Evolution of human teeth

Evolution of human teeth and how did cusps originate?

Several theories have been proposed to account for the origin of cusps and complex teeth patterns. 

The concrescence theory assumes that mammalian teeth originated by the fusion of originally separate reptilian teeth. As there were no discovered transitional forms of these teeth, this theory is not accepted.

Another theory is the differentiation theory. This hypothesis says that even the most complicated mammalian molar has originated from a simple conical reptilian tooth.

The tritubercular theory and Cope-Osborn theory are examples of modified differentiation theory. It sets out to organize the homologies of cusps, explain where the cusps came from, assigns terms to them, and traces their evolutionary history. It is much more successful with the terms than with evolution.

The tritubercular theory was first put forth by the American paleontologist Edward Drinker Cope in 1875 and modified by Henry Fairfield Osborn in 1888, known as the Cope –Osborn theory. This explains how humans have their teeth in different sizes and shapes.

The haplodont conical teeth of reptiles evolved to form molars consisting of a series of in-line cusps in the evolutionary higher animals. In the maxilla, the oldest cusp representing the original reptilian conical tooth is the protocone (Greek, proto meaning   first in time). To the mesial of the protocone, the cusp that appeared is the paracone (Greek, para meaning at the side of). Just distal to the protocone is the metacone (Greek, meta meaning in the midst of or after). In the mandible or lower jaw, the process is identical. The naming is similar, except for the addition of the suffix -id. (Greek, id meaning connected with). The terms for cusps in the lower are thus protoconid, paraconid, and metaconid.

As in reptiles, the upper and lower teeth alternate in the jaws, allowing them to interdigitate on jaw closing. Thus, so far in terms of function in evolution, this simple cone-shaped tooth has been used for piercing and tearing. The jaws function in a simple hinge fashion.

In evolution of higher animals, the accessory cusps increased in size and they rotated in relation to the principle cusp to form triangular teeth. They also acquired connecting ridges called lophs (Greek, lophs meaning crest or ridge). In the upper jaw, the protocone displaced to the lingual with the base of the triangle to the buccal side.

In the lower jaw the protoconid remained to the buccal with the base of the triangle to the lingual. The triangles were therefore reversed in relation to each other. This allowed interdigitation between the triangles in function. These trigon or /trigonid teeth thereafter could function by puncturing food as before and also by the shearing action of the crests acting against each other.

The next step in evolution was the acquisition of the distal talonid (heel) on the distal of lower molars. This formed a surface for crushing food against the surface of the protocone in the upper. These cuspal patterns form tribosphenic molars (Greek, meaning to rub / a wedge) that are ancestral to the cuspal patterns of primate molars, including humans.  

The upper molar acquired additional cusps: the hypocone on the distal aspect of the molar, completing its appearance as a four-cusped tooth. Small cusp additions to the mesial and distal of the trigon are the protoconule and metaconule. Cusps along the buccal margin are of the following styles: parastyle, mesostyle, and metastyle.

The lower molar talonid gave rise to three cusps, the hypoconid, the entoconid, and the hypoconulid producing a six-cusped molar.

In humans, the paraconid in first molars is lost, producing a five-cusped tooth. In lower second molars, the hypoconulid is lost, producing a four cusp molar. The five-cusp molar occurs in anthropoid apes and man. The Y-5 pattern named after the Y-shaped pattern of fissures separating the five cusps represents very conservative genetically determined patterns still present in 90% of lower first permanent molars of modern Homo sapiens.

Why do the teeth features change and acquire new dimensions in evolution?

There is no single explanation of the mechanisms in dental reduction. The advantage of having smaller, weaker, and more friable teeth is as follows.

1. Dental reduction is a result of facial reduction.

2. Smaller teeth conserve precious biological resources; therefore, smaller teeth are advantageous.

3. As early humans acquired culinary skills, teeth ceased to have survival value and they are just dwindling away due to the Probable Mutation Effect (PME). This hypothesis attributes the reduction of teeth in size and number to cultural factors.

4. Food preparation technology may have eliminated the forces that previously maintained tooth size.

5. Discontinuation or minimizing paramasticatory functions--such as defense, processing animal skins.

6. Due to Bio-cultural and periodic migration of the evolving humans they began to acquire change in dimension and shape of teeth.

Difference in racial features of human teeth: 

Central incisors:

1. Shoveling: The proximal marginal ridges are prominent in the lingual/ palatal surface of the anterior teeth turning the appearance of the teeth similar to that of the shovel. This trait is common in maxillary arch. Common in East Asian origin population and Native American Indians. They are especially prominent in Eskimo/Inuit who are descendants of Siberians about 4,000 years ago. The most common view regarding its function is that possibly it strengthens the structure of the incisor.
2. Double shoveling: The proximal marginal ridges are prominent in the labial and lingual/ palatal surface of the anterior teeth turning the appearance of the teeth similar to that of the shovel. This trait is common in maxillary arch. Common in East Asian origin population and Native American Indians. They are especially prominent in Eskimo/Inuit who are descendants of Siberians about 4,000 years ago.
3. Winging: An indirect trait where there is a bilateral rotation of the distal margin of the maxillary central incisors. This leads to the appearance of the incisal edge as V shaped from the incisal aspect. Common in east Asian origin population and native American Indians

Lateral incisors:

1. Shoveling:
2. Double shoveling:
3. Interruption grooves: A developmental groove on the cingulum of this teeth often extending in to the root, more common in the maxillary arch. Common in east Asian origin population
4. Peg shaped teeth: Is a common variation
5. Diminutive and T forms: Is a common variation

Canine:

1. Extra or additional prominent ridge or tubercle: observed between the lingual and distal marginal ridge in the lingual or palatal aspect. This is more common in maxillary arch.
2. Two rooted lower canines: Common in European, West and South Asian origin population.

Premolars

1. Premolar accessory lingual cusp: A common trait with two or more cusp in the lingual aspect of the premolar. This trait is more common in mandibular second premolar.
2. Enamel extension: Extension of enamel in to the cervical third of root as a ledge. Common in east Asian origin population
3. Accessory ridge: An additional ridge seen between the triangular ridge of the buccal cusp and proximal marginal ridge.
4. Accessory marginal tubercle: Seen on the proximal marginal ridges and more common in maxillary arch.
5. Odontome like: A conical projection or a tubercle on the occlusal surface with enamel, dentin and pulpal extension. It is a rare trait common in east Asian origin population
6. Tomes root: 2 roots seen in lower 1^st premolar
7.  'Uto-Aztecan' upper premolar: It is a bulge on the buccal cusp of upper first premolar, that is only found in Native American Indians, with highest frequencies of occurrence in Arizona, USA.

Upper first molar:

1. Enamel extension: Extension of enamel in to the cervical third of root as a ledge. Common in east Asian origin population
2. Carabelli’s feature/ cusp/ tubercle: A tubercle or a cusp lingual to the mesiopalatal cusp. May be rudimentary to a well-developed cusp. Common in European, West and South Asian origin population. This is the non-metric trait best known to dentists. This dental trait, the tuberculus anomalus, was first described in 1841 by Georg Carabelli, who was court dentist to the Austrian Emperor Franz. The trait when present is located on the mesiolingual corner of upper first permanent molars and second deciduous molars. The trait is found infrequently in upper second permanent molars. When present as a cusp, that feature is variable in size. The trait may express itself as a pit. The cusp version may rival the main cusps in size. In its pit version, it may appear as a small ridge, pit, or furrow. The determinants of the Carabelli trait appear early in embryogenesis; some speculate that it is quite 'old' phylogenetically. There is a demonstrated relationship between the Carabelli trait and distolingual cusp size in upper molars and between the Carabelli trait and the protostylid of lower molars. The Carabelli trait has a high incidence of expression in Caucasoid populations with a low level of frequency of shovel-shaped incisors. Conversely, the Carabelli trait is seldom fully expressed in Mongoloid populations, which possess a high frequency of shovel-shaped incisors.
3. 5 Cusp: On the occlusal surface, a tubercle on the distal marginal ridge.
4. Mesial marginal complex: A tubercle observed on mesial marginal ridge.
5. 2 rooted upper molar

Other upper molar variants:

1. Three cusp 2^nd / 3^rd molar: Distopalatal cusp (hypocone cusp) being absent. Common in European, East, West and South Asian origin population
2. Parastyle: A tubercle on the buccal surface of molars.

Lower first molar:

1. 6 Cusp form: An extra cusp seen on the distal aspect between the distolingual and distal cusp.
2. 7-cusp form: In addition to the 6^th cusp, another cusp between the mesiopalatal and distopalatal cusp.
3. Occlusal groove configuration: May appear as X, Y or +.
4. Enamel extension
5. Protostylid: A tubercle on the mesiobuccal cusp.  The protostylid is a feature on the buccal side of the lower molar crown ranging from a pit in the buccal grove, through a furrow to a prominent cusp. It is seen especially on the first or third permanent molars or in deciduous lower second molars. It may be present in up to 40% of a population.

Other lower molar variants:

1. 5 cusped 2^nd molar: Presents a distal cusp as in first molar, referred as hypoconulid.
2. 3 rooted molar: Common in east Asian origin  and North American Indian population
3. One rooted molar: Common in east Asian origin population

Prehistoric Americans display many fewer variations in their dental morphology than do Eastern Asians. Turner calls these features as sinodonty. This pattern of dental features includes shovel-shaped incisors, single-rooted upper first premolars, triple rooted lower first molars, and other attributes.

The other variants includes: canine ridges, cingulum features of the anterior teeth, variation in the main cusps of the upper molars, metaconule of upper molars, lower premolar cusps, cusp number on lower molars, molar groove and fissure patterns, deflecting wrinkle and trigonid crest, enamel extensions, and root number in molar teeth.

Australian Aborigines have the largest teeth. Europeans and Asians are at the smaller end of the scale.

The greatest sexual dimorphism is in lower canines followed by upper canines. Premolars are the least dimorphic.

Terminologies in Tooth Morphology

Terminologies in description of dentition and teeth

Homodont: Teeth are uniformly of similar shape in a dentition as in alligators.

Heterodont dentition:  Teeth are regionally specialized into classes of different types as in humans and primates.

Monophyodont dentition: The animal has a single generation of teeth only as in walrus and seals.

Diphyodont dentition: The condition of having two generation of teeth as in humans.

Polyphyodont dentition: Many generations of teeth as seen in many reptiles.   

Anodont: Absence of teeth, but they may have teeth or teeth like structures during the course of development as in certain species of whales and anteaters.

Bunodont : (Greek, meaning a mound or hill) teeth have cone-shaped tubercles or cones; they are low height occlusocervically with well-developed roots. An example is the posterior teeth in the pig.

Selenodont: (Greek, meaning the moon) teeth have cusps transformed into half-moon shapes. The teeth themselves are elongated mesiodistally. The concave side faces laterally in the upper jaw; lingually in the lower jaw. The grinding stroke is thus from the outside inward and from the inside outward. An example is in the cheek teeth of sheep.

Sectorial:  (Latin,  secare meaning to cut) teeth are blade-like teeth adapted to cutting the diet into pieces and swallowing them whole. A specialized variant in carnivores are the carnassials which consist of the last premolar in the upper jaw and the first molar of the lower jaw.

Lophodont : (Greek,  meaning a crest) molars are ridged teeth that have transverse ridges as in the tapir

Bilophodont : Refers to molars that have two sets of transverse ridges.

Polylophodont molars have many ridges as seen in the elephant molar. The power stroke is where the lower molars slide forward against the upper molars.

Brachydont (Greek, meaning a short) teeth have low crowns and well-developed roots. This condition is seen in humans.

Hypsodont (Greek, meaning height) teeth have long crowns and short roots as seen in the horse. It them, it is a function adaptation for continuous wear sustained by chewing grass with a high abrasive silica content.

Haplodont (Greek, meaning a simple) teeth have simple crowns and roots, as seen in the dolphin.

Tusks are incisors or canines of continuous growth that protrude beyond the lips when the mouth is closed. They include the following: (1) The incisors of the Elephant and Hippopotamus; (2) The left incisor of the Narwhal; (3) Canines of the Wild Boar, Wart Hog and Walrus. 

Differentiating Right and Left Human Permanent Teeth

Difference between right and left teeth Permanent teeth:

Maxillary central incisors:

When viewed from the labial aspect, the distal incisal angle is more rounded than the mesial.

In many specimens, a cross-section mid-root reveals a right triangle outline with the hypotenuse is toward the mesial.

Maxillary lateral incisors:

The distoincisal angle is more rounded than the mesial incisal angle.

The tip of the root may incline distally, but this is not a consistent finding.

Mandibular central incisors:

The symmetry of this tooth makes a judgment on right and left unreliable.

Mandibular lateral incisors:

Two significant features assist in identification, even in a worn tooth.

-The incisal edge is 'twisted' relative to a line passing from the labial to the lingual anticipating the curvature of the dental arch.

-The cingulum will be shifted toward the side from where the tooth has come.

Maxillary and mandibular canine

-The distal surface is fuller and more convex than the mesial surface.

Maxillary first premolar                                                                                          

Two distinctive traits help is distinguishing right and left.

-The mesial developmental depression and the mesially displaced lingual cups tips are consistent clues for determining right and left.

-When well defined, the mesial marginal ridge is also a clue to right and left          

Maxillary second premolar

The one consistent clue to right and left is the lingual cusp tip which is shifted mesially.

Mandibular first premolar

-The larger distal occlusal fossa and mesial lingual marginal developmental groove are consistent clues to right and left.

-The distal surface has a longer radius of curvature than does the mesial surface.

Mandibular second premolar

In the two cusp version, the lingual cusp tip is shifted mesially.

In the three cusp version, the larger of the two lingual cusps is to the mesial.

Maxillary first molar

-The large mesiolingual cusp,

-Single large lingual (palatal) root,

-Carabelli trait makes distinguishing right and left easy                                                        

Maxillary Second Permanent Molar

-The large mesiolingual cusp

-Small distolingual cusp

-Three roots (larger palatal) make distinguishing right and left easy.

Mandibular First Permanent Molar

The cervical ridge on the buccal aspect, the two buccal cusps located to the buccal along with the distal cusp provide identification of the buccal aspect.

The distal cusp is the smallest and is displaced along the occlusal aspect. These features make possible identification of right and left.

Mandibular Second Molar

When viewed occlusally, there is a distinctive prominence of enamel at the mesiobuccal--a feature shared with first deciduous molars.

Examined from the mesial or distal, the lingual surface has its height of curvature midway between the occlusal and the cervical line.

On the buccal surface, the height of curvature is at the gingival third--near the cervical line. 

Enamel-3

Answers for Enamel -3

Maximum thickness of enamel in deciduous dentition in molar cusps	1.3 mm
Volume of water in enamel	5 to 10%. This is crucial as a lot of ions cross in to tooth in liquid medium
Water in enamel is in form of	hydration layer coating enamel crystals and filling enamel defects
Prism patterns in human enamel	Three types are seen - Pattern 1 - circular; pattern 2 - prism arranged in parallel rows; pattern 3 - arranged in staggered row "key hole" pattern
Basic unit of enamel	Enamel prism
Organic material are often accommodated along enamel prism boundary because of	Microporosity
The predominant pattern of arrangement of enamel rods in human is	Type III or key hole pattern
Key hole pattern or type III pattern is made by	Large ameloblasts
Small ameloblasts produce	Pattern II or parallel arranged fashion of enamel crystallites
Enamel between prisms are called	Interprismatic enamel
Why does interprismatic enamel have different optical property	Owing to crystal deviation by 40 to 60 degrees in orientation. Abnormal abrupt change of orientation responsible for refraction of light
The angle at which interprismatic crystallites cross prisms is	60 degrees
Stacks of ______enamel prisms follows are unidirectional after which they follow different direction	10 to 13 layers
Transversely sectioned zones in Hunter-Schereger bands are	Diazones
Longitudinally sectioned zones in Hunter-Schereger bands are	Parazones
The average width of Hunter-Schereger bands are	50 micrometers
The angle between diazones and parazones of enamel is about	40 degrees
Where will be Hunter -Schreger bands be absent	when all enamel rods runs in same direction, as in outermost 25% of enamel, the bands will be absent
width of non-prismatic enamel in a newly erupted teeth	Deciduous 20 - 100 micrometers and permanent teeth 20 to 70 micrometers. Enamel crystallites are aligned at right angle to surface and hence parallel to each other.
why is the outer most layer of enamel highly mineralized	Remineralization by saliva; absence of prism boundaries.

Enamel-3

1. Maximum thickness of enamel in deciduous dentition in molar cusps...
2. Volume of water in enamel
3. Water in enamel is in form of
4. Prism patterns in human enamel
5. Basic unit of enamel
6. Organic material are often accommodated along enamel prism boundary because of
7. The predominant pattern of arrangement of enamel rods in human is
8. Key hole pattern or type III pattern is made by
9. Small ameloblasts produce
10. Enamel between prisms are called
11. Why does interprismatic enamel have different optical property?
12. The angle at which interprismatic crystallites cross prisms is
13. Stacks of ______enamel prisms follows are unidirectional after which they follow different direction
14. Transversely sectioned zones in Hunter-Schereger bands are
15. Longitudinally sectioned zones in Hunter-Schereger bands are
16. The average width of Hunter-Schereger bands are
17. The angle between diazones and parazones of enamel is about
18. Where will be Hunter -Schreger bands absent?
19. Width of non-prismatic enamel in a newly erupted tooth
20. Why is the outer most layer of enamel highly mineralized?

Tooth Morphology-1

ANSWERS FOR YESTERDAYS QUESTIONS

Questions	Answer
The postnatal period of development of deciduous dentition spans about	30 months. The second molars erupt at about 24 to 26 months and root formation completes at least 6 months later.
The primary dentition remains intact till the child is	6 years. The deciduous mandibular incisors shed usually after the first molars emerge in to the oral cavity and is usually about 6 years of age. Moreover, once the 6-year molar erupts in to oral cavity the dentition is transformed to the mixed dentition stage.
The period of mixed dentition lasts from	6 to 13 years. The mixed dentition stage is initiated once the permanent first molar erupts (6 years) to the shedding of deciduous mandibular second molar (12 to 13 years).
Mixed dentition begins with	Eruption of permanent first molars. Eruption of permanent first molars is the first occurring event in the oral cavity and usually simultaneous with the resorption/ shedding of deciduous mandibular incisors.
The teeth that replace exfoliated deciduous teeth are the	Succedaneous teeth. The Succedaneous teeth are maxillary and mandibular incisors, canine and premolars.
The teeth that do not have predecessor teeth are	The correct answer is the maxillary and mandibular molars
Pulp chamber is the space occupied by	Pulp in crown. Pulp in root is the pulp canal
Pulp cavity is made up of	Pulp canal and chamber
The bone of tooth socket is the	Alveolus. Lamina propria is a radiological image of the periodontal ligament fibers whereas maxilla and mandible hold the alveolus
The teeth that have their mesial surfaces in contact is the	Maxillary and mandibular central incisors
The teeth that has no distal contact area is the	Deciduous second molar
An elevation on the crown portion of a tooth on its occlusal surface is the	Cusp
An elevation on some portion of crown produced by abnormal enamel deposition is the	Tubercle
The lingual lobe of anterior teeth are represented as	Cingulum
A linear elevation on any surface of tooth is a	Ridge
The ridge that descends from cuspal tip towards the central part of the occlusal surface in maxillary molars are the	Triangular ridge. Marginal ridge runs along the margin of cusp while transverse ridge and oblique ridge run across the molar tooth
The triangular ridge resembles a _________side of a triangle	Two
A union of two triangular ridge is called as the	Transverse ridge
The union of triangular ridge of distobuccal cusp and distal ridge of mesiolingual cusp is the	Oblique ridge
A irregular depression on any surface of the crown is a	Fossa

Tooth Morphology -1

Questions

1. The postnatal period of development of deciduous dentition spans about
2. The primary dentition remains intact till the child is
3. The period of mixed dentition lasts from
4. Mixed dentition begins with
5. The teeth that replace exfoliated deciduous teeth are the
6. The teeth that do not have predecessor teeth are
7. Pulp chamber is the space occupied by
8. Pulp cavity is made up of
9. The bone of tooth socket is the
10. The teeth that have their mesial surfaces in contact is the
11. The teeth that has no distal contact area is the
12. An elevation on the crown portion of a tooth on its occlusal surface is the
13. An elevation on some portion of crown produced by abnormal enamel deposition is the
14. The lingual lobe of anterior teeth are represented as
15. A linear elevation on any surface of tooth is a
16. The ridge that descends from cuspal tip towards the central part of the occlusal surface in maxillary molars are the
17. The triangular ridge resembles a _________side of a triangle
18. A union of two triangular ridge is called as the
19. The union of triangular ridge of distobuccal cusp and distal ridge of mesiolingual cusp is the
20. A irregular depression on any surface of the crown is a