Fossil Friday: A Dinosaur Feather and an Overhyped New Study on the Origin of Feathers
This Fossil Friday features a feather from 100 million-year-old Burmese amber. The age and the strange ribbon-like structure of this fossil feather suggest that it could be a feather of a theropod dinosaur or primitive stem-bird rather than a modern bird (Benton et al. 2019). The fossil was acquired by me some years ago for the amber collection of the Natural History Museum in Stuttgart (Germany), where I worked as scientific curator for amber and fossil insects until 2016. I have already discussed the issue of the origin of birds and feathers in several previous articles on Evolution News (Bechly 2022a, 2022b, 2023). Today, I want to use the occasion to discuss a recent study on the origin of feathers by Cooper & Milinkovitch (2023), which was celebrated as evidence that “tweaking just a few genes transforms scales into feathers” (Starr 2023). The authors boldly claim that their “results indicate that an evolutionary leap — from scales to feathers — does not require large changes in genome composition or expression.“ This is complete hogwash.
And Here Is Why
The scientists injected chicken embryos with molecular triggers that changed the development of reticulate scales on chicken feet into that of ectopic feathers, which means that feathers developed at an abnormal place instead of scales. That‘s all. The mentioned spectacular conclusion from this experiment rests on two hidden assumptions that are both false or at least highly questionable:
- Feathers are derived from transformed reptile scales.
- The scales on bird feet are primary scales and not reduced feathers.
The first claim was the subject of a long and hot debate in modern biology. Many textbooks still suggest that bird feathers were derived from elongated reptile scales, and this was also promoted by theistic evolutionists Karl Giberson and Francis Collins (2011) in their book The Language of Science and Faith (for a critique see Luskin 2021). However, this still common claim faces several severe problems and no longer represents the consensus view in mainstream science.
One problem is that reptiles are not considered to represent a natural (monophyletic) group and include very diverse and only distantly related taxa such as turtles and tortoises, crocodiles and alligators, and lizards and snakes. These different reptile groups possess very different types of scales (e.g., compare the adjacent scales of a croc with the overlapping scales of a lizard) of dubious and disputed homology. Therefore, the claim that feathers are derived from reptile scales is rather meaningless in the first place. An even more important problem is the ontogeny of feathers, which begin as hollow tubelike filaments, with the feather forming from the disintegration and unfolding of the tube‘s wall, not as elongations of flat scales. Finally, there are significant morphogenetic and molecular differences between the various integumental structures of vertebrates that “for decades, fostered the debate on the homology, or lack thereof, among these skin appendages and led some authors to conclude that homologous skin appendages do not exist beyond amniote classes (reptiles, mammals, and birds); that is, mammalian hair and avian feather would not have evolved from reptilian overlapping scales” (Di-Poï & Milinkovitch 2016).
Common Biological Knowledge
Indeed, the recognition that feathers did not evolve directly from scales has been common biological knowledge for many years and even made it into the prestigious Encyclopedia Britannica, which unequivocally clarifies that “Feathers are complex and novel evolutionary structures. They did not evolve directly from reptilian scales, as once was thought.” An educational site on avian biology by Eastern Kentucky University makes a similarly clear statement: “Feathers, then, are not derived from scales, but, rather, are evolutionary novelties with numerous unique features.” It could hardly be more in your face than that!
Nevertheless, new evo-devo research (Di-Poï & Milinkovitch 2016) about the ontogeny of reptile scales, mammal hairs, and bird feathers was misleadingly advertised in popular media reports as “Human hair, bird feathers came from reptile scales” (Panko 2016). However, what this research really showed is a so-called deep homology of these skin structures (Benton et al. 2019), which all ontogenetically derive from thickened patches of skin (called placodes) in embryos. This means that scales, hairs, and feathers share a similar ontogenetic pathway and may share a common origin in an early precursor skin structure, but it does not demonstrate that feathers originated from modified adult reptile scales. Don’t take my word. Here is what the more recent study of Benton et al. (2019) emphasized: “Furthermore, it is inadequate to say that feathers evolved from reptilian scales, as both morphogenesis and CBPs of feathers are basal to those of avian scales, and that the molecular profiles of avian scales are similar to feathers, but different from reptilian scales.”
Concerning the second assumption, there is growing evidence that the scales on bird feet are not primary scales but reduced feathers. Here is a quote from Dhouailly (2009): “Concerning feathers, they may have evolved independently of squamate scales, each originating from the hypothetical roughened beta-keratinized integument of the first sauropsids. The avian overlapping scales, which cover the feet in some bird species, may have developed later in evolution, being secondarily derived from feathers.” Benton et al. (2019) further elaborated: “During theropod evolution, leg feathers became reduced from the foot to thigh, and scales replaced them. Likewise, such scales are present together with hair in a Cretaceous mammal, as well as over the whole body in the pangolin or along the tail in rodents, such as rats and mice. These scales are commonly interpreted as primitive holdovers from reptilian ancestors, but palaeontological and genetic evidence suggests that they are secondarily derived from feathers or hairs.”
Impressive to the Uninformed
We can therefore safely conclude that the new study by Cooper & Milinkovitch (2023) is just the most recent example of overhyped science that only sounds impressive to the uninformed, who neither know that mainstream evolutionary biology no longer supports an evolution of bird feathers from reptile scales, nor know that the scales on bird feet are believed to be reduced feathers. So, it is hardly surprising that a simple mutation can change bird leg scales back into feathers. Misleading research like this is one important reason I have lost faith in the overblown claims of evolutionary biology. It‘s mostly smoke and mirrors.
So, what about the grandiose claim that an “evolutionary leap — from scales to feathers — does not require large changes in genome composition or expression“? This is of course complete rubbish as well. In reality, the creation of feathers, which are the most complex integumental structures known in the animal kingdom, without doubt required coordinated changes in numerous genes.
This is because it involved differences in keratin structure, a sophisticated pattern formation of branches (rami) and subbranches (radii), as well as highly specific programmed cell death that sculpts the feather during ontogeny, and many more biological novelties that required new genetic code. The whole idea that a simple developmental switch could perform this trick is nothing short of ludicrous. What the simple switch does is just reactivate the already existing code for feather formation in a body region with secondarily reduced feathers. This has nothing to do with an evolutionary origin of biological novelty and has zero explanatory power for the origin of feathers. As I said, smoke and mirrors!
- Bechly G 2022a. Fossil Friday: The Temporal Paradox of Early Birds. Evolution News August 19, 2022. https://evolutionnews.org/2022/08/fossil-friday-the-temporal-paradox-of-early-birds/
- Bechly G 2022b. Educating “Professor Dave” on the Fossil Record and Genetics. Evolution News December 8, 2022. https://evolutionnews.org/2022/12/educating-professor-dave-on-the-fossil-record-and-genetics/
- Bechly G 2023. Fossil Friday: A Waiting Time Problem for Feathers. Evolution News March 10, 2023. https://evolutionnews.org/2023/03/fossil-friday-a-waiting-time-problem-for-feathers/
- Benton MJ, Dhouailly D, Jiang B & McNamara M 2019. The Early Origin of Feathers. Trends in Ecology & Evolution 34(9), 856-869. DOI: https://doi.org/10.1016/j.tree.2019.04.018
- Cooper RL & Milinkovitch MC 2023. Transient agonism of the sonic hedgehog pathway triggers a permanent transition of skin appendage fate in the chicken embryo. Science Advances 9(20), 1-13. DOI: https://doi.org/10.1126/sciadv.adg9619
- Di-Poï N & Milinkovitch MC 2016. The anatomical placode in reptile scale morphogenesis indicates shared ancestry among skin appendages in amniotes. Science Advances 2(6), 1-8. DOI: https://doi.org/10.1126/sciadv.1600708
- Dhouailly D 2009. A new scenario for the evolutionary origin of hair, feather, and avian scales. Journal of Anatomy 214(4), 587-606. DOI: https://doi.org/10.1111/j.1469-7580.2008.01041.x
- Luskin C 2021. Listen: Scale-to-Feather Evolution Doesn’t Fly. Evolution News October 17, 2021. https://evolutionnews.org/2021/10/listen-scale-to-feather-evolution-doesnt-fly/
- Panko B 2016. Human hair, bird feathers came from reptile scales. Science News June 24, 2016. https://www.science.org/content/article/human-hair-bird-feathers-came-reptile-scales
- Starr M 2023. Tweaking Just a Few Genes Transforms Scales Into Feathers. ScienceAlert May 22, 2023. https://www.sciencealert.com/tweaking-just-a-few-genes-transforms-scales-into-feathers