Toothless vertebrates have remnants of tooth genes

Teeth are very useful organs for many animals, coming in many shapes and sizes to perform a variety of functions. I imagine that most children learn early on, even if intuitively, that meat-eating animals have sharp teeth that are particularly useful for grasping and slicing up prey, whereas plant-eaters tend to have flat teeth that are useful for grinding up vegetation. Others animals have teeth for sifting through the ocean bottom to find mud-dwelling animals, some have pointed teeth adapted for crushing the exoskeletons of insects and there are even hollow teeth used to inject venom into prey or would-be predators.

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Indeed, true teeth, like those that humans have and shown in the images above, are present in the majority of vertebrates, or animals that possess a backbone or spine. Yet despite their near ubiquity in vertebrates, there exist a number of species that lack pearly whites entirely. Birds and turtles, for instance, possess a beak made of keratin, the same type of protein that makes up our hair and nails. There also exist some curious mammals that lack teeth, such as the anteaters of South America and the scaly, pinecone-like critters known as pangolins.

Furthermore, there are a number of mammals that have teeth, but lack what may be the most important part of a tooth: the enamel. Most vertebrate teeth have an outer shell of enamel, which for us is the hardest substance we have in our bodies. This makes sense, of course, as our teeth have to be tough enough to last our whole lives! Underneath the enamel is dentin, which is much softer. However, some animals, like armadillos, sloths and the aardvark only have dentin in their teeth, making them much softer than our own.

So how does this relate to evolution? Some vertebrates have teeth, some have teeth with no enamel, and some have no teeth at all. Why does this matter? Well, evolutionary theory posits that species descend from ancestral species that may have had very different anatomical features. So if most vertebrates have teeth, and some don’t, perhaps those toothless species descended from toothed ones.

Indeed, many scientists, including myself, have examined the genes of these toothless and enamelless vertebrates, and we’ve found tooth genes retained in the genomes of all of such species [1-5]. These tooth genes are known to perform a number of different functions, but are largely involved with creating conditions to allow teeth to form and grow. But what’s most striking, however, is that while these tooth genes are present in these toothless and enamelless vertebrates, they are broken and littered with mutations that render them nonfunctional.

Below are examples of two such genes, ENAM (left) and ODAM (right), with highlighting showing mutations in the genes that render them inactivated. Examples included are the toothless pangolins, anteaters and baleen whales, plus the enamelless aardvark, armadillos and sloths.

Furthermore, many of these groups of organisms share identical mutations, implying that the gene was broken and then inherited by all of their descendants. For instance, scientists found that 48 bird species, as different as penguins, eagles, ostriches and hummingbirds, share disabling mutations in five different genes critical for tooth development and function (DSPP, ENAM, AMELX, AMTN, MMP20) [3]. For all 48 bird species to coincidentally have the same mutations seems unlikely, but inheriting them from a common ancestral bird makes a lot of sense.

And if that isn’t convincing enough,  some of the toothless species have a fossil record suggestive of descent from toothy ancestors. Birds seem to have descended from toothed dinosaurs, and lost their teeth some 65 million years ago, and turtles appear to have had proto-turtle ancestors with a full set of teeth before disposing of them entirely some 200+ million years ago.

Questions for Creationists

So this raises some questions: why would the Creator create toothless and enamelless animals with nonfunctional remnants of tooth genes?  Why do some of these vertebrates have identical disabling mutations? Is it just a coincidence that scientists have found fossils of some of these vertebrates that have teeth but modern species have broken tooth genes?

References

1. Meredith, R. W., Gatesy, J., Murphy, W. J., Ryder, O. A., & Springer, M. S. (2009). Molecular decay of the tooth gene enamelin (ENAM) mirrors the loss of enamel in the fossil record of placental mammals. PLoS genetics5(9), e1000634.

2. Meredith, R. W., Gatesy, J., & Springer, M. S. (2013). Molecular decay of enamel matrix protein genes in turtles and other edentulous amniotes. BMC evolutionary biology13(1), 20.

3. Meredith, R. W., Zhang, G., Gilbert, M. T. P., Jarvis, E. D., & Springer, M. S. (2014). Evidence for a single loss of mineralized teeth in the common avian ancestor. Science, 346(6215), 1254390.

4. Springer, M. S., Emerling, C. A., Gatesy, J., Randall, J., Collin, M. A., Hecker, N., … & Delsuc, F. (2019). Odontogenic ameloblast-associated (ODAM) is inactivated in toothless/enamelless placental mammals and toothed whales. BMC evolutionary biology, 19(1), 31.

5. Springer, M. S., Starrett, J., Morin, P. A., Lanzetti, A., Hayashi, C., & Gatesy, J. (2016). Inactivation of C4orf26 in toothless placental mammals. Molecular phylogenetics and evolution, 95, 34-45.

2 thoughts on “Toothless vertebrates have remnants of tooth genes

  1. Pingback: Toothless, baleen whales have remnants of tooth genes – Evolution For Skeptics

  2. Pingback: Fetal baleen whales develop with remnants of toothy past – Evolution For Skeptics

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