Hooke. He is simply describing the natural world and finding certain conclusions inescapable, including some to which creationists today still can’t adjust — and note that he is writing this more than 300 years ago.

There have been many other Species of Creatures in former Ages, of which we can find none at present; and that ’tis not unlikely also but that there may be divers new kinds now, which have not been from the beginning.

And then there’s Baron Cuvier (1769-1832) and Alexandre Brongniart (1770-1847) who studied the rocks of the Paris Basin. There were many quarries situated around Paris that cut deep into the hills to provide building stone, and they gave these two the opportunity to look into the structure of the rocks. They identified five major layers, and by examining the fossils, worked out what kinds of animals and plants lived there when the layers were deposited. They found that layers with saltwater species were interleaved with layers containing freshwater species — Paris had been under the sea at least twice!

Cuvier was not an atheist. In fact, he was even adamant that the earth was relatively young, but in a way that contradicts what Answers in Genesis would tell you. He had worked out that there were different assemblages of animals in each layer, and proposed an explanation: a series of ages, each very different, with the most recent major catastrophe occurring five or six thousand years ago (to bring it in line with the literal interpretation of the Bible) and sweeping away prior forms to allow for the flourishing of human beings.

It is certain that we are now at least in the fourth succession of terrestrial animals. The age of reptiles was followed by that of the palaeotheres [primitive mammals], then the age of mammoths, mastodons, and megatheria. Finally we arrive at the age of the human species together with domestic animals. It is only in the deposits subsequent to the beginning of this age, in turf-bogs and alluvial deposits, that we find bones all of which belong to animals now existing…None of these remains belong either to the vast deposits of the great catastrophe or to those of the ages preceding that wonderful event.

If you want someone who was willing to assert that the earth was very, very old, we have to look to the Scottish geologist James Hutton (1726-1797), who was accused of atheism for his ideas, but they were backed up entirely by hard-earned evidence. He postulated that the geology we see was created by multiple cycles of sedimentary deposition, volcanic uplift, and erosion, and he mapped and documented complex unconformities and intrusions that demonstrated that the history of the earth was complex and required great time for the formation and distortion of rock. He also found that the evidence of the time was insufficient to even show the history of the beginning of the earth, which is why he closed his book, Theory of the Earth, with the famous line, “The result, therefore, of our present enquiry is, that we find no vestige of a beginning,—no prospect of an end.”

Again, his conclusion was dictated by the evidence, not some atheistic philosophy.

At the same time Cuvier and Brongniart were exploring the Paris Basin, William Smith (1769-1839) was walking all over England, building up his geological map. We know what his motivation was: it was economic. He worked in mines, and was eager to capitalize on the opportunities opened up by the Industrial Revolution. Railroad and canal cuts exposed the strata of English geology all over the place, and being able to assess good locations for coal mines was a profitable skill — much like petroleum geology now. Smith observed consistent features of geology, like the way rocks were layered, and what fossils were present in specific layers, and could see that a layer was a slice of time, and that each slice contained different animals (which led to his Principle of Faunal Succession). He worked out the first geological map of Britain on the basis of his surveying.

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There is a pattern to geology: we can see that the strata are not purely local phenomena, but part of formations that often extend continent-wide. These strata also have a predictable order that reflects the timing of their formation. These observations are not reconcilable with the simplistic dogma of the creationists.

Charles Lyell was also an important geologist, who was also very influential on Charles Darwin. He was not an atheist, but rather, a devout Christian, which caused him considerable discomfort since he was never able to accept the full implications of Darwin’s work. Lyell’s key dictum was that the present is the key to the past, that what you needed to do was work out mechanisms in action right now and use those to explain what must have happened in the past.

Darwin himself applied this principle to estimate a minimum age for the earth. He knew from published observations that a rapid rate of sedimentary deposition was 600 feet in 100,000 years; he also knew that the known strata in England had a depth of over 72,000 feet, which implied that the earth had to be at least 12 million years old.

It’s so widely accepted that even creationists use it — it’s the basis for their arguments that the ocean sediments and moon dust say the earth is young. Unfortunately, the way they accomplish that is by either using the wrong numbers for accumulation or ignoring the multiple processes that affect the rate.

It is simply ludicrous to claim that 18th or 18th century geologists bent their interpretation to fit some imaginary godless worldview — in general, the scholars of that period were more concerned with avoiding conflicts with religion, since the majority of them were doctrinaire church-going Christians themselves. What led them to the conclusion that the earth was millions, and then billions of years old was the evidence, not their ideology.

And now, of course, the evidence is even more overwhelming, and it’s mostly physics at its heart. Trying to salvage Bishop Ussher’s weird numerological and biblical 17th century chronology in the 21st century by invoking the incomplete understanding of 19th century scholars is exactly the kind of inanity we’ve come to expect from creationists.

Arthrodires got penises!

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This is the skull of an arthrodire, an armored placoderm from the Devonian.

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Somehow, 20 foot long predatory fish with a mouth lined with razor-edged bony shears has never made me think of sexy time…until I ran across this comparison image.

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Oh, schwiiing. It really doesn’t take much to get a mammal to associate just about anything with sex. And then, what do you know, the latest Nature has a short article on an interesting fossil: it’s the pelvic region of an arthrodire, Incisoscutum ritchiei, and look what it’s got: an ossified clasper, comparable to the erectile organ of modern sharks. This is a bony rod that would have been the core of an intromittent organ in the living animal, so what we have here is a small relic of the sex life of a big fish from a few hundred million years ago.

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a, Pelvic girdle in dorsal view; b, pelvic girdle restored.

Think about this, you over-sexed apes: what will be left of your manhood 300 million years from now?

Ahlberg P, Trinajstic K, Johanson Z, Long J (2009) Pelvic claspers confirm chondrichthyan-like internal fertilization in arthrodires. Nature 460:888-889.

Gene regulatory networks and conserved noncoding elements

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We miss something important when we just look at the genome as a string of nucleotides with scattered bits that will get translated into proteins — we miss the fact that the genome is a dynamically modified and expressed sequence, with patterns of activity in the living cell that are not readily discerned in a simple series of As, Ts, Gs, and Cs. What we can’t see very well are gene regulatory networks (GRNs), the interlinked sets of genes that are regulated in a coordinated fashion in cells and tissues.

[Read more…]

The future is roaring your way…

Edge hosted an amazing session that described the looming future of biology — this is for the real futurists. It featured George Church and Craig Venter talking about synthetic genomics — how we’re building new organisms right now and with presentiments for radical prospects in the future.

Brace yourself. There are six hours of video there; I’ve only started wading into it, but what I’ve seen so far also looks like a lot of material that will be very useful for inspiring students about the future of their field. There is also a downloadable book (which is a dead link right now, but I’m sure will be fixed soon) if you don’t want to watch the talks…but the talks are pretty darned good. Somehow, I’m going to have to make time to soak these up. Here’s the overview of the six sessions:

  • Dreams & Nightmares
    Overview, safety/security/policy, nanotechnology, molecular manufacturing

  • Smaller than life
    What is life, origins, in vitro synthetic life, mirror life, computing and DNA, computing with DNA

  • Engineering microbes
    Bio-petrochemicals & pharmaceuticals, accelerated lab evolution

  • Engineering humans
    Electronic-biological interfaces, bioengineered personal stem cells, humanized mice, bringing back extinct species

  • The sorceror
    The diversity of life, constructing life, from Darwin to new fuels

  • The near future, big questions
    Terraforming earth, creating extraterrestrials, the singularity, human nature

There goes your weekend.

The evolution of Hedgehog

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PLoS has recently published a highly speculative but very interesting paper on how a particular signaling pathway, the Hedgehog pathway, might have evolved. It’s at a fairly early stage in hypothesis testing, which is one of the things that makes it interesting — usually all you see published is the product of a great deal of data collection and experiment and testing, which means the scientific literature gives a somewhat skewed view of the process of science, letting the outsider mainly see work that has been hammered and polished, while hiding the rougher drafts that would better allow us to see how the story started and was built. It’s informative in particular for those who follow the creationist “literature”, which often crudely apes the products of actual working science, but lacks the sound methodological underpinnings. In particular, creationism completely misses the process of poking at the real world to develop ideas, since they begin with their conclusion.

So take this description as a work in progress — we’re seeing the dynamic of building up a good working model. As usual, it starts on a sound foundation of confirmed, known evidence, makes a reasonably hypothesis on the basis of the facts, and then proposes a series of research avenues with predicted results that would confirm the idea.

[Read more…]

Brian Goodwin, 1931-2009

It’s sad to see that we’ve lost Brian Goodwin, one of the genuinely original (but not always right!) thinkers of our time. There aren’t many left of the old structuralist tradition in biology, the kind of non-genetic purists who tried to analyze development in terms of the fundamental physical and chemical properties of the organism—they’ve been swallowed up and lost in a triumphal molecular biology research program.

Edge has a nice interview with and essay by Goodwin — they’re good places to start. If that whets your appetite, you should also read his book, How the Leopard Changed Its Spots : The Evolution of Complexity(amzn/b&n/abe/pwll), which is aimed at general audiences and is a good overview of why we should look at more than just genes to explain form.

He was an advocate for one view of nature, and I think he missed the mark by neglecting genes as much as he did; we know now that a lot of details of morphology are directly affected in subtle and not-so-subtle ways by the genetics of the organism. But I think we can also make a case that the modern molecular biological approach is also missing a significant element. Every biologist ought to read a little Goodwin, just to leaven their picture of how biology works with his special perspective.

Good TV

I must thank the reader known to me only as CAC for sending me DVDs of the Inside Nature’s Giants programs. I’ve been enjoying the dissections of an elephant and a whale in the evening — most of the organisms I cut into are millimeters long and require very sharp, thin instruments, so it’s interesting to see ones that require hip waders and backhoes.

You should all lobby your local PBS stations and tell them these would be wonderful additions to the lineup! You might also suggest that broadcasting them during the dinner hour might not be recommended.

What caused the Cambrian explosion? MicroRNA!

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No, not really — my title is a bit of a sensationalistic exploitation of the thesis of a paper by Peterson, Dietrich, and McPeek, but I can buy into their idea that microRNAs (miRNAs) may have contributed to the pattern of metazoan phylogenies we see now. It’s actually a thought-provoking concept, especially to someone who favors the evo-devo view of animal evolution. And actually, the question it answers is why we haven’t had thousands of Cambrian explosions.

In case you haven’t been keeping up, miRNAs are a hot topic in molecular genetics: they are short (21-23 nucleotides) pieces of single stranded RNA that are not translated into protein, but have their effect by binding to other strands of messenger RNA (mRNA) to which they complement, effectively down-regulating expression of that messenger. They play an important role in regulating the levels of expression of other genes.

One role for miRNAs seems to be to act as a kind of biological buffer, working to limit the range of effective message that can be operating in the cell at any one time. Some experiments that have knocked out specific miRNAs have had a very interesting effect: the range of expressed phenotypes for the targeted message gene increases. The presence or absence of miRNA doesn’t actually generate a novel phenotype, it simply fine-tunes what other genes do — and without miRNA, some genes become sloppy in their expression.

This talk of buffering expression immediately swivels a developmental biologist’s mind to another term: canalization. Canalization is a process that leads organisms to produce similar phenotypes despite variations in genotype or the environment (within limits, of course). Development is a fairly robust process that overcomes genetic variations and external events to yield a moderately consistent outcome — I can raise fish embryos at 20°C or at 30°C, and despite differences in the overall rate of growth, the resultant adult fish are indistinguishable. This is also true of populations in evolution: stasis is the norm, morphologies don’t swing too widely generation after generation, but still, we can get some rapid (geologically speaking) shifts, as if forms are switching between a couple of stable nodes of attraction.

Where the Cambrian comes into this is that it is the greatest example of a flowering of new forms, which then all began diverging down different evolutionary tracks. The curious thing isn’t their appearance — there is evidence of a diversity of forms before the Cambrian, bacteria had been flourishing for a few billion years, etc., and what happened 500 million years ago is that the forms became visible in the fossil record with the evolution of hard body parts — but that these phyla established body plans that they were then locked into, to varying degrees, right up to the modern day. What the authors are proposing is that miRNAs might be part of the explanation for why these lineages were subsequently channeled into discrete morphological pathways, each distinct from the other as chordates and arthropods and echinoderms and molluscs.

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