Monday, 15 February 2016

Shark Bite For Creationists

An ancient dental gene set governs development and continuous regeneration of teeth in sharks | ScienceDirect

Astonishing news today that the creationist industry's intelligent (sic) designer favours sharks over humans.

Although humans notoriously suffer from all manner of dental problems, including dental caries caused by bacteria, abscesses formed in the gums and jaw when these caries erode the tooth to the point where the dentine and root canals are exposed, and gum diseases leading to the teeth actually falling out, sometime by middle age, the intelligent (sic) designer apparently designed a system for replacing teeth but switched it off in humans, and most of our vertebrate relatives, way back in our evolutionary history. It left the genes there, not doing anything useful though.

We know that sharks are fearsome predators and one of the main reasons they are so successful at hunting prey is because of their rows of backward pointing, razor-sharp teeth that regenerate rapidly throughout their lifetime, and so are replaced before decay.

The Jaws films taught us that it's not always safe to go into the water, but this study shows that perhaps we need to in order to develop therapies that might help humans with tooth loss.

But in sharks, which it seems to favour, the genes for tooth replacement are not only working perfectly well, churning out new teeth like a factory production line as the old ones fall out, but they have been working for some 450 million years, way before the intelligent (sic) designer got round to designing even the terrestrial ancestors of humans, so not knowing how to do it is not an excuse open to it. The only explanation is that the intelligent (sic) designer wants humans to suffer with dental problems and lose their teeth permanently as they get older.

This was demonstrated a few days ago in an open access paper published in Developmental Biology by a team from the University of Sheffield's Department of Animal and Plant Sciences, Sheffield, England, led by Dr Gareth Fraser and funded by Natural Environment Research Council.

Comparative phylogeny of tooth replacement modes in toothed vertebrate clades. A. the shark dentition (elasmobranchs) continuously regenerates throughout life (polyphyodont) with a many-for-one mechanism (many teeth made in advance of function for each functional tooth position in the jaw). B. Bony fishes offer a diverse range of tooth replacement mechanisms, with continuous replacement with a one-for-one system (e.g. cichlid fishes, in B (Fraser et al., 2013); one replacement tooth made in advance of function for each functional position), although a many-for-one system is observed in many bony fish species. C. Reptiles show a range of diverse mechanisms for tooth replacement with a continuous polyphyodont system in some species e.g. the snake, in C (with some species showing a more restricted dentition without replacement (monophyodont; e.g. chamealeon; Buchtová et al. 2013). D. The mammalian dentition also offers a range of diverse replacement or renewal phenotypes from monophyodont mammals e.g. mice, where the only regenerative potential is observed in the continuously growing incisor (Di), and molars are never replaced. In most mammals the dentition is diphyodont with two tooth generations e.g. humans (D). A phylogenetic reduction of tooth generations are generally observed toward more higher vertebrates i.e. mammals.
Highlights
  • Tooth development is highly conserved from sharks to mammals, over 450 million years.
  • Sharks regenerate their teeth in a continuous conveyor belt-like manner.
  • A core set of dental genes is utilised by all toothed vertebrates.
  • The dental lamina is a developmental prerequisite for continuous tooth regeneration.
  • All vertebrates share common signalling centres that direct tooth shape morphogenesis.

Abstract
The evolution of oral teeth is considered a major contributor to the overall success of jawed vertebrates. This is especially apparent in cartilaginous fishes including sharks and rays, which develop elaborate arrays of highly specialized teeth, organized in rows and retain the capacity for life-long regeneration. Perpetual regeneration of oral teeth has been either lost or highly reduced in many other lineages including important developmental model species, so cartilaginous fishes are uniquely suited for deep comparative analyses of tooth development and regeneration. Additionally, sharks and rays can offer crucial insights into the characters of the dentition in the ancestor of all jawed vertebrates. Despite this, tooth development and regeneration in chondrichthyans is poorly understood and remains virtually uncharacterized from a developmental genetic standpoint. Using the emerging chondrichthyan model, the catshark (Scyliorhinus spp.), we characterized the expression of genes homologous to those known to be expressed during stages of early dental competence, tooth initiation, morphogenesis, and regeneration in bony vertebrates. We have found that expression patterns of several genes from Hh, Wnt/β-catenin, Bmp and Fgf signalling pathways indicate deep conservation over ~450 million years of tooth development and regeneration. We describe how these genes participate in the initial emergence of the shark dentition and how they are redeployed during regeneration of successive tooth generations. We suggest that at the dawn of the vertebrate lineage, teeth (i) were most likely continuously regenerative structures, and (ii) utilised a core set of genes from members of key developmental signalling pathways that were instrumental in creating a dental legacy redeployed throughout vertebrate evolution. These data lay the foundation for further experimental investigations utilizing the unique regenerative capacity of chondrichthyan models to answer evolutionary, developmental, and regenerative biological questions that are impossible to explore in classical models.

Rasch, L.J., et al., An ancient dental gene set governs development and continuous regeneration of teeth in sharks.
Dev. Biol. (2016), http://dx.doi.org/10.1016/j.ydbio.2016.01.038i

Copyright © 2016 The Authors. Published by Elsevier Inc. Reprinted under Creative Commons license (CC-BY 4.0)

Alternatively, you have to believe that it favours sharks over its supposedly favourite and special creation, humans.

Humans also possess these specialised cells but only ever develop milk teeth followed by adult teeth which you have for life, or at least until they have to be extracted or they fall out; then the specialised cells are lost. There appears to be no biological reason why teeth can't be replaced as they are lost since not only is the genetic mechanism still intact but the cells could be retained. If you believe in intelligent (sic) design, you have to accept that the intelligent (sic) designer deliberately turned this mechanism off, but for some reason decided to keep it and reproduce it in every vertebrate, including humans for no reason whatsoever.

Dr Gareth Fraser, lead author
Quoted in ScienceDaily
I appreciate that this is a problem and another embarrassment for the creation industry but I'll ask it anyway. Why would an intelligent designer give humans and all other mammals the ability to replace lost teeth, then switch it off? Just bad planning and a lack of foresight, maybe?

Luckily, for those who are interested in real biology, evolution by natural selection is more than capable of explaining why it would have been a waste of resources to continually replace teeth when it was more efficient not to, especially when there would have been a very low evolutionary pressure to retain this mechanism.

Now that we have the problem of dental caries, and now that humans have evolved to live way beyond their reproductive years, unlike most other mammals, because that allows us to share in the care of grandchildren so helping to ensure the genes they carry that they got from their grandparents are more successful, there is a case for resurrecting this mechanism. The evolutionary pressure is still very low, though, like the evolutionary pressure to overcome cancers and other degenerative diseases.

For an intelligent designer who loves humans, though, switching this mechanism back on again would be the work of a moment.

Maybe it just favours sharks and enjoys watching old people struggling to eat.

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