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.