This wiki page on “Recapitulation” theory says this is a common misunderstanding (ie that embryonic similarities during fetal development point to common evolutionary origins), but I’m not following exactly why this is a misunderstanding. Why doesn’t striking embryonic similarities between divergent species indicate some sort of common biological origin?
Use small words if possible.
The point is that there AREN’T similarities between developing embryos in different species. Haeckel made diagrams of developing embryos in various species, but they don’t accurately reflect reality. One of the most notorious scientific frauds, in fact.
I managed to get a B.S. in biology without ever hearing about this theory, but it looks like recapitulation theory means (to sum up the article): the evolutionary history of a species is observable in its embryonic development. If you look at the chart in the article, you will see that Haeckel made the developing human embryo look, in different phases, like a fish, a chicken, and a mouse.
This is all errant nonsense because humans did not evolve from fish, chicken, or mice. Assuming that life began only once, then every single organism alive on earth today is equally “evolved”, and none of them evolved “from” each other.
The fact that Haeckel’s drawings don’t represent what a developing human fetus actually looks like falsifies his idea that we resemble other creatures, in our early development.
Now. On to kosher evolution (and I’m sure that many others will come after me and fine-tune what I’m saying. Have patience. I specialized in neuroscience.) Darwin came up with his theory of evolution based on observable physical similarity between organisms. This was a good way to do science at the time he was doing it, without knowledge of DNA or powerful microscopes or all that fun stuff we did in sophomore labs. It is, however, a “common sense” approach and also very subjective–one can try to make the process of observing physical similarities into something quasi-scientific by taking prescribed measurements of certain body parts etc, and a lot of work is still done that way, buuuuuut…
DNA analysis is such a powerful tool that a great deal of the phylogenetic tree is being turned topsy-turvy by it. It’s being turned upside down because, it turns out, sometimes evolution is convergent. That means that two organisms with separate evolution histories happen to develop in the same way. This makes them appear–to the biologist working by physical observation–to be more closely related than they actually are. DNA sequencing is nifty because it appears that the DNA mutates at a steady and predictable rate. Therefore, looking at the DNA of two creatures that are very physically similar can reveal that they are in fact quite different. Often there are significant physiological differences, etc, also. It’s all about how powerful your tools of observation are.
To sum up and try to make a point, dismissing recapitulation theory as nonsense does not hurt the theory of evolution because all that throws out is Haeckel’s wacky idea that developing embryos somehow “relive” their species’ evolutionary history–which is a twisted exaggeration of what really happens, if not an outright fabrication. Now. Having dismissed recapitulation theory, we are left with observable physical similarity and with DNA evidence. If we throw out observable physical similarity–which a lot of biologists are doing–we lose much of the “common sense” force of evolution’s argument, and also lose most of the fossil record of extinct species, which I agree, is a significant blow to the evidence in evolution’s favor.
One still has observable instances of microevolution, DNA evidence, and other things to support the theory though–all of it more than enough to keep the scientific community convinced that evolution theory is correct.
Of course, one can still argue about its mechanism. Or something.
Such similarities do indicate a common biological origin. That is what the Wiki article meant when it noted that “modern biology does recognize numerous connections between ontogeny and phylogeny”. Embryonic similarities between starfish and humans is the best evidence we have for a recent common origin. Embryonic similarities between Onycophorans and insects is the best evidence we have for a recent common origin.
The point the Wiki article is making is that the old saw that “ontogeny recapitulates phylogeny” is untrue.
“Ontogeny recapitulates phylogeny” was still being taught when I was an undergrad, and what it means in simple terms is that the way an organism develops literally retraces the evolutionary development of the organism. IOW the reason why a tadpole looks like a larval sea squirt is because it is descended from a larval sea squirt and it is repeating that evolutionary development in its physical development.
At its most extreme Haekel and his successors suggested that the human embryo first looks like a segmented worm, then it looks like a lancet, then it takes on the physical features of a fish, then the features of a lizard, and an insectivore and a monkey and so forth.
IOW “ontogeny recapitulates phylogeny” means in simple terms that we can see how an organism evolved by simply looking at its development because the organism will progress through the physical forms of all its ancestors.
Of course that is a load of dingoes kidneys. Many of the traits of juvenile organisms result from new gene expressions that were never present in any ancestor and most importantly many juvenile traits only exist in juveniles. Kangaroo foetuses in late development for example have massive forearms and no legs at all. Such developmental traits are simply developmental traits, not evidence of the existence of some ancestral kangaroo that walked on it hands.
So the recapitulation theory is simply wrong insofar as the way an organism develops doesn’t in any way reflect the body form of its ancestors. That doesn’t prevent embryonic similarities between organisms being valid evidence of shared ancestry, it just means that those similarities don’t mean that some ancestor actually looked like that embryonic form.
In case I wasn’t clear. Haeckel’s theories were widely accepted, and often cited as evidence of evolution. His theories have now been discredited, hence using them as proof of evolution is invalid.
It is totally indisputable that there ARE similarities between developing embryos in different species. Surely you don’t deny that a human embryo more closely resembles a goat embryo than a spider embryo? Or that spider embryo more closely resembles a scorpion embryo than a jellyfish embryo?
You are the first person I have ever heard dispute that there are similarities between developing embryos. Even the most rampant Fundamentalist evolution deniers won’t do that.
Would you care to present some evidence to support this extraordinary claim?
No, but I don’t quite see what that has to do with Peter Morris’ claim that Haeckel’s drawings were fraudulent and don;t accurately reflect reality.
It is possible that some traits are shared by embryos because they were found in ancestral species, but in the vast majority of cases this is not true. I won’t say that Britannica is wrong in this issue but I will say that it presents in an ambiguous way, and as uncontroversial fact something that is strongly disputed by evolutionary and developmental biologists. I suspect that a majority of evo-devo biologists would dispute that Britannica article as it is written. Most notably the idea that vertebrates share gill slits because of a shared ancestry with fish when the gill slits are in fact homologous to structures found in lancets. I guess that since the last common ancestor of all vertebrates was probably a fish it is correct, but it is presented in a manner that is easy to misinterpret.
I would ask: Why would it indicate common biological origin?
If it were true, it wouldn’t seem to me to indicate… much of anything, really. It would just be weird.
-FrL-
Right, I was slightly careless in my choice of words. There ARE similarities between developing embryos in different species, but nowhere near the degree of similarity Haeckel laimed. See article here reproducing his original diagram.
Note that he shows a fish and a human starting from nearly identical embryos. AIUI, his diagrams have been discredited. The embryos don’t look much like that in reality.
As the Wiki article says:
Bolding mine.
It should be stressed that only the extreme form of the idea has been discredited; developmental similarities between organisms do provide evidence of evolution.
Haekel’s major error was to contend that adult ancestral forms are recapitulated during development. As has been indicated by others, this is false. Stephen Jay Gould, in his book on the subject, *Ontogeny and Phylogeny, * says the principle might better be stated “ontogeny recapitulates ontogeny.” That is, the early developmental stages of descendent forms resemble the early developmental stages of ancestral forms; major evolutionary changes are more likely to affect later developmental stages. As the Wiki article says:
Peter’s claim was worded as follows: “There aren’t similarities between embryos of different species.”
I think he misspoke, that he didn’t mean to say what he said, exactly. But still, it is what was said, so its valid to ask for an explanation so that Peter can clarify.
-FrL-
Yes. Human embryos do not have gill slits. What they do have are a series of blocks of tissue called pharyngeal arches, which are separated by deep grooves, or pharyngeal clefts. My copy of Langman’s Medical Embryology, 6th ed., has this to say:
Haeckel was wrong in saying that the various stages of embryonic form reflect the evolutionary morphological history of the organism, but it is still true that the study of the development of analogous embryonic structures can shed light on the evolutionary development of the organs or features into which those structures develop in the mature organism.
In other words, that a mammalian embryo looks a bit like a tadpole doesn’t demonstrate that the ancestors of mammals looked like tadpoles, but that the mammalian ear bones develop from structures that, in reptiles, merely develop into part of the jawbone, demonstrates that the bones in mammalian middle ears evolved from adaptations of the jawbone.
Unfortunately, creationists have latched onto this and find it very easy to loudly misrepresent, saying “Haeckel was a fraud! Evolution is false!”. They’ve done the same thing with the Peppered Moth (which is still a good, valid example of natural selection).
Why should the early developmental stages of organisms as disparate as turtles, chickens, and elephants show strong similarites if they were not related by descent? Surely, if you were “building” a turtle or a chicken from scratch there would be no need to begin with a fish-like form.
“Just be weird” is not a scientific hypothesis. 
Why should common ancestry give rise to similarities between embryos? It’s hard to see how the former is supposed to explain the latter. That means its hard to see how the latter is supposed to be evidence for the former.
I know. That was my point.
-FrL-
*Not that I’m saying they’re not…
I’m sorry, this seems so obvious to me I am having trouble understanding why you do not get it. Common ancestry doesn’t “give rise to” similarities between embryos; these similarities are instead an indication of shared evolutionary history, in the same way that anatomical homologies and the fossil record also provide evidence of shared evolutionary history.
Sorry, that *'ed comment was from an earlier “draft” of the post.
-FrL-
What’s the distinction your making here. In my ideolect, if A indicates B, it can only be because either B gives rise to A, or because some third factor gives rise to both. (“Gives rise to” means, roughly, “cause” in my ideolect, though it’s broader than my use of the word “cause.” “Give rise to” allows for the possibility that the one simply allows for the other to happen, rather than actively “causing” it to happen.)
Also, in my ideolect, “shared evolutionary history” means “common ancestry.”
I can easily envision how shared evolutionary history would lead to anatomical homologies, and how it could lead to the fossil record we find to actually exist. I can not envision, much less easily envision, how shared evolutionary history would lead to embryonic similarities: neither the old fake Haekel style similarities, nor the actual similarities that have been described in this thread.
In other words, when I wonder what anatomies would be like assuming evolutionary theories were true* I figure common evolutionary history would lead to shared anatomal features. Also, when I assume certain theories of evolution are true, and wonder what the fossil record should look like on that assumption, I imagine the record would look much like it actually does look.
But on the assumption that evolutionary theories are true, when I wonder what embryos should be like, I have no intuition or reasoning which would lead me to expect embryos from disparate species should “recapitulate” each others’ ontogenies. I don’t see why they would, no matter whether I assume evolution to be true or false.
-FrL-
*Which by the way I do so assume in the course of normal life
Read the paragraph I quoted from Wiki in post number 11. If that doesn’t explain it to you, I’m afraid I can’t help you.