Teeth are the appendages located on the doorway of the alimentary canals of vertebrates. Teeth are highly mineralized and they serve the functions of gripping and food processing. Humans also use teeth in phonetics processing. The prototypes of the present vertebrae teeth were tooth – like projections found on ancient jawless fish that lived around 460 million years ago in the Orduvum period (1). The teeth like projections named odontodes on the body surface of ancient fish served the functions of protection, perception and sensation. These odontodes encroached on the mouth cavity in the course of evolution over 500 million years to form buccal teeth that covered the entire mouth cavity and later became restricted to the margins of the jaw cavity (2).
The endoderm and the ectoderm are the tissues in which the teeth originate in most vertebrates through a process of invagination. Some teeth also have a mixed origin of both the ectoderm and ectoderm. The tooth creating capacity of neural crest cells of the ectoderm and the endoderm are therefore the major factor that influenced evolution of teeth rather than the epithelial tissues(3). Mutations in the genome of ancient fish are the factors that resulted in the invasion of the odontodes into the mouth cavity. The development of different phenotypes of vertebrate teeth resulted from Mutations in the genome of ancient vertebrates through years of evolution. The process of teeth like projections on the body of vertebrates evolving into teeth is clear in some type of sharks where the scales present on the body of the shark change to form teeth in their mouth cavity. The dietary habits and other ecological adaptations made the vertebrates teeth to evolve into many physiological shapes like canines, molars, premolars and incisors (4).
In the course of evolution, the number of teeth per dentition in vertebrates decreased substantially. This decrease in the number of teeth is called oligodonty. The variations in the number of teeth represent an important factor in the diversity brought by evolution to vertebrates.Evolution also led to increase in the morphological characteristics of vertebrate teeth. This characteristic of variability in morphological characteristics is called heterodonty. Heteredonty was present in some ancient mammal-like reptiles relatives of mammals. In the course of evolution, some vertebrates lost the ability to replace continually teeth in their lifetime (5).
Evolution led to loss of teeth in some vertebrates like birds and in some types of whales, a phenomenon called adontia while some vertebrates in the course of evolution retained homodont. For example, dolphins have a set of 200 similar teeth in their dentition (6). Changes in morphology and the number of teeth was a major result of evolution in mammals. Mutations that occurred during evolution resulted in complexity in dentition of vertebrate teeth. For example, some mammals like rats have no incisors and premolars while Ruminants like sheep have no canines and incisors on their upper jaws.
These complexities occurred due to mutations in tooth genes over the course of evolution (7). Mastication and diet were key factors in guiding the process of development of complexity in vertebrate teeth since there is correlation in the type of vertebrae teeth and diet. For example, Cyonodata reptile mammals changed their dentition from one that was adapted to catching their prey to one that was better suited for chewing food (8),
The most notable morphological feature of vertebrate teeth is the variability in the crowns of the molars and premolars that appear in different and complex shapes. The basic units of the crowns are cusps or conical projections on the crown of molars and premolars, the number morphology and orientation of the cusps vary greatly, even in the same animal, depending on the feeding habits. These cusped teeth originated from single cusped teeth of ancient mammal like reptiles. The cusps grow on the crown of molars and premolars and are responsible for the great variability there is in teeth even on the same animal and species of vertebrates. The cusps grow the top of the teeth down and have dentine and enamel in them. The evolution of the molar and premolars in vertebrate teeth in mammals follows a system of complexity where in the Pertmain period the ancient reptile mammals had only single cusped teeth but evolved later to cause the development of multi cusped teeth having many patterns (9)
Mammalian jaws and teeth evolved to create occlusal surfaces on the crowns of teeth adapted to varied diet. These cusps are important in aiding the better mastication of food, making mammals have the ability to feed on varied diet. For example , Trocodont mammals had teeth with cusps in a straight angle and other surrounding cusps on a central arrangement(10).This arrangement improves the ability to chew food which is still present in mammals to date but more complexed into an array of three cusps on the molars and pre molars. Evolution has therefore resulted in diversity and complexity in the phenotype of vertebrate teeth. The understanding of the evolution of teeth is important in providing insights in treatment and prevention of teeth malformations and diseases (11).
Notes
- Kousso Lakou, Despina & Margaaritis Lukas. A curriculum vitae of teeth : evolution ,generation ,regeneration international journal of biological sciences (2005) v 5 ,226
- Tea ford Mark, Moya Ferguson, development function and evolution of teeth Cambridge: Cambridge university press ,2007.page 152
- Donoghue Philip 2002 evolution of development of the vertebrae dermal and oral skeletons unraveling concepts regulatory theories and homologies paloeobiology 2002 vol. 28 (4) 475
- Thesleff I the genetic basis of tooth development and dental effects American journal of medical genetics (2006) vol 140, 2530
- Huysseune A & Sire J. 2010 revised hypothesis on the evolution origin of the vertebrate dentition journal of applied ichthyology (2010) volume 26 (2 )152
- Koussoolakou Despina & Margaaritis Lukas. A curriculum vitae of teeth : evolution generation regeneration international journal of biological sciences (2005) v 5 ,225
- Donoghue Philip evolution of development of the vertebrae dermal and oral skeletons unraveling concepts regulatory theories and homologies paloeobiology (2002) vol. 28 (4 ) 476
- Huysseune A & Sire J revised hypothesis on the evolution origin of the vertebrate dentition journal of applied ichthyology (2010) volume 26( 2 ) 9153
- Donoghue Philip 2002 evolution of development of the vertebrae dermal and oral skeletons unraveling concepts regulatory theories and homologies paloeobilogy (2002) v 28 (4),500
- Koussoolakou Despina & Margaaritis Lukas. A curriculum vitae of teeth : evolution generation regeneration international journal of biological sciences (2005) v 5 ,226
- Donoghue Philip 2002 evolution of development of the vertebrae dermal and oral skeletons unraveling concepts regulatory theories and homologies paloeobiology (2002) v. 28 (4 )506
References
Donoghue Philip evolution of development of the vertebrate dermal and oral Skeletons Unraveling concepts regulatory theories and homologies Paloebiology (2002) v 28, (4) (474-507)
Huysseune A. & Sire J.A revised hypothesis on the evolution origin of the Vertebrate Dentition journal of applied ichthyology (2010) volume 26 (2), (99152 – 9155)
Koussoulakou ,Despina & Margaaritis, Lukas. Curriculum vitae of teeth evolution Generation regeneration international journal of biological sciences (2009) vol 5 (226 -243)
Teaford, Mark, Moya, Fergusson Development function and evolution of teeth Cambridge; Cambridge university press, (2009) page 152-155
Thesleff I. the genetic basis of tooth development and dental effects American Journal of Medical genetics (2006) vol 140 (2530-2535)