The ASUDAS (Arizona State University Dental Anthropology System) is a reference system for collecting data on

human tooth The human teeth function to mechanically break down items of food by cutting and crushing them in preparation for swallowing and digesting. As such, they are considered part of the human digestive system. Humans have four types of teeth: incis ...

morphology and variation created by

Christy G. Turner II Christy G. Turner II (November 28, 1933, Columbia, Missouri – July 27, 2013, Tempe, Arizona) was an American anthropologist known for his research on dental anthropology, perimortem taphonomy, and his theories about the populating of the Amer ...

, Christian R. Nichol, and G. Richard Scott. The ASUDAS gives detailed descriptions for common crown and root shape variants and their different degrees of expression. It also comprises a set of reference plaques illustrating dental variants as well as showing their expression levels in 3D. The ASUDAS was designed to ensure a standardized scoring procedure with minimum

error An error (from the Latin ''error'', meaning "wandering") is an action which is inaccurate or incorrect. In some usages, an error is synonymous with a mistake. The etymology derives from the Latin term 'errare', meaning 'to stray'. In statistics ...

in order to warrant comparability between data collected by different observers. The ASUDAS currently comprises a set of 42 dental variants that can be observed in the permanent adult dentition. The majority are crown and root shape variants, although the system also includes some

skeletal A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...

variants of the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...

and

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

. Most of the variants occur at different frequencies in human populations around the world. Examples of dental variants listed in the ASUDAS are

shovel-shaped incisors Shovel-shaped incisors (or, more simply, shovel incisors) are incisors whose lingual surfaces are scooped as a consequence of lingual marginal ridges, crown curvature or basal tubercles, either alone or in combination. Shovel-shaped incisors are ...

, Carabelli cusps, or

hypocone A cusp is a pointed, projecting, or elevated feature. In animals, it is usually used to refer to raised points on the crowns of teeth. The concept is also used with regard to the leaflets of the four heart valves. The mitral valve, which has two ...

Genetics

It is hypothesized that most of the dental variants listed in the ASUDAS are

heritable Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic informa ...

and selectively neutral and that the worldwide dental diversity was generated by random evolutionary processes consisting of

founder effect In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, using ...

s and

genetic drift Genetic drift, also known as allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene variants to disappear completely and there ...

. Several studies have also demonstrated that

genetic distance Genetic distance is a measure of the genetic divergence between species or between populations within a species, whether the distance measures time from common ancestor or degree of differentiation. Populations with many similar alleles have sma ...

s across modern human populations derived from neutrally evolving

SNPs In genetics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently larg ...

microsatellite A microsatellite is a tract of repetitive DNA in which certain DNA motifs (ranging in length from one to six or more base pairs) are repeated, typically 5–50 times. Microsatellites occur at thousands of locations within an organism's genome. ...

s are highly correlated with distances derived from dental variants listed in the ASUDAS. Additionally, dental variation within populations decreases with increasing geographical distance from

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

, a signature also found in neutral genetic datasets as a result of a serial founder effect originating in Africa. Some dental variants listed in the ASUDAS are also likely to be associated with non-neutral evolutionary processes, such as

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...

. For example, shoveling and double-shoveling of upper first incisors and the presence of hypoconulids of lower second molars have been found to be linked to the

ectodysplasin A receptor Ectodysplasin A receptor (EDAR) is a protein that in humans is encoded by the EDAR gene. EDAR is a cell surface receptor for ectodysplasin A which plays an important role in the development of ectodermal tissues such as the skin. It is structural ...

gene (EDAR). EDAR is a functional genomic region and has a range of

pleiotropic Pleiotropy (from Greek , 'more', and , 'way') occurs when one gene influences two or more seemingly unrelated phenotypic traits. Such a gene that exhibits multiple phenotypic expression is called a pleiotropic gene. Mutation in a pleiotropic g ...

effects on ectodermally derived structures, such as

hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...

mammary gland A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in primat ...

s, and teeth, and is most likely under positive selection in

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...

n populations. It is possible that dental variants linked to EDAR are not direct targets of positive selection but rather 'hitchhiking' when selection acts on another phenotype.

Applications

The enamel which covers a tooth crown is the hardest tissue in the human body and generally well preserved in

taphonomic Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientist Ivan Efremov t ...

contexts, even when associated skeletal and DNA preservation is relatively poor. Therefore, dental morphological data collected with the ASUDAS are commonly used for inferring the

biogeographical Biogeography is the study of the distribution of species and ecosystems in geographic space and through geological time. Organisms and biological communities often vary in a regular fashion along geographic gradients of latitude, elevation, i ...

origin of deceased humans when no other biological markers are available. For example, ASUDAS data are typically used for identifying unknown individuals in

forensic Forensic science, also known as criminalistics, is the application of science to Criminal law, criminal and Civil law (legal system), civil laws, mainly—on the criminal side—during criminal investigation, as governed by the legal standard ...

cases, for examining past

migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...

and

mobility Mobility may refer to: Social sciences and humanities * Economic mobility, ability of individuals or families to improve their economic status * Geographic mobility, the measure of how populations and goods move over time * Mobilities, a contemp ...

in bioarchaeological contexts, and for reconstructing

hominin The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus ''Gorilla'' (gorillas). The t ...

phylogenies A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...

in paleoanthropological studies. As a rule of thumb, dental inferences about neutral genetic affinities based on many ASUDAS variants are more reliable than those based on only a few variants. Nevertheless, the best performance is achieved when using specific combinations of highly diagnostic variants, and not the full ASUDAS set.

References

{{Reflist Teeth Human migration Biological anthropology