More evidence for evolution: Horse embryos start forming five toes, and four primordia disappear

February 10, 2020 • 9:00 am

When I started this website in 2009, my intention was just to publicize my new book, Why Evolution is True. On the advice of my publishers, I created a site with the idea of occasionally posting new evidence for evolution to complement what was in the book. I expected to post about once a month or so.  Well, what a monster this has become!

But today I’m writing about some new work that fits perfectly with the original aim of this site. It’s a paper in the Proceedings of the Royal Society by Kathryn Kavanagh et al. that gives developmental evidence for the five-toed ancestry of modern one-toed horses. You can read the paper by clicking on the screenshot below or reading the pdf here ; a full reference is at the bottom. If you want a short but less informative piece, the New York Times has a report

Lots of organisms show developmental evidence for their evolution from very different ancestors; I describe some of this in chapter 3 of Why Evolution is True.  Embryonic dolphins, for example, develop hindlimb buds, which in their four-legged ancestors went on to become legs, but in the dolphin the buds regress before birth, leaving newborns with no hind limbs. We humans, like all terrestrial vertebrates, begin development by forming what go on to become gill slits in our fishy ancestors. In reptiles, amphibians, and mammals, though, those gill slits are transformed into other stuctures, like our esophagus.

In my evolution class I talk about the lanugo: the thick coat of hair that human fetuses develop at about six months after conception. It’s shed before birth—but not in chimps, our closest relatives, who are born hairy (remember: we are the “naked ape”). The transitory formation of that coat of hair in our species, which is of no use to the embryo, can be explained only by our descent from a primate with hair.

So if these transitory features disappear, why do we see them at all? We’re not sure, but their appearance may be necessary to provide developmental “cues” for the appearance of features that do remain. Remember, development is a very complex process which requires a nexus of coordinated features appearing at the right times. In the dolphin, for instance, the hind limb buds may provide cues for the development of other skeletal structures, and then disappear because they are no longer needed, for natural selection would remove them because they’re cumbersome, a waste of resources, and unnecessary in a marine mammal.

Horses are another example. We know from the fossil record that modern horses evolved from five-toed ancestors, but what we have left, the lower leg and hoof, are the remnants of only the middle toe. The other four toes disappeared over time (we can see this in the fossil record), though the two toes flanking the middle one remain as vestigial “splint bones” on the horse’s leg. The outer two toes are gone completely. Here’s a drawing of one of the vestigial toes in a modern horse: a splint bone (lateral view; there’s one on the other side, too):

Source: Atlanta Equine Clinic

But you can see all five toes if you look closely during development, as reported in this new paper. If you get horse embryos at the right early stage of development, you can see the primordia for all five toes forming, with the outer ones fusing and shrinking to leave only the middle toe, which becomes the lower leg and hoof. Previously, nobody had been able to see this evidence of ancestry in embryos, but Kavanagh et al. managed to get the right material.

The authors procured (don’t ask me how) horse embryos from artificially inseminated mares, and analyzed four of them by making tissue sections of embryos between days 29 and 35 after copulation. (There is a very narrow window of time to see the primordia for all five digits, as shrinkage and disappearance of the four superfluous ones is fast.)

First, here’s a diagram of what the primordia look like. On the right side you see the evolutionary progression of horse ancestors (also shown below), starting with five, then four, and then three, with the two side toes gradually being reduced to the splint bones. We’re not sure why this happened, but a likely explanation is that at the time these horses were evolving—and they evolved in what is now North America—the climate was drying up and the forests of the West were giving way to grasslands. While toes are good for running fast around trees and vegetation, if you’re escaping predators in a featureless grassland, you want a hoof to run fast and straight.

On the left side, in blue, are the toe primordia in four embryos; those blue bits are regions where cartilage would normally condense and then bone would form. (“FL is “foreleg” and “HL” is “hindleg”). You can see that two of the embryos have five primordia early on, with the central one, which becomes the lower leg, being the largest, as it’s the toe that will become the hoof. The other two embryos have three primordia, with the central one the largest.

Figure 1. (a) Illustration of arrangement and relative sizes of pre-cartilaginous condensations in developing Equus FL and HL digits based on reconstructions of histological sections of 30–35 dpc embryos from this study. (b) Fossil transition series of adult horse FL digits (isometrically scaled) showing the sequence of reduction of anterior and posterior digits and increasing dominance of central digit III. (i) Phenacodus (AMNH 4369), (ii) Hyracotherium (AMNH 4832), (iii) Mesohippus (AMNH 39480 and AMNH 1477), (iv) Hypohippus (AMNH 9407), (v) Hipparion (AMNH 109625), (vi) Dinohippus (AMNH 17224). Illustration from Solounias et al. [6]. (Online version in colour.)
Here’s a photo of the two embryos showing five toe primordia at different points along the legs. You can see five shadowy primordia, with the largest in the middle in (b) of embryo 1 and (g) of embryo 2. (“Proximal” is toward the body and “distal” is toward the future hoof.) In later embryos studied by other people, the big primordium in the middle goes on to develop into the lower leg and hoof, and the others disappear.

 

Now of course we don’t need this kind of evidence to show evolution, or even to show evolution of toe loss in horses, as we have an excellent fossil record of horses and a good idea of their “family tree”. Here’s a figure from the Encyclopedia Brittanica:

But the developmental evidence is a nice confirmation of what the fossils tell us, and add the information that the evolutionary sequence of toe loss is mirrored in the developmental sequence of modern one-toed horses. This is a version of the “biogenetic law” stating that “ontogeny recapitulates phylogeny” (i.e., development mimics evolutionary history). That law has many exceptions, for sometimes the ancestral stages are competely lost in embryos, but it does hold for horse toes. First five, then three, then one—in both development and in the fossil record.

By the way, sometimes the side toes don’t disappear, but, probably through a screwup in development, form rudimentary adult toes, producing polydactylous horses like this one:

Similarly, sometimes the dolphin’s hind limbs don’t disappear and we get dolphins with little legs sticking out of its rear, like this one that I show in my “evidence for evolution” talk:

UPDATE: I forgot to include the authors’ point that many vertebrates have lost toes from the ancestral five, and that these species are ripe for embryological investigations of the type shown here. They give a table of some of these animals. Would embryos of the camel or the three-toed jerboa, for instances, show five toe primordia that are then lost? We don’t know if this is the case, and the absence of five primordia wouldn’t disprove evolution, for the retention of toe primordia is a lucky (for us) feature of development, but isn’t expected in every case.

____________

Kavanagh, K. D., C. S. Bailey, and K. E. Sears. 2020. Evidence of five digits in embryonic horses and developmental stabilization of tetrapod digit number. Proceedings of the Royal Society B: Biological Sciences 287:20192756.

 

44 thoughts on “More evidence for evolution: Horse embryos start forming five toes, and four primordia disappear

    1. It wasn’t always this way. In the Devonian tetrapods had as many as 13. Why pentadactyly became part of the bauplan for terrestrial vertebrates is unknown.

  1. Love this and other similar examples! At my daughter’s recent baby shower, for example, I was discussing the recurrent laryngeal nerve with a high school biology teacher… on Michael Behe’s birthday, no less!

    Larry Smith

  2. Delightful info. Not really surprising though. Teeth in chicken embryos (sonic hedgehog), comes to mind. There are many examples. Creation Wiki is a kind of Answers in Genesis page that I think is connected to the odious Ken Ham outfit. I’m sure they are now frantically working on a section trying to show how this paper actually supports Genesis. Fortunately, it will only fool a few low information fools.

    1. I’ve had a student very matter-of-factly tell me that such things were of course because embryos still show their state from ‘before the Fall’. This sort of thing poses no difficulty for Creationists.

      1. What’s “before the Fall”? Before Eve ate the knowledge pomegranate? I thought everything was perfect before the Fall so why the imperfection of vestigial bits? I guess the real question is why am I trying to make sense of or debunk a Creationist’s delusion?
        Moving along…

  3. The National Museum of Natural History in DC used to have (may still have) a nice display of the fossils of this transition along with a discussion of the concurrent environmental changes, linking a progression from a four toed animal in a forest environment to a one toed plains/grassland animal.

    The fossil collection has been reorganized and I couldn’t find the display last time I was there.

    1. Last time I was at AMNH in New York, they had arranged their horses in a nice cladistic sequence. No longer a simple linear array.

      Some of the horses likely came from Oregon’s Painted Hills. Although Oregon has just about zilch exposures for pre-Cenozoic fossils, there is a wonderful record since mid-Eocene. Fifty mission years of regular volcanic ash-falls.

  4. Love this, love your blog, love you! This blog is my favorite daily read. Keep bringing pieces like this, and on free will and determinism, and calling out the woke and apologists. We need it!

    1. Only if we swim too close to a boat’s propeller…

      Apparently you have to learn how to walk again if you lose a big toe (not sure about other toes).

      1. Lol. Teacher was referring to the smaller toes on humans feet; apparently we don’t really need them. There was some theorizing about appendix and gall bladder also.

        I find it interesting that dolphins ancestors walked on land, indicated by the tiny feet in embryos, whereas human embryos similarly had “gills” indicating we once were aquatic creatures, yet we are both air breathing mammals. Wonder where upon the evolutionary path our species were more alike.

      1. I heard about Alexander the Great’s horse, Bucephalus (Cow’s head??), having additional toes. Was unaware of Caesar’s.

  5. This is cool; another facet to the classic O. C. Marsh horse series. A great teaching tool, to be able to bring back the same story several times during a course to add new levels of understanding. (Sickle-cell anemia is another good story in that way.)

    Eastern box turtles (Terrapene carolina) would be another good place to look for digit-reduction patterns. They have 4 toes on the hind legs, except for the three-toed subspecies (T. c. triunguis), and in contrast with the 5 of closely related emydid turtles.

  6. The biogenic law, as originally stated by Haeckel, is a bit problematical because it asserted that embryos recapitulate adult forms of ancestors. So a mammal embryo has the anatomy of a fish. But of course one soon runs into problems with that. We mammals never develop scales or a lateral line system; that sort of thing.
    But one can somewhat salvage the law if we say that embryos pass thru stages that resemble the embryos of ancestors. So “ontogeny recapitulates… ontogeny“.

  7. “The transitory formation of that coat of hair in our species, which is of no use to the embryo, can be explained only by our descent from a primate with hair.”

    For several months now I’ve been doing my annual grumbling about that loss of hair. If only evolution could anticipate that some dumb ancestor of mine would move to this frozen wasteland, and some dumb me would stay there! I feel like a subprime-mate.

  8. Science has so many ‘decent explanations’ constantly being discovered … that it boggles the mind why so many humans still believe silliness. Yet there are also ‘decent explanations’ as to why so many believers believe their silliness.

  9. Thanks for more evidence for evolution. I enjoy a world where evidence matters. Another reason I find refuge here on WEIT.

  10. Natural selection was just making life easier for farriers and horseshoe throwers, who said it does not have a goal. NS seeks advantage, employing people and bringing them together, marvelous stuff.

  11. “Toes will always be at the end of your feet, but at the start of your journey.”

    ― Anthony T. Hincks

    [ https://www.goodreads.com/quotes/9136531-toes-will-always-be-at-the-end-of-your-feet ]

    In my evolution class I talk about the lanugo: the thick coat of hair that human fetuses develop at about six months after conception. It’s shed before birth—but not in chimps, our closest relatives, who are born hairy (remember: we are the “naked ape”). The transitory formation of that coat of hair in our species, which is of no use to the embryo, can be explained only by our descent from a primate with hair.
    So if these transitory features disappear, why do we see them at all? We’re not sure, but their appearance may be necessary to provide developmental “cues” for the appearance of features that do remain. Remember, development is a very complex process which requires a nexus of coordinated features appearing at the right times.

    While we do have little evidence of timing of hair loss, I think it has been correlated with the timing of clothes (hide implements). Presumably the null hypothesis works – it is near neutral drift of an unnecessary character – and the developmental timing of loss isn’t very telling.

    Though if one is fishing for an adaptive theory to test, it could possibly be that generic absence of hair at birth would be adaptive. Children would remind more of adults and presumable be more acceptable. Else long hair or plumage in young animals is often considered “cute” by humans (and likely the parents as well, in cases of parenting).

  12. Theories propounded to support the concept of evolution is subjective.I believe for a theory to be objective,it really must exhibit consistency in all ramifications. If all organisms came into existence through evolution, we should have them still following such Patten in our contemporary era,if humans being the closest relatives to chimps evolve from chimps,we should still have such scenery play aswell.the concept of evolution will always remain hypothetical.

    1. And here we have it, ladies and gentleman: someone weighing in for the first time (and the last) showing complete ignorance about evolution coupled with an inability to express any clear thoughts. Not a single sentence in this post makes any sense.

      1. Ken Ham would be proud. He’s got at least one follower! So is the new offense saying evolution is a “concept” not even a theory…damn we’re hitting the mantle here, let alone rock bottom.

      2. Bottom dwellers comes to mind lol. Best leave them to their closed minds. They could have at least read some of Darwin’s “Evolution of the Species” before they commented. Evolution is all about species adapting to their environments; natural selection. Some species are evolving due to climate change; corals for example.

        1. ‘They could have at least read some of Darwin’s “Evolution of the Species” . . . .’

          And perhaps also Strunk, Pinker and Harbrace College Handbook.

  13. Note that even in highly derived tridactyl equids (e.g., Pliohippus, that’s reducing its side toes on the way to becoming monodactyl) small nubbins of digits 1 and 5 can be seen at the top of the metapodial, flanking the splint bones. I’m not sure exactly when those disappear, they may even be retained in early species of Equus. (I’ll check when I’m back looking at horse fossils in NYC in April.)

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