Teeth
From EvoWiki
Teeth (singular, tooth) are structures found in the jaws of many vertebrates, with the exception of extant birds where teeth have been lost to lighten body weight. The primary function of teeth is to tear and chew food, and in some animals, particularly carnivores, for fighting and/or defense.
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Anatomy
The tooth can be divided into two parts:
- Crown - the part above the gum, which is subject to wear
- Root - the part embedded in the gum and/or jaw
Adult teeth naturally darken with age as the pulp within the tooth shrinks and dentin is deposited in its place.
Morphology
Teeth are among the most distinctive features of mammal species and fossils. Paleontologists use them to identify fossil species and their relationships. Mammalian teeth are classified as one of four types, (most rostral listed first):
- Incisors - blade shaped, for cutting and clipping.
- Canines (or cuspid) - conically shaped with a pointed tip, used for seizing and piercing.
- Premolars (or biscuspid) - flattened with prominent crest, used for both chewing and tearing
- Molars - flattened, used for crunching and grinding
The shape and size of the teeth present in an animal is related to the animal's diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing the food before swallowing. Meat is easier to digest but harder to tear, so Carnivores need large canines and pointed premolars to kill and tear meat.
The teeth of some animals have evolved into highly modified structures, such as the tusks of elephants, which evolved from incisors.
Development
Most vertebrates develop new teeth throughout their life, so are described as polyphyodont. Sharks, for example, grow a new set of teeth every two weeks. New teeth develop beneath the old and as the new tooth grows, the old one is pushed out and is then replaced. Most mammals (with the exception of toothed whales) only have two sets of teeth throughout their life, and so are described as diphydodont. Some teeth, such as those of rodents, continually grow and wear away continually through the animal's gnawing, maintaining approximately constant length.
Wisdom Teeth and Relevance to Evolution
Wisdom teeth are third molars that usually appear between the ages of 17 and 24 (although they may appear when older, younger, or may not appear at all). Due to the decreased jaw size of modern humans compared to those of early hominids (associated with a more refined diet), the jaw is often too crowded to allow the wisdom teeth to emerge. They are commonly extracted when they affect other teeth (this impaction is colloquially known as "coming in sideways").
Wisdom teeth are considered by many scientists to be a vestigial structure, and the tendency towards absence of this structure (known as hypodontia) is considered a step forward in human evolution.
References
- Liem K.F, Bemis W.E, Walker W.F Jr, and Grande L. (2001), Functional anatomy of the vertebrates : an evolutionary perspective, Harcourt College Publishers, Third Edition, pp 537-40
