A vestigial trait of birds that may have been functional in ancestors: remote-sensing of vibrations in the bill (still active in the kiwi)

December 11, 2020 • 10:00 am

A new scientific paper from the Proceedings of the Royal Society Series B (first screenshot below) tells a rather complex story that I’ll deliberately simplify to save space. The paper is behind a paywall, but a pdf may be found via judicious inquiry, and the reference is at the bottom.

The article above is aptly summarized by Veronique Greenwood in the New York Times‘s “Trilobite” column

Three groups of birds have evolved a remarkable feature: the ability to remotely sense prey (i.e., detecting prey without touching them) by sticking their bills in the ground and sensing vibrations. These groups are the kiwis, the ibises, and some shorebirds. The detection can occur either through the direct sensing of vibrations of prey movement, or the reflection of sound waves off hard-shelled prey as the bird sticks its beak into the ground. This feature is called “remote touch.”

It turns out that the bill tips of “remote touch” birds are pitted with small depressions that contain cells called “Herbst corpuscles”, which are the motion-detecting organs. aThese birds also have an expanded area of the brain that is used to process the extremely important touch signals.

Other species have different ways of using their bills to detect prey by touch. Ducks and geese have a bony organ at the tip of their bills that also have pits with Herbst corpuscles, but they are organized differently, with mechanoreceptors beside them. These are what ducks use in “dabbling”—turning their butts up and sticking their beak into the dirt or sediments to forage. Finally, parrots have a different kind of bill-tip organ with receptors not located in the bone.

Below are photos from the paper showing the different types of bill tips. The authors also examined skulls of hundreds of living species and dissected beak tissue from many to see if there were Herbst cells associated with the bill pits.

First, a bird without remote sensing, as with most birds. It’s a kelp gull (Larus dominicus). There are a few pits at the tip of the bill, but soft tissue analysis showed no receptor cells. It does not forage by touch.

Here are two birds with remote sensing. First, the hadeda ibis (Bostrychia hagedash). Note the highly pitted bill tip organ (enlarged). It also has the Herbst cells as well as an enlarged bit of the brain for detecting touch. (This bird, like all other birds save the ratites and tinamous, falls into the large group of species called neognaths.)

Ditto for the kiwi, which falls into the other group of birds, the paleognaths, a small group that contains only the large flightless birds or ratites (emus, ostriches, etc.) plus the tinamous, which can fly, but not well. Its remote sensing organ with Herbst cells is located at the very tip of its long bill. Indeed, the ratio of bill length to skull size is one of several keys to diagnosing whether these birds have remote sensing.

Finally, the tinamou, a paleognath that= has a remote-sensing organ containing the pits but no Herbst corpuscles in them. But this species doesn’t feed by probing the ground. Other ratites, like the ostrich and emu, also have a pit-studded bill, but no vibration-detecting cells. The pits seem to be a leftover from an ancestor which had pits that were useful because they contained vibration-detecting cells. In other words, they’re a vestigial trait.

The other ratites also lack the expanded brain regions for processing information from the touch receptors. This makes sense, for while it may not cost much to retain some pits in the bill when you don’t need them, brain tissue is metabolically expensive, and if you’re not using it it would pay to divert those resources to other functions that would help you reproduce.

As I said, the presence of the pits in birds that don’t use them suggest that this trait is a vestigial trait carried over from an ancestor.  One can distinguish the remote-touch birds from other species by a combination of bill length/skull size ratio, number of pits, and spacing between the pits.

But which ancestor? It turns out that we have fossil skulls of ancient extinct birds, the lithornithids, which are very early paleognaths. Although soft tissue wasn’t available for these birds, some of the species show the mechano-sensing organ—as evidenced from the number and spacing of the pits, as well as the bill/skull ratios characterized by remote foragers. Here’s a photo of the two lithornithid skulls; captions under the photos are from the paper (click to enlarge photo).

Cranial fossils of two species of lithornithids, showing high degree of pitting on the surfaces of their beaks, similar to all extant palaeognathous birds, potentially indicative of a bony bill-tip organ. (e) Lithornis promiscuus: (i) skull and attached maxilla (USNM 391983) showing the shape of the beak relative to the skull; (ii) distal portions of maxilla and mandible (USNM 336535). ( f ) Paracathartes howardae: maxilla (USNM 404758) and distal portion of mandible (USNM 361437).

The conclusion is that putative ancestors of the paleognaths were remote-touch-sensing species. The fact that living paleognaths like emus and tinamous still retain the pits suggests that this nonfunctional “organ” is a useless remnant of a trait inherited by all paleognaths from a lithornithid ancestor.  Indeed, the authors think that the ancestor of all birds might have been a remote-sensing prober (my emphasis):

Our analyses corroborate that the basal palaeognaths, the small, volant lithornithids, had a tactile bony bill-tip organ enabling them to use remote touch to locate buried invertebrate prey items. This finding, combined with our understanding of the evolution of the lithornithids, suggests a Cretaceous origin of the remote-touch sensory system in modern birds before the palaeognathneognath split.

As for why among living paleognaths only the kiwi has a functional touch organ when it was present in an extinct ancestor, that could be explained by either of two scenarios. The first involves, the organ’s loss in a more recent ancestral species and then the re-acquisition of the organ in just the kiwi lineage. The second possibility is that the kiwi kept an ancestral remote-probing organ while all the other paleognaths lost it. The authors are unable to distinguish between these two scenarios.

What about the neognaths that have remote-sensing organs, like the ibis or shorebirds? Did they retain the ancestral touch organ while all other neognaths—the vast majority of living birds—lost it? Probably not; as the authors say, this is an independent case of evolution.

What is even more fascinating is the possibility that this ability to detect prey remotely may have been present in the reptilian ancestors of birds, which may scientists think are the theropod dinosaurs:

Interestingly, there is increasing evidence that some non-avian theropods had specialized sensory structures located on the distal portion of their rostra, based on a high degree of external foramina/pitting preserved on their mandibles. We speculate that perhaps such sensitive snouts in non-avian theropods may have been precursors to the evolution of remote touch in their avian relatives.

It’s interesting to note that alligators and crocodiles also have touch-sensitive “dome receptors” in their upper jaws, also associated with pitting in the bones.  The archosaurs are a group of early reptiles ancestral to both birds and crocodilians, and maybe the receptors we see in crocs and gators are related, in some way, to the pits in the beak of the kiwi.

This is all speculative, but what seems pretty solid is that the bill pitting and useless “touch organs” in non-kiwi ratites and tinamous are vestigial remnants of a functional organ in an ancient ancestor. And that’s evidence for evolution.

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Toit, C. J. d., A. Chinsamy, and S. J. Cunningham. 2020. Cretaceous origins of the vibrotactile bill-tip organ in birds. Proceedings of the Royal Society B: Biological Sciences 287:20202322.

An atavistic claw in a duckling?

June 2, 2020 • 12:30 pm

The other day I took a picture of this juvenile mallard—one of Honey’s babies—and a friend noticed it had what appeared to be an atavistic claw on its wing. At least I think it’s on its wing; it could be on a  foot tucked behind the bird. But I doubt it.

Here I’ve circled it:

And enlarged it:

The question is whether this is an atavistic claw: the remnant of the claw that was on the reptilian forelimb, and was also prominent in early birds (ignore the labeling of Archaeopteryx as “the earliest known bird”.

Birds also have “spurs“, which are outgrowths of bone that aren’t developmentally homologous to a true claw. But the duckling above seems to have a true claw; it doesn’t look like a bone spur, but is recurved and apparently made of keratin.

Real bird claws, as in the hoatzin,  grow from the digit that’s in the bird wing; in this case it would be the “thumb”. Here’s a “normal” bird.

 

But birds like hoatzins have true claws, especially in the chicks, which use them to climb back into trees when they fall in the water. I’ve put an Attenborough video of this behavior below the picture, and what it shows is that the claim that a “vestigial” character has to be nonfunctional to be considered vestigial is incorrect. Vestigial traits are simply remnants of traits that evolved earlier but have been coopted for a different function (“exaptations”, Steve Gould might call them). The hoatzin’s claw, very useful for the bird, is certainly a vestigial trait, and is just as much evidence for evolution (of birds from reptiles) as if it were completely nonfunctional.

Some species of waterfowl are known to have these claws (see here and here), but I can’t find something explicitly on mallards.

So we have a mystery here, and I’ve asked a few experts to weigh in. This is either a true atavistic claw in the wing, or one of the claws (nails) in the duck’s rear foot, which could be tucked behind it. You can weigh in, or wait for an answer. Stay tuned.

h/t: Nicole, Greg

 

My talk in Tallahassee in late March

February 25, 2020 • 12:00 pm

In almost exactly one month, I’m speaking to the Tallahassee Scientific Society in Tallahassee, Florida. My talk is on Thursday, March 26, and I think the time and venue are the same as those for the previous speaker: 7 p.m. at Tallahassee Community College’s Center for Innovation on Kleman Plaza. The topic is “Why Evolution is Still True”, and I’ll give a brief rundown of the evidence for evolution (updated in light of new discoveries), followed by discussion of why Americans remain so resistant to this scientific truth.

I’ll give one more announcement in mid-March or so, and all are welcome to come. I believe they’ll also have my two trade books on sale, which I’ll be glad to autograph. And, if you tell me the genus and species of any felid besides the house cat, I’ll draw a cat in it.

Here’s a photo I sent them to use for advertising the talk; the picture is from Wikipedia so it’s in the public domain. Toes, teeth, and size!

40bd9fb2-c3b2-4206-a1fa-4d888b06006f-Coyne_Evolution
Photo credit: H. Zell (from Wikimedia Commons; CC license CC BY_SA 3.0).

 

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.

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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.

 

Nathan Lents on the imperfection of the human body (it’s evolution, of course)

January 10, 2020 • 12:45 pm

UPDATE:  I found out that the well-known evolutionary geneticist John C. Avise published a related book in 2010, but one that concentrates on a different line of evidence for evolution. John’s book (screenshot of cover below with link to Amazon) lays out the many suboptimal features of the human genome. He thus concentrates on molecular evidence, noting the many features in that bailiwick whose imperfection gives evidence for evolution and against intelligent design.  Lents’s and Avise’s books thus make a good pair, since the former seems to deal mostly with anatomy and physiology and the latter with molecular data. I’ll be reading both of them.

***************

Biologist Nathan Lents, whose abbreviated c.v. is given below, has been featured on this site before, both as a critic of creationism (good), but also as a defender of the Adam-and-Eve apologetics pushed by his religious friend Josh Swamidass (bad). But chalk up another two marks on Lents’s “good” side.  First, he’s written a book (click on screenshot below) that lays out all the suboptimal features of the human body—features whose imperfection gives evidence for evolution. I’m getting the book for teaching purposes, and here’s the Amazon summary:

Dating back to Darwin himself, the “argument from poor design” holds that examples of suboptimal structure/function demonstrate that nature does not have a designer. Perhaps surprisingly, human beings have more than our share of quirks and glitches. Besides speaking to our shared ancestry, these evolutionary “seams” reveal interesting things about our past. This offers a unique accounting of our evolutionary legacy and sheds new light on how to live in better harmony with our bodies, in all their flawed glory.

Nathan Lents is Professor of Biology at John Jay College and author of two recent books: Not So Different and Human Errors. With degrees in molecular biology and human physiology, and a postdoctoral fellowship in computational genomics, Lents tackles the evolution of human biology from a broad and interdisciplinary perspective. In addition to his research and teaching, he can be found defending sound evolutionary science in the pages of Science, Skeptic Magazine, the Wall Street Journal, The Guardian, and others.

And here’s a half-hour Center for Inquiry talk, clearly based on his book, in which Lents discusses how the flaws in the human body instantiate evolution. It’s not just that there are flaws—which support the notion that natural selection doesn’t produce absolute perfection, but simply the best result available given the existing genetic variation—but, more important: those flaws are understandable as the result of our evolution from ancestors who were different from us.

Some of Lents’s examples (like our broken gene in the Vitamin C synthesis pathway), are discussed in WEIT, but others, like the bizarre configuration of our nasal sinuses, aren’t. I haven’t seen the book, but it looks like a good compendium of evidence for evolution using something that everyone’s familiar with: the glitches and bugs in the human body.

It’s a good talk, and Lents is an energetic and lucid lecturer. I recommend that you listen to this, for you’ll learn stuff that will stay with you, and also serve to help you argue with creationists.

h/t: Michael

Vestigial limb muscles in human embryos show common ancestry—for the gazillionth time

October 6, 2019 • 9:00 am

There are three kinds of vestiges that constitute evidence for evolution, or rather its sub-claim that modern species share common ancestors. I discuss all three in Why Evolution is True:

1.) Vestigial traits that persist in modern species but either have no adaptive function in a species or a function different from the one served in that species’ ancestors. The vestigial ear muscles of humans are one, the flippers of penguins (functional, but not for flying in the air) is another, and the coccyx in humans (sometimes with attached “tail muscles” that can’t move it) is a third.

2.) Vestigial genes that are functional in our relatives (and presumably in our ancestors) that have been inactivated in some modern species. There is no explanation for these “dead genes” save that they were useful in ancestors but aren’t useful any longer. Examples are “dead” genes that code for egg yolk proteins in humans (but don’t produce them); a dead gene for vitamin C synthesis in humans (we don’t make the vitamin because that gene is inactivated, but rather get it from our diet; and the many dead “olfactory receptor” genes in cetaceans (whales, dolphins, etc.)—genes that were active in their terrestrial ancestors but became inactivated because “smelling” underwater uses different genes and traits.

3.) Features in development that are transitory, and whose appearance makes sense only under the supposition that those features were present in common ancestors and persist in some descendants but not others. The lanugo (a transitory coat of hair in human embryos) is one.

Today’s paper, which just appeared in the journal Development, shows several other “transitory” evolution-attesting features. Diogo et al. show that human embryos develop muscles that disappear as development proceeds, but those muscles don’t disappear in some of our relatives, including closely related ones like other primates as well as distant relatives like reptiles.

Moreover, these muscles, which disappear in most human embryos, sometimes don’t disappear, persisting in adults as rare and nonfunctional variants. Or they appear in malformed individuals, with both phenomena often seen in “vestigial traits”. For example, some people are born without wisdom teeth, considered a vestigial holdover from our ancestors; and the functionality of human vestigial ear muscles that move the ears in our relatives, like cats and dogs, is variable: some people like me are able to move those muscles and wiggle their ears, while others can’t.

Click on the screenshot below to access the paper, and the pdf is here (reference at the bottom of this post).

The authors visualized the muscles in the embryonic arm and leg by doing immunostaining—using antibodies that would affix to proteins in the muscles and also carried ancillary molecules that would make those muscles more easily visualized under the microscope in a three-dimensional way. The authors used 70 antibodies, but the main ones bound to muscle-specific proteins like myosin and myogenin.

They stained the mounted limb sections of 13 embryos (presumably from abortions) ranging from nine to thirteen weeks after gestation (quantified as “gestational weeks”, or GWs), and with the standard measurement “crown-rump length” (CR) ranging from 2.5 cm to 8.0 cm (about 1 to 3 inches). These were thus very small embryos, but the sophistication of the technique, and the efficacy of the stain, combined with our knowledge of embryonic development and tetrapod muscle anatomy, enabled the authors to produce pictures like these: the muscles in the hands of a 10 and 11-GW fetus:

 

What they found is that human embryos show a number of muscles present in the adults of some other tetrapods (including our closest relatives, the chimps), but that disappear during human development, with a few of these “atavistic muscles” fusing with other muscles in human fetuses although remaining distinct in our tetrapod relatives.

Here’s how the authors describe the main results, listing some of the atavistic muscles in the embryos (I’ve put them in bold):

As summarized in Tables 2-5 and also noted above, various atavistic muscles that were present in the normal phenotype of our ancestors are present as the normal phenotype during early human ontogenetic stages and then disappear or become reduced and completely fused with other muscles, thus not being present/distinguishable in human adults. These include the upper limb muscles epitrochleoanconeus (Fig. 3), dorsoepitrochlearis, contrahentes 3-5 (Fig. 4) and dorsometacarpales 1-4 (Figs 3-5), and the lower limb muscles contrahentes 3-5, dorsometatarsales 1-4 (Fig. 6) and opponens digiti minimi (Fig. 6). These muscles are present in some other tetrapods, as shown in Tables 6 and 7, which summarize the comparisons with other limbed vertebrates. Of all these muscles, only the dorsometacarpales often remain in adults, fused with other muscles: all the others are normally completely absent in human adults. Fascinatingly, all these atavistic muscles are found both as rare variations of the normal adult population and as anomalies in individuals with congenital malformations such as those associated with trisomies 13, 18 and 21, reinforcing the idea that such variations and anomalies can be related to delayed or arrested development.

Here are two of the fetal atavistic muscles. First, the dorsometacarpales in the hand, which are present in modern adult amphibians and reptiles but absent in adult mammals. The transitory presence of these muscles in human embryos is an evolutionary remnant of the time we diverged from our common ancestor with the reptiles: about 300 million years ago. Clearly, the genetic information for making this muscle is still in the human genome, but since the muscle is not needed in adult humans (when it appears, as I note below, it seems to have no function), its development was suppressed:

 

Here’s a cool one, the jawbreaking “epitrochleoanconeus” muscle, which is present in chimpanzees but not in adult humans. It appears transitorily in our fetuses. Here’s a 2.5 cm (9 GW) embryo’s hand and forearm; the muscle is labeled “epi” in the diagram and I’ve circled it:

This muscle must have become nonfunctional, and reduced in development, over the last six million years or so, when the common ancestor of humans and chimps gave rise to our separate lineages.

An interesting sidelight of this study is that some of these vestigial muscles occur as rare variants in adult humans, either via developmental “accidents” or as part of congenital malformations. Presumably these screwups in development block the genetic changes that normally lead to the suppression and disappearance of muscles in embryos. Variable expression of vestigial traits is common in organisms where the traits haven’t evolved into something else that’s useful. (For more on human vestigial traits, see the Wikipedia article on “human vestigiality”). The authors note that when the muscles do appear in adults, they are “functionally neutral, not providing any type of major functional advantage or disadvantage.”

The presence of these vestigial muscles is pretty irrefutable evidence of evolution and common ancestry, for there’s no reason why either God or an Intelligent Designer (a pseudonym for “God” to ID advocates) would put a transitory muscle in a human fetus that’s of no use whatsoever, but just happens to resemble the fetal muscles that goes on to develop into adult muscles in our relatives.  I wonder how creationists, including IDers, will explain this as the work of a designer. Will they say the muscles are really functional in a fetus? If so, why do they disappear? And doesn’t the fact that they go on to develop into functional muscles in our relatives like chimps and reptiles say something about common ancestry?

Two more points:

1.) The order of appearance of these muscles in development doesn’t completely comport with their order of evolution. This shows that the “recapitulation theory”—that the order of development mimics the order of evolution—isn’t completely obeyed. But we’ve known that for a long time. The time of appearance of a trait in development can be changed by other factors, like its usefulness in “priming” the development of other features. But this doesn’t overturn the very strong conclusion that the presence of transitory muscles in the human fetus that remain in adults of our relatives is evidence for evolution.

2.) Finally, muscles in the arms and legs that appear “homologous” (i.e., have the same evolutionary origin) may have had independent evolutionary origins, and may involve different genes, so they’re not really “homologous” in the way evolutionists use that term. As the authors note,

These differences support the emerging idea that the topological similarities between the hand and foot of tetrapods, such as humans, are mainly secondary (see recent reviews by Diogo et al., 2013, 2018; Diogo and Molnar, 2014; Sears et al., 2015; Miyashita and Diogo, 2016). This idea is further supported by the fact that the order of developmental appearance of the hand muscles is markedly different from that of the corresponding foot muscles (Tables 6, 7). As an illustrative example, whereas the lumbricales are the first muscles to differentiate in the hand, together with the contrahentes (Table 6), in the foot the lumbricales differentiate only after most other foot muscles are already differentiated (Table 7). Thus, these developmental data and evidence from comparative anatomy and from the evolutionary history of human limb muscles (see Tables 6, 7) indicate that several of the muscles that seem to be topologically similar in the human upper and lower limbs actually appeared at different evolutionary times; appear in a markedly different ontogenetic order; derive from different primordia; and/or are formed by the fusion of different developmental units in each limb.

Now the authors didn’t do this study to demonstrate evolution; like most rational people, they accepted it long ago. Rather, their stated aim was to “build an atlas of human development comprising 3D images. . . that can be used by developmental biologists and comparative anatomists, as well as by professors, students, physicians/pathologists and the broader public.” But one of the bonuses, especially for the broader public, is the very clear demonstration of the common-ancestry tenet of modern evolutionary theory.

h/t: Liz

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Diogo, R., N. Siomava, and Y. Gitton. 2019. Development of human limb muscles based on whole-mount immunostaining and the links between ontogeny and evolution. Development 146:

How the whale lost its genes

September 27, 2019 • 8:30 am

The evolution of whales, porpoises, and dolphins—the “cetaceans”—is well understood thanks to a plethora of fossils, mostly found in recent years (for a good general summary of the data, go here). Starting from a small, deerlike artiodactyl living around 48 million years ago (Indohyus may be related to the common ancestor of whales), this evolution proceeded rapidly, with the two groups of modern whales—Odontocetes (toothed whales, including dolphins) and Mysticetes (baleen whales), diverging only about 12 million years later. In other words, in a mere 12 million years—only about twice the time since we diverged from the lineage that led to modern chimps—evolution went from a terrestrial artiodactyl to a fully marine whale. That’s surely macroevolution, however you define it, and gives the lie to the creationist claim that major transitions aren’t seen to have occurred over time. (I discuss much of the fossil evidence in Why Evolution Is True).

Here’s a diagram of the evolutionary sequence of some of the forms, and the times they appear in the fossil record, taken from the UC Berkeley site Understanding Evolution. 

During this brief period, cetacean ancestors lost their hind legs and developed a fluke, the body became streamlined for swimming, body hair was lost (not needed in a fully marine whale), the nostrils evolved backward into a single blowhole, a lot of adaptations for diving evolved, as well as the ability of the species to collapse their lungs when diving, a layer of blubber evolved, and there were many other physiological and anatomical changes. These are described in a new paper in Science Advances (click on screenshot below, pdf here, reference at bottom of the post).

But the authors are not so concerned with the well-documented morphological changes, for they wanted to see what genes had changed, in particular, which genes in the ancestors of whales had been inactivated during whale evolution—inactivated because they were of no use to fully marine mammals.

To find these genes, the authors looked at whole genomes of cetaceans (bottlenose dolphins, killer whales, minke whales, and sperm whales) and compared them to the genomes of 62 placental mammals. They found 2472 genes in cetaceans that were broken: genes having deletions, stop codons, splice-site mutations, “frame-shift” mutations, and other changes that kept these genes from being expressed. They then carved out a subset of these genes that were not inactivated in 95% of the terrestrial mammals they used for comparison, so the broken genes were largely unique to all cetaceans. That took the sample of broken cetacean genes down to 350.

They then excluded genes already known to be broken in the cetacean lineage, including olfactory receptor genes (I discuss the loss of “smelling genes” in whales in WEIT) and keratin-associated genes, most involved in hair formation.

Finally, they excluded genes known to be broken in the closest living relative of whales—the hippopotamus, whose “broken genes” are not associated with a marine way of life. This left the authors with a final sample of 85 genes that were inactivated in all sampled whales (mysticetes and odontocetes) but not in their living relatives; these were presumably genes that got broken in the common ancestor of the two groups of whales, and whose broken state was passed on to all living cetaceans.

Why would a gene become inactivated in a group? Well, presumably because it’s not needed. But there are then two ways that a non-useful gene could become broken via the accumulation of inactivating mutations. First, an inactivation could be “neutral”: a gene that’s not needed and becomes nonfunctional may not have a selective advantage or disadvantage over the active form, and could eventually become “fixed” (present in all individuals in a population) via random genetic drift.

Alternatively, a broken gene could increase in frequency because it has a selective advantage over its functional competitors. That is, the non-production of a gene product could save energy that could be diverted to other functions, or it could reduce an unneeded organ or feature that could be damaged (both of these arguments have been used to explain why eyes largely disappear in cave animals who don’t “need” them). The authors posit that most of the broken genes in cetaceans accumulated by neutral processes, but it’s very hard to distinguish that scenario from an increase-by-selection argument, as this involves comparing DNA sequences and looking for a “signature of selection”: nearly impossible in such data.

But this is a side question. What’s important are two things. The first one I emphasized in WEIT:

1). The presence of nonfunctional genes in whale genomes—genes that are functional in their living relatives—is strong evidence for common ancestry of whales from terrestrial organisms and against any creationist scenario. It’s also evidence for macroevolution. You’d be hard pressed indeed to give reasons why an intelligent designer or a god would install useless genes in a genome that remain useful in the “outgroup” relatives. But this is exactly what is expected if whales evolved from terrestrial species in whose descendants those genes remain useful. But we’ve known about broken genes for a long time (e.g., olfactory genes), and IDers and creationists still can’t explain them.

2.) The broken genes give evidence about what the genes were used for in the ancestors, and why they weren’t needed in cetaceans. Thus, the authors looked at what the genes do when they’re functional, which helps tell us why they might not be needed in cetaceans. The broken genes fall into several classes; I’ll highlight just three:

a. Genes involved in blood coagulation.  When cetaceans dive, peripheral blood flow is reduced, making it more likely that damaging blood clots could form, especially when nitrogen microbubbles form in the blood (this is what causes “the bends” in divers). The authors found two genes involved in blood coagulation that were broken in whales. Like all of the broken genes, this scenario for why genes are inactivated is speculative, but can still prompt further research.

b. Genes involved in DNA repair. When tissues become short of oxygen, as when cetaceans are diving, but then get a surge of oxygen later, forms of “reactive” oxygen accumulate that can damage DNA. Cetaceans have lost an enzyme, POLM, that repairs DNA, but does so by inducing many errors in the repaired DNA. Since there are other less error-prone ways of repairing DNA, the authors speculate that the loss of POLM is a way to avoid a “mutagenic risk factor” in cetaceans, which are especially prone to damaged DNA.  The idea is that it’s better, if you’re susceptible to damaged DNA from diving, to get rid of a system that repairs quickly but makes errors, and rely instead on another system that repairs more slowly but with fewer errors.

c. Genes involved in melatonin biosynthesis. Whales, like ducks, sleep with only half of their brain at a time, with the other half active and awake to watch for danger and, in whales, to keep the animal swimming, surfacing, and breathing, maintaining body heat. The sides alternate over time so that the entire brain eventually gets a rest. (This shows how important sleep is, though we don’t yet know why.) Melatonin is a hormone synthesized by the pineal gland that helps keep animals entrained to daily (circadian) rhythms. The authors found that four genes involved in melatonin synthesis (AANAT, MTNR1A, MTNR1B, and ASMT) were inactive in cetaceans but not in their relatives.

The authors speculate that the loss of melatonin synthesis “helps decouple sleep-wake patterns from daytime,” as whales sleep with half their brains during both day and night. Further, since melatonin synthesis inhibits body-temperature regulation, its absence may help whales maintain their high body temperature in a chilly environment.

There were other pathways in which cetaceans showed broken genes, including those involved in transporting amino acids to the kidneys and genes expressed in the lungs, which may facilitate non-damaging lung collapse  that occurs in diving cetaceans. You can read about these in the paper; again, the reasons for their loss are plausible but speculative.

Finally, the authors looked at two other groups of aquatic mammals whose ancestors independently invaded the sea: manatees (sirenians, related to elephants and hyraxes) and pinnipeds like seals and sea lions (descended from terrestrial carnivores). Their goal was to see if there was independent “convergent” loss of similar genes between these groups and cetaceans. They found two genes, including AANAT, that were inactivated in manatees or pinnipeds, but not in their terrestrial relatives.

What does it all mean? As I said above, this paper gives further evidence for evolution in the form of dead genes, genes not needed in some groups of animals but needed (and “alive”) in their terrestrial relatives and presumably ancestors. This gives further evidence for evolution and especially common ancestry, though the evidence (even in the form of dead genes) is at this point somewhat superfluous.

More important, the work tells us what genes may have been useless—and therefore inactivated—in the ancestors of all cetaceans: genes that would be a hindrance to adapting to a fully marine way of life. Now we don’t know that the genes mentioned above were definitely inactivated because of new way of life, but the authors at least provide a suggestive but useful list for further investigation. Are these genes inactivated in all cetaceans? Do they do what they’re said to do when they’re active, and is their inactivation useful in cetaceans? This is all grist for further research.

Finally, what mechanism led to the gene inactivation? The authors posit mutation followed by the generation of “neutral” gene forms, with the inactive forms fixed in cetaceans by genetic drift:

Many of these gene losses were likely neutral, and their loss happened because of relaxed selection to maintain their function.

Well, that might be true, but such fixations of broken genes take a long time, and if they’re neutral they’re likely to keep both active and inactive forms of a gene around for a long time, especially in large populations. And why would the broken gene always be “fixed” in cetaceans rather than coexisting with the active form?

Given that the authors speculate that the loss of gene function might be adaptive in cetaceans, it seems more likely that natural selection swept the broken genes to fixation because they were adaptively superior to active genes (see above for reasons why). It’s a challenge for future work to try to determine, through DNA-sequence analysis, whether broken genes come to be fixed by positive selection or by random genetic drift due to “relaxed selection” (i.e., no selection either way on broken versus non-broken genes). That kind of analysis, as I said, is hard, but it helps us understand how genes get broken when they’re not useful.

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Huelsmann, M., N. Hecker, M. S. Springer, J. Gatesy, V. Sharma, and M. Hiller. 2019. Genes lost during the transition from land to water in cetaceans highlight genomic changes associated with aquatic adaptations. Science Advances 5:eaaw6671.

 

A 43 million-year-old transitional form: an amphibious whale

April 7, 2019 • 9:45 am

The evolution of whales from a small, deer-like artiodactyl took about ten million years: from about 50 million to about 40 million years ago. That’s remarkably fast evolution, especially when you consider the amount of morphological and physiological change that occurred, and the fact that the divergence between chimps and modern humans from their common ancestor, which (I think) took far less morphological and physiological change, took more than 6 million years.

Fortunately, we have a good fossil record of whales from Egypt, Pakistan and intervening areas, and so we can document this rapid change. Although the closest living relative of whales are hippos, the ancestor of modern cetaceans might have looked something like the first photo below, a reconstruction of the fossil Indohyus,  a terrestrial, raccoon-sized artiodactyl (note hooves) similar to the modern chevrotain (“mouse deer”), which is known to stay underwater for long periods to escape predators.

Indohyus (reconstruction)

Here’s a living chevrotain, the Lesser mouse-deer (Tragulus kanchil). It’s hard to imagine that something like this could evolve into the mighty whale!

At any rate, a new paper in Current Biology by Oliver Lambert et al. documents a transitional form between ancient and modern whales, a species they name Peregocetus  pacificus, dating from about 43 million years ago. It was found not in the Middle East but in Peru, and so also provides information about how whales colonized the Atlantic Ocean after their presumed origin off Southeast Asia. You can read the paper for free by clicking on the screenshot below, or by getting the pdf here. If somehow your access is blocked, judicious inquiry will get you a pdf.

This “whale” (if it can be called that) was dug up on the southern coast of Peru, with a large proportion of its skeleton remaining (see below; solid lines are recovered bones). From this skeleton we can conjecture that it was amphibious—it could swim and also walk on the land. Here’s the tally of found bones, and a reconstruction of how they were used to both swim and walk. As you can see from the scale, it was about 4 meters long:

(From paper): Schematic drawings of the articulated skeleton of MUSM 3580 showing the main preserved bones, in a hypothetical swimming and terrestrial posture. For paired bones, the best-preserved side was illustrated (sometimes reversed), or both sides were combined (e.g., mandible). Stippled lines indicate reconstructed parts and missing sections of the vertebral column; cranium, cervical vertebrae, and ribs based on Maiacetus inuus.

Some questions:

Why do we think it could swim? Peregocetus had a well-formed pelvis, attached to the rest of the skeleton, and well formed legs that would have stuck well outside the body.  It had a long tail, and the configuration of the tail vertebrae, compared to those of other swimming mammals like otters and beavers, suggest that the tail was somewhat paddle-like, powerful, and thus could be used for swimming. (This is seen in the reconstruction at the bottom.). It’s not known if the tail ended with a boneless “fluke” (as in modern whales), as that would not have been preserved.

The rear feet were long, as were the rear digits, and those digits were flattened with flanges on the side, indicative of webbed rear feet. Based on the foot anatomy, the authors suggest that “Propulsive movements were either alternate or simultaneous hind-limb paddling or body and tail undulations, as observed in modern river and sea otters, alternating between lift-based propulsion via pelvic undulations, including tail and hind limbs, and drag-based propulsion via independent strokes of the hind limbs.”

Here’s the distal part of the front leg:

Here’s the rear foot with flattened digits (I believe “3” are the hooflets):

Why do we think it could walk? I’ll quote the authors here: “The fore- and hind-limb proportions roughly similar to geologically older quadrupedal whales from India and Pakistan, the pelvis being firmly attached to the sacrum, an insertion fossa for the round ligament on the femur, and the retention of small hooves with a flat anteroventral tip at fingers and toes indicate that Peregocetus was still capable of standing and even walking on land.”

What else is interesting about this fossil? It had sharp and robust teeth, some shearing teeth, and a long snout, indicating that it probably ate large bony fish. Here is a mandible with teeth:

Further, this fossil suggests to the author a route for migration of ancestral whales from their origin in the Pacific to the Atlantic Oceans and into the New World.

Based on recent finds of whale fossils in West Africa, as well as this find from Peru, the authors suggest that whales made their way through the Tethys sea (the Mediterranean, which was contiguous with the Indian Ocean then), south along the West African coast, then hopped over the South Atlantic (much narrower at that time due to continental drift) along the coast of South America and then south through the Isthmus of Panama (open to the sea then) to get back to the eastern Pacific. It also is thought to have spread north along the east coast of North America. Below you can see the route of migration, with the authors’ hypothesis denoted by the black arrows below. (A trans-Pacific route can’t be ruled out, but the Pacific was much wider then and there are no early whale fossils on the Pacific coasts.)


Finally, here’s a reconstruction of the beast on land and see from the paper’s Supplemental Information:

(From paper): Figure S4. Artistic reconstruction of the middle Eocene (about 42.6 Ma) protocetid whale Peregocetus pacificus gen. et sp. nov., Related to Figure 2 and Data S2. Life reconstruction of two individuals of P. pacificus, one standing on the rocky shore and the other hunting sparid fish, along the coast of nowadays Peru. The presence of a caudal fluke in P. pacificus remains hypothetical and should be tested with the discovery of a more complete specimen, including posteriormost caudal vertebrae. Reconstruction by A. Gennari.

 

Here we have a true intermediate form, a transitional species which occurs when it’s supposed to: after the earliest whales but before the appearance of modern whales. Given this find, as well as the panoply of other fossil whales showing progression from ancestral to modern forms, creationists will be hard pressed to explain this.

h/t: Kevin

 

 

The recurrent laryngeal nerve as evidence for evolution

March 2, 2019 • 12:00 pm

On pages 82-84 of Why Evolution is True I discuss the recurrent laryngeal nerve of humans (and other tetrapods) as an example of evolution. It’s evidence via “retrodiction”, which is what I call the situation when a previously unexplained and puzzling phenomenon can be understood only in light of a theory, thus supporting that theory—in this case, evolution.

Rather than describe it again, here are two videos showing it and explaining how the configuration of that nerve supports evolution.

Creationists have an explanation for it, too (there’s nothing they can’t explain via God’s will, except perhaps the peculiar species composition of oceanic islands), but the goddy story is unconvincing and less parsimonious. Whereas the evolutionary explanation tells us why only one of the twinned cranial nerves does its crazy loop, and why it’s completely comprehensible via the known evolution of tetrapods, the creationist explanation is based solely on how the nerve works: a post facto “functional” explanation of why the creator would create the nerve’s tortuous path. But it doesn’t explain why the creator made that big loop to enervate the larynx when he could have sent a branch directly to the larynx without the loop.

Here Dr. Rohin Francis, a cardiologist and researcher in London, uses his expertise to show how the nerve supports the “tinkering” aspect of evolution:

Below Richard Dawkins attends the dissection of a giraffe, which has an extraordinarily long (5 meter) recurrent laryngeal nerve. Rohin, however, notes above that some long-necked sauropod dinosaurs certainly had a recurrent laryngeal nerve about 28 meters (92 feet) long! I believe I’ve posted this video before, but it goes well with the video above:

In my only visit ever to a human anatomy lab (I get freaked out by corpses), I myself watched the dissection of this nerve by an anatomy professor. And it’s just like the one above, only shorter.

h/t: Scott

You have vestigial muscles that moved the whiskers of your ancestors

June 19, 2017 • 9:30 am

This is the kind of post I envisioned writing—once every few weeks or so—when I started this website. My intention was to use the site to publicize new evidence for evolution. Not that we need any to show that that well evidenced theory is true, of course, but to support the book and alert people to cool new findings. But, as I’ve said, things got out of hand, and so we have cats, food, travels, religion, and so on. So let’s go back to our roots today. . .

Matthew, on the job as always, sent me this tw**t and asked me if I’d mentioned this in the “vestigial structures” section of Why Evolution is True.

I told him I hadn’t even heard of this, but, sure enough, I found the following in the “Human vestigiality” article in Wikipedia (worth looking at):

In many non-human mammals, the upper lip and sinus area is associated with whiskers or vibrissae which serve a sensory function. In humans, these whiskers do not exist but there are still sporadic cases where elements of the associated vibrissal capsular muscles or sinus hair muscles can be found. Based on histological studies of the upper lips of 20 cadavers, Tamatsu et al. found that structures resembling such muscles were present in 35% (7/20) of their specimens.[50]

Naturally I went to the cited source: a short paper by Yuichi Tamatsu et al. in Clinical Anatomy in 2007 (reference and link below; free access if you have the legal Unpaywall application). That paper shows what to me (and I’m not an anatomist) looks like vestigial muscles that are the remnants of muscles that move the whiskers in our mammalian relatives—and in our whiskered ancestors.

Mammalian whiskers are called “vibrissae” and most are are movable (their function is at least partly tactile–touch–though they may have other functions). As for how and why they move, here’s the Wikipedia entry:

The follicles of some groups of vibrissae in some species are motile. Generally, the supraorbital, genal and macrovibrissae are motile, whereas the microvibrissae are not. This is reflected in anatomical reports that have identified musculature associated with the macrovibrissae that is absent for the microvibrissae. A small muscle ‘sling’ is attached to each macrovibrissa and can move it more-or-less independently of the others, whilst larger muscles in the surrounding tissue move many or all of the macrovibrissae together.

Amongst those species with motile macrovibrissae, some (rats, mice, flying squirrels, gerbils, chincillas [sic], hamsters, shrews, porcupines, opossums) move them back and forth periodically in a movement known as whisking, while other species (cats, dogs, racoons, pandas) do not appear to. The distribution of mechanoreceptor types in the whisker follicle differs between rats and cats, which may correspond to this difference in the way they are used. Whisking movements are amongst the fastest produced by mammals. In all whisking animals in which it has so far been measured, these whisking movements are rapidly controlled in response to behavioural and environmental conditions. The whisking movements occur in bouts of variable duration, and at rates between 3 and 25 whisks/second. Movements of the whiskers are closely co-ordinated with those of the head and body.

You might remember that we evolved from a shrewlike ancestor, and thus probably an ancestor that could move its whiskers.

In their paper, Tamatsu et al contrast the whisker muscles (muscles of the “sinus hairs”, another name for vibrissae) with those of regular body hairs. The latter have smooth “arrector pili” muscles that can erect each hair involuntarily during times of cold or fear, giving us goose bumps. As I note in WEIT, these are probably vestigial in humans, as we have no need to look bigger by erecting our hairs (versus cats, who bush out when they’re threatened), and erecting our body hair in the cold doesn’t provide much thermal insulation since we’re “naked apes”. Arrector pili appear to be remnants from mammalian ancestors who could really use these muscles adaptively, and thus they give testimony to our evolution.

Here’s a drawing showing the arrector pilus, the orange-red band affixed to the hair follicle at center left:In contrast, whiskers are attached to special “capsular muscles” and can be moved voluntarily; unlike the smooth arrector pili, they are striated, or “striped”, as voluntary muscles are.

Tamatsu et al. looked for these capsular muscles by dissecting 20 cadavers (11 males and 9 females) and doing scanning electron microscopy of sections of the upper lip.  They found what looked like capsular “whisker muscles” in 4 males and 3 females, or 35% of the sample. I won’t go into detail, but will just show a few of the photos they present as evidence, along with their captions (indented). Note the striated muscle in (b), which you can see better two pictures down:

Sections through the upper lip. a: Light micrograph of a section of the upper lip region of a 76-yearold female. The right side of the figure is medial and the left side is lateral. Arrows indicate the location and direction of muscle fascicles. The fascicles diverge from the orbicularis oris layer and course to the dermis (D). SC, subcutaneous tissue; ML, muscle layer. Azan-Mallory stain. b: Magnified light micrograph of the rectangular, outlined area shown in (a). The arrows indicate the course of a muscle fiber having a striated pattern and collagen fibers. This bundle courses to a hair follicle (HF). [Scale: bar = 1 mm in (a) and 0.5 mm in (b)]

Higher magnification light micrograph (c) and SEM micrograph (d) of the rectangular, outlined area shown in (b). Arrows indicate an attachment area between the hair follicle and collagen fibers continued from the muscle fiber located in the subcutaneous tissue. [Scale bars: 250 microns in both photos]

Here the striated muscle is seen clearly:

High magnification light micrograph of the small rectangular outlined area shown in (b). A striated pattern can be seen in the muscle fiber. Scale bar = 25 microns

The authors conclude that these muscles are similar to those that move the whiskers in mammals that have them:

. . . in the sections that displayed vertical sections of hair follicles it was observed that these muscle and collagen fascicles surrounded the outer half of a hair follicle. This configuration is different from arrector pili muscles of body hairs that attach to the follicle at a single point, but shows similarity to muscle slings of mystacial vibrissae reported by Dorf (1982). According to his report, muscle slings of mystacial vibrissae embraced the follicles. Furthermore, we found many blood vessels containing blood cells near these follicles. Because of their thin walls and large diameter, these blood vessels were assumed to be veins, which are known to be associated with sinus hair follicles. Our findings of the structural characteristics of these muscles and follicles, which bear similarities to those of sinus hairs, led us to conclude that the observed muscle fascicles are a vestigial muscle of sinus hair. A 35% incidence supports this conclusion, given that regressive organs do not exist in all individuals.

Vestigial structures like wisdom teeth and ear-moving muscles often are missing in many individuals—one of the signs that the feature is of no or little use.  Now you’re probably asking yourself, if you’re a man, “If I had a mustache, could I move it if I were one of the individuals that had these muscles?” Or, if you have a mustache, you’ve probably already tried to move it as you read this. But no dice, for a mustache is not the equivalent of mammalian whiskers. A ‘stache is made of regular body hair and thus, while it could be moved by arrector pili, its hairs cannot be moved voluntarily.

Let’s take the finding of Tamatsu et al. as tentative but very suggestive, as the authors knew what kind of muscle to look for and found it. And we have no vibrissae. And the muscle is present in only a fraction of individuals, and in females, too. This looks to be an anatomical remnant of our whiskery ancestry: a vestigial trait that testifies to the fact of evolution.

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Tamatsu, Y.; K. Tsukahara, Kazue; M. Hotta and K. Shimada. 2007. “Vestiges of vibrissal capsular muscles exist in the human upper lip“. Clinical Anatomy. 20(6): 628–31.