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!)