"A Very Short Introduction: Teeth"

Posted on Sat 06 October 2018 in books

A Very Short Introduction: Teeth is my 28th VSI book read, and I enjoyed it. I had picked it up to read before my dentist's appointment, so that I would understand teeth terminology better. Since the book gives a nice wide introduction to teeth, I found that I ended up learning many things as well. I'll have to see if there's a VSI on dentistry.

This VSI has a well-defined topic (teeth), covers multiple aspects of teeth (function, evolution, humans), and is a clear and understandable read. What I found most interesting was the evolution of teeth, how teeth cusps slowly reconfigure themselves. Once in a while I feel the sharpness of my teeth and recall that certain animals have self-sharpening teeth. While the knowledge may not be practical, but it was still interesting for me to learn.

Teeth matter

In the introductory chapter, the author outlines why teeth are interesting. I find that he does a good job here.

His first argument is that teeth are cool. Teeth are part of a "perpetual death match in the mouth" because they continue to evolve to be sharper, tougher and stronger to chew plants and animals, who sometimes evolve to counteract teeth (ie: harder insect exoskeleton).

Teeth are noticeable. Whether it's the small teeth of a kitten or the large teeth of a predator, or the smile of another human being, our attention is drawn to teeth. Personally I find it fascinating to see cats' small incisor teeth since they're so tiny compared to ours.

My cat's teeth

Teeth have an evolutionary significance too. Before teeth, organisms would filter nutrients and not much else. What teeth allowed was the crushing of food sources to get at the nutrients within. Teeth further evolved in terms of efficiency (better tearing flesh) or in response to the defenses of prey.

Teeth are also important to our lives. Mammals are endoterms so they rely a lot on teeth to get at nutrients. If human beings did not have good teeth, they would be a lot less effective at extracting energy from food. Human beings could not simulatenously tear meats, chew vegetables, and crunch candies without well-developed teeth.

Teeth are often the only fossils of extinct animals such as dinosaurs. The teeth are hard bones with extra protection, so they are preserved better.

Types and parts of teeth

The author covers a lot of technical information and terminology. When the chapter got into more specific teminology I got lost. The authors talks about the history and controversy of paracones, metacones, and tribosphenic molars for a few pages.

Different organisms have different teeth. They may either grow their teeth once, twice, or multiple times. The first teeth are called baby, milk or deciduous, and for some animals these are permanent. For organisms with multiple sets, the baby teeth can either fall off in the womb or later in life. Teeth may all be identical, which are called homodont, or they can be different, which are called heterodont.

Mammalian teeth are heterodont, meaning that they're different. Incisors are the flat front teeth used for cutting. Canine are sharper needle-like teeth for tearing (their shape vary). Pre-molars are the first of the "cheek teeth" (or post-canines) and vary in usage. The molars seem to mostly be for crushing. These can be quite different in different mammals. In cats the pre-molars are for slicing and the canines are longer. In herbivores the canines can be flatter, like extra incisors.

The amount of these types of teeth in a mammal vary. Humans are identified by I2/2, C1/1, P2/2, M3/3 (2+2 incisors, 1+1 canines, 2+2 pre-molars and 3+3 molars). Note that wisdom teeth are the third set of molars, and you may have had yours removed.

There are terms used to identify locations on teeth.

  • For the front teeth, the front side is labial and the back side is lingual.
  • Towards the middle of the front teeth is called mesial and away from it if distal.
  • For the back or post-canine teeth, towards the front is anterior and towards the back is posterior.
  • Tongue side of the back teeth is lingual and away from the tongue is buccal.
  • The biting side (towards the biting) is called the occlusal surface.

The vocabulary for occlusal surfaces is quite complicated, and unfortunately, partly misleading. Terms such as paracone, metacone, protocone and hypocone were supposed to correspond to how the cusps of the teeth evolved, but the names turned out not to match exactly. There is more information in the book.

Teeth are made of enamel, dentine, cementum and pulp. Enamel is the toughest material, at 95% mineral content, and it forms the biting surface. Dentine constitutes most of a tooth and is still a tough material. Cementum is found at the root and sometimes at the crown. The pulp fills out a tooth with blood vessels, nerves and other soft tissue.

What teeth do: food and feeding

This is a fun chapter. The author starts by going over what kind of nutrients and micronutrients are found in different food sources and how these different food sources require different teeth. By associating food with teeth and teeth with animal, the author gives examples that readers can easily follow and even visualize.

Plant matter can be leaves, fruits, roots, and grasses, and these different items require different kinds of teeth to eat them. Fruits can be pulped, but grasses might need to be eaten twice. Some animals regurgitate food in order to chew it a second time. For carnivores, meat must be sliced and chewed, and it tends to be easier to digest; however, teeth might need to be specialized for attacking and immobilizing prey.

Teeth and mastication interact with the digestive system. More chewing exposes more nutrients to the digestive system, but it also tends to make food move faster through it. For certain foods that are not digestible directly, slow movement through the digestive system encourages more fermentation. With ruminants, for examples, grasses are chewed and then passed through multiple stomachs. It's important to "choose" the appropriate amount of chewing so that the digestive system gets the optimal input.

The teeth of animals are therefore useful for infering the diet of animals. Primates that husk large fruits have larger incisors. Grazers that eat higher volumes of grasses have broader incisors than more selective grazers. Cats have sharper canines than dogs. Some animals have tusks, and some animals use their teeth for digging or grooming. Teeth give helpful clues to the characteristics of these animals.

Teeth before the mammals

This chapter is a historical summary of teeth. The author lists examples and includes visual aids to help the reader follow along.

There are two possibilities of the origin of teeth for mammals, either the outside-in or the inside-out hypotheses. In the outside-in possibility, teeth would have originated as sharp bony plates near the mouth, similar to shark scales. Ostracoderms are such ancient fish with these bony scales. In the inside-out hypothesis, conodonts had teeth-like things in their throats to help process food, and these proto-teeth would have migrated into the mouth.

Moving forward through time, the trend as been for teeth to migrate to the jaw and attach themselves there. Fish (and reptiles to a lesser extent) can have teeth spread out through the mouth and sometimes in the throat. But mammals keep their teeth at the margin of the mouth.

How teeth are attached also differ. Fish have their teeth attached to the bone. Crocodiles have teeth growing out of bone sockets. If I understand right, mammals have some kind of ligament and bone sockets that fill themselves out in adulthood.

Over time the trend is towards fewer and more specialized teeth, but there is a lot of variety out there. Fish have lots of different types of teeth. While crocodiles just have lots of pegs, some dinosaurs had what the author calls "dental division of labor", different types of teeth in the same mouth.

The evolution of teeth in mammals

The chapter starts with an interesting fact: the amniotes evolved an egg that could be laid and hatched on dry land. These amniotes split into groups distinguished by the amount of holes on the sides of their skulls; anapsids without holes, synapsids with one (mammals), and diapsids with two (reptiles). The pelycosaurs (a group that includes the reptile-like creatures with a fan on their backs) were synapsids that were successful until the arrival of the dinosaurs. Wikipedia has some more information about this; birds are diapsids

What made mammals develop specialized teeth was the higher energy requirements of having to generate heat and supply energy to brains needing to analyze better senses of taste and smell. Throughout the chapter the impression is that mammalian teeth evolution was a free-for-all.

Mammalian skulls and jaws evolved to better attach muscles for chewing. Muscles needed to work together so that chewing motions could go from side to side and shift around. The jaw joint evolved into the temporomandibular joint (TMJ), which had the side benefit of letting a few bones split off and become the bones of the inner ear.

Another evolution was the hard bony palate, which makes separating the nasal passage a surer thing. It also helps food move around the mouth with some help from the tongue.

When dinosaurs ruled the land and mammals became small nocturnal insectivores, their teeth specialized to eat insects. This changed things and I think wikipedia even called this the nocturnal bottleneck of mammalian evolution.

The rest of the chapter is spent talking about the evolution of particular tooth shapes. The takeaway message for me is that there was a lot of change. Certain shapes appeared and dissappeared forever, like the saber-toothed cat's cheek teeth the shape of single long horizontal blades. I think that since mammalian teeth are so specialized (and also crucial to breaking down food extra efficiently) it makes sense they'd evolve quickly.

I enjoyed this chapter. I've been drawn to dinosaur teeth because they're large, monstrous and cool, like the author alluded to in the introductionary chapter. Nevertheless the adult me now appreciates the subtleties of mammalian teeth.

Mammalian teeth today

The author jumps back to the present, after having spent two chapters in history.

Mammalian teeth are diverse because properly chewed food is crucial to satisfy the energy needs of endothermy. When at rest, warm-blooded animals use about 5 to 10 times the amount of energy of a cold-blooded animal.

This chapter goes over the different teeth of the marsupials, and the placental xenarthrans, afrotherians, laurasiatherians, and euarchontoglirians. The laurasiatherians have the most variety in teeth and include whales, pigs, cats, dogs, horses, and shrews. The euarchontoglirians include rodents and primates and have the greater number of species despite having smaller teeth variation; rodents have chisel incisors and flat cheek teeth.

Human teeth and their history

The chapter's message seems to be that most of the difference is between hominid teeth and the rest of the primates. Chimpanzees and gorillas have large canines for fighting and cheek teeth for leaves and fruits, while humans are more omnivorous.

Apparently humans have thicker enamel, but what this means is unclear.

The author says that current human diet has changed faster than evolution can keep up. Humans have a lot more dental caries than other primates and other animals. Humans also have overbite because of smaller jaws, likely because we are chewing softer foods. Apparently the changes in caries and overbite can happen in a generation from when foragers adopt a western diet.

I found it interesting to hear that human foragers don't have the overbite we do, and their front teeth "occlude" better. Each time I read that paragraph, I push my jaw forward and try to figure out how "normal" teeth would fit together.

Endless forms

This is just a summary of things already dicussed in the book, such as the evolution of teeth, how they're made, and the relationship between diet, circumstance and teeth.