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.
  1. Smaller teeth conserve precious biological resources;
    therefore, smaller teeth are advantageous.
  1. 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.
  1. Food preparation technology may have eliminated the
    forces that previously maintained tooth size.
  1. Discontinuation or minimizing paramasticatory
    functions–such as defense, processing animal skins.
  1. Due to Bio-cultural and periodic migration of the
    evolving humans they began to acquire change in dimension and shape of
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
    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
  3. Interruption
    : 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
    : Is a common variation
  5. Diminutive and
    T forms
    : Is a common variation
  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.
  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 of enamel in to the cervical third of root as a
    ledge. Common in east Asian origin population
  3. Accessory
    : 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 1st 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 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
Other upper molar variants:
  1. Three cusp 2nd
    / 3rd 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 6th cusp, another cusp between the
    mesiopalatal and distopalatal cusp.
  3. Occlusal
    groove configuration
    : May appear as X, Y or +.
  4. Enamel
  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 2nd
    : Presents a distal cusp as in first molar, referred as
  2. 3 rooted
    Common in east Asian origin 
    and North American Indian population
  3. One rooted
    : 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.


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