إدارة الموسوعة 1: مراجعة واحدة

2016-11-14T04:01:33Z

مراجعة واحدة

صفحة جديدة

'''Post-canine megadontia''' is an enlargement of the molars and premolars, which is found in early [[hominid]] ancestors such as ''[[Paranthropus aethiopicus]]''.<ref name="Smithsonian">{{cite web | title = ''Paranthropus aethiopicus'' | publisher = The Smithsonian Institution | url = http://humanorigins.si.edu/evidence/human-fossils/species/paranthropus-aethiopicus | accessdate = March 2011}}</ref>

== Archaeological Evidence ==
[[File:Paranthropus bonsei compared to Homo sapiens.png|thumb|''Paranthropus boisei'' (left) vs. ''Homo sapiens'' (right). The average post-canine tooth area for ''P. boisei'' was 756mm2 versus 334mm2 in ''H. sapiens''.]]''[[Australopithecus]]'', dated to have lived 2 to 3 million years ago, is the earliest hominid genus to demonstrate post-canine enlargement.<ref>{{Cite journal|title=McHenry, H. M. (1984), Relative cheek-tooth size in Australopithecus. Am. J. Phys. Anthropol., 64: 297–306. doi:10.1002/ajpa.1330640312|journal=|volume=}}</ref> The greatest canine enlargement was seen in ''[[Paranthropus boisei]]'' and a steady decline in post-canine size was observed in the ''Homo'' genus.<ref>{{Cite journal|last=Lacruz|first=Rodrigo S.|last2=Dean|first2=M. Christopher|last3=Ramirez-Rozzi|first3=Fernando|last4=Bromage|first4=Timothy G.|date=2008-08-01|title=Megadontia, striae periodicity and patterns of enamel secretion in Plio-Pleistocene fossil hominins|url=http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7580.2008.00938.x/abstract|journal=Journal of Anatomy|language=en|volume=213|issue=2|pages=148–158|doi=10.1111/j.1469-7580.2008.00938.x|issn=1469-7580|pmc=2526111|pmid=19172730}}</ref>

Studies of premolar size in hominid species that predate [[Australopithecus afarensis|''Australopithecus'' ''afarensis'']] show long, unicuspid teeth at the P3 location, while species dated after ''A. afarensis'' have been shown to have wider, bicuspid teeth at same location, which is hypothesized to show the beginnings of canine to premolar evolution in hominids.<ref>{{Cite journal|last=Delezene|first=Lucas K.|last2=Zolnierz|first2=Melissa S.|last3=Teaford|first3=Mark F.|last4=Kimbel|first4=William H.|last5=Grine|first5=Frederick E.|last6=Ungar|first6=Peter S.|date=2013-09-01|title=Premolar microwear and tooth use in Australopithecus afarensis|url=http://www.sciencedirect.com/science/article/pii/S0047248413001310|journal=Journal of Human Evolution|volume=65|issue=3|pages=282–293|doi=10.1016/j.jhevol.2013.06.001}}</ref>

''[[Homo floresiensis]]'', a hominid species from the late Pleistocene found in cave deposits in Liang Bua, Indonesia, shows a smaller molar size that is closer to the hominid lineage. However, the remaining teeth show similarities to the bigger tooth sizes of the earlier genuses ''Australopithecus'' and ''Homo.''<ref>{{Cite journal|last=Brown|first=Peter|last2=Maeda|first2=Tomoko|date=2009-11-01|title=Liang Bua Homo floresiensis mandibles and mandibular teeth: a contribution to the comparative morphology of a new hominin species|url=http://www.sciencedirect.com/science/article/pii/S0047248409000876|journal=Journal of Human Evolution|series=Paleoanthropological Research at Liang Bua, Flores, Indonesia|volume=57|issue=5|pages=571–596|doi=10.1016/j.jhevol.2009.06.002}}</ref>

== Timeline ==
[[File:Post-Canine Megadontia Timeline.jpg|thumb|A timeline depicting observed evidence of post-canine megadontia from 3 million years ago to the hominid species Homo floresiensis|left]]The origins of post-canine megadontia dates back to about 4-5 million years ago with the discovery of ''[[Ardipithecus ramidus]]'' in the Middle Awash region of Ethiopia. Distinctive features in ''A. ramidus'' such as dentition with reduced canines, the skull, hindlimb and forelimb reveals it to be the transitional species between chimpanzees and hominid species.<ref>{{Cite web|url=http://www.becominghuman.org/node/ardipithecus-ramidus-essay|title=Ardipithecus ramidus essay {{!}} Becoming Human|last=|first=|date=February 2009|website=www.becominghuman.org|publisher=Institute of Human Origins|access-date=2016-11-13}}</ref>

It was the arise of ''[[Australopithecus africanus]]'' found in several regions of South Africa ([[Taung Child|Taung]], Sterkfontein, Makapansgat) 2-3 million years ago that first demonstrated the enlargement of the pre-molars and molars. In terms of morphology, ''A. africanus'' shares many similar characteristics with ''A. Afrensis'' as well as other geneses in ''[[Paranthropus]]''. <ref>{{Cite web|url=http://www.becominghuman.org/node/australopithecus-africanus-essay|title=Australopithecus africanus essay {{!}} Becoming Human|last=|first=|date=February 2009|website=www.becominghuman.org|publisher=Institute of Human Origins|access-date=2016-11-13}}</ref>

The first species of ''Paranthropus aethiopicus'' was discovered in Lake Turkana, Kenya and its successor, ''[[Paranthropus robustus|Paranthropus robust]]'', was seen in the northern parts of South Africa; Swartkrans, Kromdraa and Drimolen (also referred to as the [[Cradle of Humankind]]).<ref>{{Cite web|url=http://www.becominghuman.org/node/human-lineage-through-time|title=The Human Lineage Through Time|last=|first=|date=February 2009|website=www.becominghuman.org|publisher=Institute of Human Origins|access-date=2016-11-13}}</ref> ''[[Paranthropus boisei]],'' the last species included in the genus Paranthropus, was first found in Olduvai Gorge, Tanzania and around Ethiopia and Kenya.<ref name=":5">{{Cite web|url=http://www.becominghuman.org/node/paranthropus-boisei-essay|title=Paranthropus boisei essay {{!}} Becoming Human|website=www.becominghuman.org|access-date=2016-11-13}}</ref> ''P. bosei'' was known for massive facial and dental bones and structure, primarily larger mandibles, molars, and premolars, which was an adaptation allowing them to consume hard plant foods with the ability of high force chewing. <ref name=":5" />

==Evolutionary Implications==

Postcanine megadontia is commonly associated with the repeated consumption of tough plant-like material, which can be referred to as "low-quality food stuffs".<ref name=":3">{{Cite journal|title=http://journals.plos.org/plosone/article/asset?id=10.1371/journal.pone.0023095.PDF|url=http://journals.plos.org/plosone/article/asset?id=10.1371/journal.pone.0023095.PDF|doi=10.1371/journal.pone.0023095.pdf}}</ref><ref name="hindawi" /> The substances were integral to the diet of extinct [[Hominidae|hominids]], and their [[Molar (tooth)|molars]] were subject to the constant occlusal attrition from the stress of vigorous [[mastication]].<ref name=":4" /> The development and evolution of this trait was characterized by a thick coating of [[Tooth enamel|enamel]] surrounding the molars and premolars, mitigating the detrimental effects of the tough diet.<ref>{{Cite journal|last=Lacruz|first=Rodrigo S|last2=Dean|first2=M Christopher|last3=Ramirez-Rozzi|first3=Fernando|last4=Bromage|first4=Timothy G|date=2016-11-13|title=Megadontia, striae periodicity and patterns of enamel secretion in Plio-Pleistocene fossil hominins|url=http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2526111/|journal=Journal of Anatomy|volume=213|issue=2|pages=148–158|doi=10.1111/j.1469-7580.2008.00938.x|issn=0021-8782|pmc=2526111|pmid=19172730}}</ref> As such, this postcanine dentition is capable of “crushing and grinding” the tough shoots and leaves common to the diet of an early hominid.<ref>{{Cite journal|last=Constantino|first=Paul|last2=Wood|first2=Bernard|date=2007-03-01|title=The Evolution of Zinjanthropus boisei|url=http://onlinelibrary.wiley.com/doi/10.1002/evan.20130/abstract|journal=Evolutionary Anthropology: Issues, News, and Reviews|language=en|volume=16|issue=2|pages=49–62|doi=10.1002/evan.20130|issn=1520-6505}}</ref> Australopithecus [[Paranthropus]], for example, was perhaps the most noteworthy hominid to display this trait, an adaption perhaps due to its varied and encompassing diet .<ref name="hindawi" /> Note, postcanine megadontia is hypothesized to have no correlation to [[Durophagy|durophagy,]] but is rather a crucial development in hominids that allowed for preservation of occlusal quality.<ref name=":3" />

Increased postcanine size can be correlated with the evolution of other physiological traits <ref name="hindawi">{{cite journal|last1=Jiménez-Arenas|first1=Juan Manuel|title=On the Relationships of Postcanine Tooth Size with Dietary Quality and Brain Volume in Primates: Implications for Hominin Evolution|journal=BioMed Research International|date=30 January 2014|volume=2014}}</ref>
<ref name="scielo">{{cite journal|last1=Jiménez-Arenas|first1=Juan Manuel|title=Tooth size and metabolic requirements in Primates: The ‘equivalence between exponents’ under discussion|journal=Int. J. Morphol|date=2013|volume=31|issue=4|pages=1191–1197}}</ref>
<ref name="book">{{cite book|last1=Ungar|first1=Peter|title=Mammal Teeth: Origin, Evolution, and Diversity|date=2010|publisher=Johns Hopkins University Press|location=Baltimore, Maryland}}</ref>
.<ref name="Jolly">{{cite journal|last1=Jolly|first1=C|title=The seed-eaters: a new model of hominid differentiation based on a baboon analogy|journal=Man|date=1970|volume=5|issue=1|pages=5–26}}</ref> Inverse trends of brain enlargement and molar size point to diet and food processing as a linking factor; [[encephalization]] is a crucial consideration in the development of [[Tool use by animals|tool usage]] and extraoral food processing that was observed in [[Homo]] species, but not in [[Australopithecines]].<ref name="hindawi" /> Post canine enlargement has also been significantly positively correlated with [[basal metabolic rate]], independently of body size.<ref name="scielo" /> Larger primates tend to need larger teeth to process more food to meet the energy requirements of a larger body,<ref name="book" /> however the evolution of postcanine megadontia is more likely due to the quality of the diet. The tough, “low quality” foodstuff consumed by robust Australopithecines coupled with their lack of food processing technology, lead to an enlargement of the occlusal surface of the molars<ref name="hindawi" />
<ref name="scielo" />
<ref name="Jolly" />

A study that analyzed the development of molars in hominids and Miocene apes found that a larger “ratio of the areas of [molar 1] and [molar 3]” could correlate to an increase in fruits found in the diet of these species.<ref name=":2">{{Cite journal|last=Teaford|last2=Ungar|title=Diet and the evolution of the earliest human ancestors|url=http://www.pnas.org/content/97/25/13506.full|journal=Proceedings of the National Academy of Sciences of the United States of America|volume= 97| issue = 25}}</ref> An increased ratio of the areas of molars was found to have a negative correlation with the amount of “leaves, flowers, and shoots” in the diet, suggesting that species like Ardipithecus, which had a greater ratio of areas of molars, had gradually transitioned to including more fruit in their diet as the size of their molars increased.<ref name=":2" />

== Form and Function ==
'''Functionality'''

Post-canine megadontia is associated with specific food material properties. This allows for great insight into the diets of early hominins that exhibited the trait. Post-canine megadontia is most commonly linked to diets rich in foods that are “small, chemically sealed, and resistant to bolus formation.”<ref name="Jeremiah Scott">{{cite journal|last1=Scott|first1=Jeremiah E|title=Molar size and diet in the Strepsirrhini: Implications for size-adjustment in studies of primate dental adaptation|journal=Journal of Human Evolution|date=December 2012|volume=63|issue=6|page=796-804|url=http://www.sciencedirect.com/science/article/pii/S0047248412001583|accessdate=7 November 2016}}</ref> Having these larger teeth in comparison to the oral cavity size breaks down food particles more, which will increase the effectiveness of the natural processes of food processing that occur within the mouth. <ref name="Jeremiah Scott" />For example, the large transverse dimensions of corpses of [[Homo floeresiensis]] from the [[Liang Bua]] cave on Flores Island in Indonesia suggest that these early hominids had postcanine megadontia and a diet with great masticatory stress.<ref name="David Daegling">{{cite journal|last1=Daegling|first1=David J.|last2=Patel|first2=Biren A.|last3=Jungers|first3=William L.|title=Geometric properties and comparative biomechanics of Homo floresiensis mandibles|journal=Journal of Human Evolution|date=1 March 2014|volume=68|pages=36–46|doi=10.1016/j.jhevol.2014.01.001|url=http://www.sciencedirect.com/science/article/pii/S0047248414000104|accessdate=7 November 2016}}</ref> However, as Homo evolved, the amount [[mastication |masticatory]] stress involved in eating decreased as “behavioral adaptations for extraoral food processing” were further developed. Thus, tool making Liang Bua corpses would have a comparatively smaller transverse dimensions of the skull, meaning their “mascilatory functional morphology” deviates greatly from the [[Pleistocene]] Homo.<ref name="David Daegling">{{cite journal|last1=Daegling|first1=David J.|last2=Patel|first2=Biren A.|last3=Jungers|first3=William L.|title=Geometric properties and comparative biomechanics of Homo floresiensis mandibles|journal=Journal of Human Evolution|date=1 March 2014|volume=68|pages=36–46|doi=10.1016/j.jhevol.2014.01.001|url=http://www.sciencedirect.com/science/article/pii/S0047248414000104|accessdate=7 November 2016}}</ref>'

'''Biomechanics'''

Megadontia works by increasing the ratio of minimum to maximum [[Second moment of area|second moments of area]] or the efficiency of a shape to resist bending or malformation.<ref name=":0">{{Cite journal|last=Daegling|first=D.J.|last2=Grine|first2=F.E.|date=June 26, 1991|title=Compact Bone Distribution and Biomechanics of Early Hominid Mandibles|url=http://onlinelibrary.wiley.com/doi/10.1002/ajpa.1330860302/epdf|journal=American Journal of Physical Anthropology|volume=}}</ref> This resistance is tied to whichever specific axis about which the bending is being applied.<ref name=":1">{{Cite book|title=Evolution of the Human Diet|last=Daegling|first=David|publisher=Oxford University|year=2007|isbn=978-0-19-518347-4|location=|pages=89–92|quote=|via=}}</ref> Also called mandible robusticity, this characteristic allows for much stronger [[mastication]] of food. However, calculations made using biomechanical models does not necessarily perfectly predict the efficiency of different mandibular compositions in resistance to bending forces. [[Shearing (physics)|Shearing]] is also an important factor in mastication effectiveness, and resistance to shear is proportional to the cross sectional area of the teeth. In postcanine Megadontia specimens, the cross sectional area is on average much larger than modern hominids which implies greater shear stress resistance.<ref name=":1" />

'''Genetics'''

One hypothesis proposes that an inactivation of the [[MYH16 gene]], which resulted in an increase in brain size, decreased temporal muscle mass.<ref name=":0" /> The decrease in muscle allowed for the brain to grow, which might have allowed early hominids to develop tools.<ref name=":0" /> A second hypothesis suggests that the [[SRGAP2|SRGAP2 gene]] is responsible instead.<ref name=":0" /> The inhibition of this gene allows for an increase in brain development.<ref name=":0" />

== Comparative Biology ==
'''Post-canine Megadontia Compared with Modern Human Dental Morphology'''

Compared to present day humans, early hominids such as ''Paranthropus aethiopicus'' and ''Australopithecus garhi'' <ref>{{Cite journal|last=Strait|first=David S.|last2=Grine|first2=Frederick E.|date=1999-08-20|title=Cladistics and Early Hominid Phylogeny|url=http://science.sciencemag.org/content/285/5431/1209.3|journal=Science|language=en|volume=285|issue=5431|pages=1209–1209|doi=10.1126/science.285.5431.1209c|issn=0036-8075}}</ref> had significantly larger [[Hominid dental morphology evolution|dental morphology]] in their molars and premolars and smaller incisors. The hominids possessing post-canine megadontia had thick molar enamel, premolars with molarized roots, and lower molars that had additional capsules.<ref>{{Cite journal|last=McCollum|first=Melanie A.|date=1999-04-09|title=The Robust Australopithecine Face: A Morphogenetic Perspective|url=http://science.sciencemag.org/content/284/5412/301|journal=Science|language=en|volume=284|issue=5412|pages=301–305|doi=10.1126/science.284.5412.301|issn=0036-8075|pmid=10195892}}</ref> Rather than inheriting their early hominid ancestors’ large sized [[Molar (tooth)|molars]], human molars evolved significantly, reducing instead to a size more similar to their front teeth. Contrary to megadont hominins’ dominant second molars, modern humans’ first molar is the largest, and their [[mandible]]s can rarely fit a third molar.<ref>Emes, Y., Aybar, B., Yalcin, S. (2011). On the Evolution of Human Jaws and Teeth: A Review. Bull Int Assoc Paleodont. 5(1): 37-47.</ref>

'''Post-canine Megadontia Compared to Primates and Other Species'''

The [[Dental morphology|morphology of teeth]] among species offers insight into their diet and [[Phylogenetics|phylogeny]]. The constant chewing in primate’s diets created a selection in primate molar shape, notably possessing cusp tips to ingest seeds.<ref name=":4">{{Cite book|url=https://books.google.com/books?hl=en&lr=&id=q8TH6NOOs9cC&oi=fnd&pg=PA39&dq=functional+morphology+of+molars&ots=vx6EDzm2L7&sig=uF30E7E5U9AmZBfTnqmMPzRmAn8#v=onepage&q=functional%20morphology%20of%20molars&f=false|title=Evolution of the Human Diet: The Known, the Unknown, and the Unknowable|last=Ungar|first=Peter S.|date=2007-01-01|publisher=Oxford University Press, USA|isbn=9780195183467|language=en}}</ref> In order to achieve maximum chewing efficiency as the food toughness increases, [[Folivore|folivorous]] primates tend to have larger post-canines than [[Frugivore|frugivorous]] primates.<ref>{{Cite book|url=https://books.google.com/books?hl=en&lr=&id=YcN5HBeHEM8C&oi=fnd&pg=PR9&dq=functional+morphology+of+molars&ots=t5K6oggZ4m&sig=Op36B8Y-xNrOVXVEigy0kaFHtGs#v=snippet&q=primates&f=true|title=Dental Functional Morphology: How Teeth Work|last=Lucas|first=Peter W.|date=2004-06-03|publisher=Cambridge University Press|isbn=9780521562362|language=en}}</ref> In primates, positive allometry exists between the size of post canine teeth in primates and cranial length. This relationship has also been suggested in other groups of mammals, but the differences in postcanine size in primates are less variant compared to other mammals.<ref>{{Cite journal|last=Kanazawa|first=Eisaku|last2=Rosenberger|first2=A. L.|title=Interspecific allometry of the mandible, dental arch, and molar area in anthropoid primates: Functional morphology of masticatory components|url=http://link.springer.com/article/10.1007/BF02380880|journal=Primates|language=en|volume=30|issue=4|pages=543–560|doi=10.1007/BF02380880|issn=0032-8332}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=0w0E2ElaD9sC&printsec=frontcover&dq=primate+dentition:+an+introduction+to+the+teeth+of+non-human+primates&hl=en&sa=X&ved=0ahUKEwiIsMrX7pnQAhVHiFQKHaPBCsYQ6AEIJjAA#v=snippet&q=molars&f=true|title=Primate Dentition: An Introduction to the Teeth of Non-human Primates|last=Swindler|first=Daris R.|date=2002-02-21|publisher=Cambridge University Press|isbn=9781139431507|language=en}}</ref> Other species with [[Herbivore adaptations to plant defense|herbivorous diets]] have adaptations in their post canines in order to eat plant material,<ref>{{Cite book|title=The Vertebrate Story|last=Romer|first=A. S.|publisher=University of Chicago Press|year=1959|isbn=|location=|pages=|quote=|via=}}</ref> but the term postcanine megadontia typically refers to the dental adaptation in the hominid group.<ref>{{Cite journal|last=McHenry|first=Henry M.|date=1984-07-01|title=Relative cheek-tooth size in Australopithecus|url=http://onlinelibrary.wiley.com/doi/10.1002/ajpa.1330640312/abstract|journal=American Journal of Physical Anthropology|language=en|volume=64|issue=3|pages=297–306|doi=10.1002/ajpa.1330640312|issn=1096-8644}}</ref>

== References ==
{{reflist}}

[[Category:Teeth]]
[[Category:Evolution]]

Post-canine megadontia - تاريخ المراجعة

إدارة الموسوعة 1: مراجعة واحدة