I’m teaching about this today in my introductory evolution class, and thought I’d include a video showing evidence for evolution that comes from the development of the human kidney. The video is in French, but I’ll explain what’s happening.
The evolutionary/developmental phenomenon of “recapitulation,” in which developing organisms are supposed to go through stages that reprise their evolutionary ancestry, has been in bad repute, but some of that is undeserved. (Recapitulation is often characterized with the phrase “ontogeny recapitulates phylogeny”, with “ontogeny” meaning development and “phylogeny” meaning “order in which ancestors evolved.”)
No, our own development does not successively resemble that of an adult fish, amphibian, and reptile before arriving at our own mammalian characteristics, but we do show developmental features resembling those of young ancestors. And yes, in some cases the order in which developmental features appear often corresponds to the order in which the ancestors with those features evolved.
One example is the development of the human kidney, which is pretty much the same as the development of any mammalian kidney. It turns out that, in utero, we develop three separate kidneys in succession, absorbing the first two before we wind up with the embryonic kidney that will become our adult kidney. The first two of these reprise embryonic kidneys of ancestral forms, and in the proper evolutionary order.
The video below shows the production and disappearance of the two kidneys. (Note: there may be a few errors in what I say, since the information is gleaned from many different sources and was sometimes conflicting. Kidney-savvy readers can weigh in.) Note too that the development of structures associated with the kidney—the urogenital system—differs between males and females. I show the male development, but you can see that of females here.)
Pronephric kidney (0-12 seconds in the video): This appears first in the video, and begins to form at about three weeks into human development. It consists of an organ that, in lampreys and hagfishes (primitive jawless vertebrates), filters wastes from the coelom (body cavity) and excretes them to the outside. But the pronephric kidney does not function in human and other mammalian embryos. It begins to disappear shortly after formation and gives rise to the
Mesonephric kidney (14-40 seconds). This kidney filters wastes from the blood, not the body cavity, and excretes them to the outside of the body via a pair of tubes called the mesonephric ducts (also “Wolffian ducts”). The mesonephric kidney goes on to develop into the adult kidney of fish and amphibians. This kidney does function for a few weeks in the human embryo, but then disappears as our final kidney forms, which is the
Metanephric kidney (42 seconds into the video until the end): This begins developing about five weeks into gestation, and consists of an organ that, like the mesonephric kidney, filters wastes from the blood, but excretes them to the outside through a pair of new tubes, the ureters. In the embryo, the wastes are excreted directly into the amniotic fluid. The metanephric kidney is the final adult kidney of reptiles, birds, and mammals.
It was Darwin, of course, who first noticed the phenomenon of recapitulation and used it as evidence for evolution: it forms part of Chapter 13 of The Origin, though, as I recall, Darwin doesn’t mention kidneys.
This bizarre formation of three successive kidneys, with the first not functioning at all and the first two degenerating completely, begs explanation. It doesn’t make a lot of sense under a creationist hypothesis: why would a creator bestow the embryo with three kidneys, trashing the first two (one of which doesn’t do anything) before making the final one? The explanation involves the fact that the first two kidneys resemble, in order, those of primitive aquatic vertebrates (lampreys and hagfish) and aquatic or semiaquatic vertebrates (fish and amphibians): an evolutionary order. The explanation, then, is that we go through developmental stages that show organs resembling those of our ancestors. For we are, after all, descended from fish and amphibians (though cladists might argue with those terms).
Why do we still retain those early developmental forms? We’re not sure, but many suspect that development is such an integrated process that it’s “easier” for natural selection to remodel existing features than to form new ones de novo. The pronephric kidney, for example, may provide a key morphological or chemical stimulus for the formation of the mesonephric kidney, and the mesonephric for the metanephric kidney. So the first two kidneys appear in a transitory way to provide those stimuli. This doesn’t always happen, of course: many features form without having to first reprise the ancestral condition of those features. Recapitulation is a phenomenon, not a law.
This ordering of developmental events that mimic those of our ancestors is not unique to the kidney: it also occurs, for example, in the way our blood vessels form, and Darwin gives other examples. One of my favorite examples, which I’ll also teach about today, is the lanugo, the coat of hair that all human embryos develop and then shed about a month before birth (see my explanation here). The lanugo forms because we carry the genes for a full coat of hair, inherited from our primate ancestors. We briefly express those genes in utero, and at about the same relative time of development as do embryonic chimps (who don’t lose the hair). Here’s the lanugo on a premature infant. It’s shed soon after birth.
Embryonic baleen whales, which don’t have teeth as adults, form tooth buds in the embryos, which then disappear. The same is true for toothless anteaters, which, like baleen whales, are descended from ancestors that had teeth.
Buried in our embryology are innumerable signs of our ancestry—innumerable proofs of evolution. As Darwin said in The Origin,
“Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals.”
Connecting the events of development with evolution was one part of Darwin’s genius, and one reason why On the Origin of Species is such a fantastically insightful book. But that connection forms only one section of a single chapter out of fourteen. In the rest of the book, Darwin also connected evolution to the fossil record, to biogeography, to animal and plant breeding, and to the existence of vestigial organs. And he also produced the correct explanation for the way evolution molded adaptations—natural selection.
The Origin is one of the monumental achievements of the highly evolved human brain—the best science book ever written. I’ve always said that it’s the one science book you must have read to be considered truly educated. (If you read WEIT, I’ll consider you fairly educated!)
Interesting stuff for the layman this. Sean B Caroll’s ‘Endless Forms’ was a good read on this and other Evo Devo stuff. Hopefully it hasn’t been completely trashed by those in the know.
Just to clarify on “fish”: Which kind of kidneys do sharks and rays have, pro or meso?
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Just a small but important correction: The mesonephros does not morph into the metanephros (watch the video again!)
Yep, a misstatement. I’ve corrected it, thanks!
Great animations! Readers should watch the linked female video, too. One thing not shown in the video is that all of the kidneys develop in the nephric ridge, not de novo within the body. The nephric ridges run paraxially along the dorsal side of the body. Kidney development consists largely of the sequential development of tubules from the anterior to the posterior part of the nephric ridge. In answer to Ant, in many fish (including sharks and rays) and amphibians the mesonephros continues a posterior proliferation of kidney tubules, forming an elongate kidney that includes the mesonephric and metanephric regions of the nephric ridge (but lacking the ureter or metanephric duct of the metanephros, which is an antero-dorsal outgrowth of the distal part of the mesonephric (= pronephrric = archinephric) duct). This elongate mesonephric kidney is called an opisthonephros. Most adult vertebrates have either an opisthonephros (fish, amphibians) or a metanephros (reptiles, birds, mammals). Hagfish and a few teleosts retain a pronephros, although whether it is functional is debated (those with it also have a functional opisthonephros). The pronephros is called the “head kidney”, because it is so far forward in the body.
Thanks, Greg! Even if that does just complicate things. I was trying to map kidney development to the vertebrate cladogram, which seemed to be straightforward…
+– Amniotes ~ meta
|
+–+– Other jawed vertebrates ~ meso
|
–+—– Lampreys and hagfishes ~ pre
… but now you’ve made it more complicated.
Who are opisthonephrotic?
• All sharks and rays
• Some but not all ray-finned fishes
• Coelocanths?
• Lungfishes?
• Some but not all amphibians
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So… <tt> tags don’t work here… 🙁
This is for adults only; embryos and larvae often have transitorily ‘earlier’ kidneys:
Amniotes- metanephros.
Amphibians- opisthonephros.
All fish (inl. coelacanths, lampreys, hagfish)- opisthonephros.
Hagfish and a few teleosts- pronephros (in addition to their opisthonephros).
The hagfish opisthonephros is more primitive in that the tubules are segmented throughout their length, while in other opisthonephros only the anterior tubules are segmented. So, it matches the tree fairly well, but not at the agnathan/gnathostome transition. There’s some variation in how the kidneys are described in various comparative anatomy texts (some, for example, describe the hagfish kidney as purely pronephric), but this is the basic story.
another interesting feature not shown in the female video is the relationship of the structures with the coelom. The paramesonephric duct begins as a fold of the coelom and maintains the connection anteriorly. This is why the uterine tubes open into the coelom in females and why there can occur peritoneal (abdominal coelom) ectopic pregnancies in females – some of these have developed far enough for successful c-section. Its also a useful exercise for students to ponder what happens to all the sperm that make it into the peritoneal cavity. The male peritoneum is completely closed because the paramesonephric duct degenerates.
The possibility of ectopic pregnancies–yet another marker of “unintelligent design!”
I remember feeling frustrated 20 years ago, when I began debating Creationists. I wanted to learn the evidence for evolution. Do you know that there isn’t really much of that in Biology textbooks? At least those accessible to lay readers. None bother to explain. They all pretty much assume evolution is true and talk about natural selection.
Blame Darwin. He did such a stunning job of explaining the why evolution is true that no-one in the 150 years since has ever bothered to do better.
Though some have come close 🙂
“I wanted to learn the evidence for evolution. Do you know that there isn’t really much of that in Biology textbooks?”
American biology textbooks? As designed in Texas? And you are surprised?
Have you read “Dr Tatiana’s Sex Advice to All Creation”? If they ever require teaching creationism I’d use that since it has the word ‘Creation’ in the title.
(Heh, heh, heh).
These were Undergrad introductory texts. They didn’t “prove” evolution because, firstly, an honest proof requires one to start from the position that evolution is not proven. I suspect that is extremely difficult for a professional biologist.
The second reason is the Origin. All the textbooks kept referencing it. When I finally understood that they were serious, that a 150 years out-of-date book had somethimng useful to say, I opened it. And have known why evolution is true since.
My niece was born full term with lanugo on her shoulders and upper arms. I used to call her trog. No one else in my family got it.
So the lanugo hair is shed into the amniotic fluid, and broken down there?
Teaching the development and the histology of the kidney was one of my favorite teaching activities. I does seem that the mesonephros is given a bit of short shrift given its importance in development of gonads after the arrival of the endodermal primordial germ cells. This makes the gonads one of only a few organs with major components derived from two different embryonic germ layers.
I want my full coat, dammit!
Can I also get a long, furry tail, like my cat’s? =)
I’d hold out for a prehensile one, like a New World monkey.
I want a prehensile tail with stripes like a lemur, and I also want skin like a cuttlefish and the ability to see ultrviolet like a bird. Those genetic engineers are just lying round on the job if you ask me.
Since nephros means kidney, “-nephric kidney” is a tautological redundancy.
“tautological redundancy” is redundant
You’re right, it’s a tautologically redundant pleonasm.
Fascinating.
Playing devil’s advocate a bit, one can imagine that this gives the impression of whole kidneys forming instantly out of nothing. Surely there was some smooth transition from pro- to meso- types, but taking the idea of “ontogeny recapitulating phylogeny” why should one disappear before the next forming rather than one transitioning smoothly into the next?
I wonder how many universities still teach comparative vertebrate anatomy?
Pretty much all of them.
I’ll use he kidney evidence next time I’m lecturing. Thanks
Wonderful stuff, nice vid!
Totally unrelated association. My roommate in grad school (a psych student) was excited about attending her first departmental party. She returned a bit nonplussed; seemed they’d played Charades, and she’d drawn the phrase “ontogeny recapitulates phylogeny” to act out . . .
The early stages of the larval lamprey (ammocoete) have a functional pronephros.
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As a pediatric nephrologist, I deem your description suitable. The video is lovely.
I remember as a medical student suggesting that embyonic development paralleled evolution and getting shot down by the prof. I have felt so justified in recent years as evo-devo has regained a degree of respectability.
Of course, if you use this as an example with a true creationist, they will tell you that god just does this to see who is faithful enough to disregard evidence (have done this experiment a few times).
Your professor in med school was probably reacting to Haeckel’s Law (“ontogeny recapitulates phylogeny”) which is only occasionally true. For ontogeny to recapitulate phylogeny (i.e. for embryos to resemble ancestral adults), evolution would have to occur by what’s been called terminal addition– new structures or modifications being added only at the end of development. This does occur, but it’s by no means a universal phenomenon. What is generally true is von Baer’s Law: more generally distributed (primitive) features appear earlier in development, so you can tell an embryo is a vertebrate earlier in development than you can tell it’s a mammal, and you can tell it’s a mammal before you can tell it’s a horse. This leads to embryos resembling ancestral embryos, which they generally do. Darwin’s understanding of this was quite good and von Baerian, and he also understood what conditions would lead to von Baer’s law not holding (see Chap. 14 of the Origin).
I knew about the kidneys (through PZ) but not the other examples. Cool!
There is no measure for that.
But as an achievement without much before, that combined various fields (like math and physics), started new areas (calculus and mechanics) and kick started a new science I give you Philosophiæ Naturalis Principia Mathematica by Newton.
Without math to quantify results, there would be no science. Newton did that, as well as set the mold for testable theories. “Today the two methodological aspects that Newton outlined could be called analysis and synthesis.”
I’ve read small bits of Darwin (Newton is unreadable today) and no doubt he got analysis and synthesis down pat. It is also a tremendously important book, but Wallace was on the case as Hooke and Leibniz was on the cases of mechanics and calculus.
I think Newton did as much heavy lifting as Darwin, if not more. But again, how to measure? It all comes down to personal preference as it is, and then we can just state them.
I’ve heard that one of the main rooms of the Royal Observatory in Greenwich is flanked by two statues–Newton & Darwin.
Is that an illustrious heritage, or what?
This is amazing stuff…embryological vestigal organs are powerful pieces of evidence indeed. My favorite has to be the tail on human embryos.
But, I have a question. Do we actually know the genes that are responsible for a coat of hair, like lanugo? I know we’ve found the genes that regulate tail growth in humans, but I’d like to know if we’ve also found the genes for a “fur coat” so to speak.
Is there any link between the reabsorption of these kidneys and bilateral renal agenesis?