Post-canine megadontia is a relative enlargement of the molars and premolars compared to the size of the incisors and canines. This phenomenon is seen in some early hominid ancestors such as Paranthropus aethiopicus. [1]
The evidence for postcanine megadontia comes from measuring postcanine tooth surface area of hominid specimens and comparing these measurements to other hominid species. Australopithecus , dated to have lived 2 to 3 million years ago, is the earliest hominid genus to demonstrate postcanine enlargement, with average postcanine tooth area ranging from approximately 460mm2 and going all the way up to the largest tooth area, 756mm2, which is seen in Paranthropus boisei . [2] After Australopithecus, a trend of steady decline in postcanine size is observed, starting in the genus Homo and culminating with Homo sapiens which has an average postcanine tooth area of only 334mm2. [2] [3]
Studies of premolar size in hominid species that predate Australopithecusafarensis show long, uni-cuspid teeth at the P3 location, while species dated after A. afarensis have been shown to have wider, bicuspid teeth at the same location, which is hypothesized to show the beginnings of canine to premolar evolution in hominids. [4]
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 of H. floresiensis show similarities to the bigger tooth sizes of the earlier genera Australopithecus and Homo. [5]
The shift towards postcanine megadontia dates back to about 4-5 million years ago with the discovery of Ardipithecus ramidus in the Middle Awash region of Ethiopia. [6] Distinctive features in A. ramidus such as dentition with reduced canines, the skull, hindlimb and forelimb suggest it to be near the split between the chimpanzee and hominin lineages. [7]
It was the origin of Australopithecus africanus , found in several regions of South Africa (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. afarensis as well as other genera in Paranthropus . [8]
The first specimen of Paranthropus aethiopicus was discovered in Lake Turkana, Kenya and its successor, Paranthropus robustus , was found in the northern parts of South Africa (Swartkrans, Kromdraai and Drimolen). [9] Paranthropus boisei, the last species included in the genus Paranthropus, was first found in Olduvai Gorge, Tanzania and around Ethiopia and Kenya. [10] P. boisei 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. [10]
The first species in the genus Homo, Homo habilis , has been found in Tanzania and Kenya at sites dating between 2.1 and 1.5 million years ago. [11] Species within the genus Homo showed no difference within molar size up until Homo floresiensis , where smaller molars were beginning to be expressed. [12] Species after H. floresiensis such as H. heidelbergensis , H. neanderthalensis , and H. sapiens began to show instead wider frontal teeth and a decrease in size of the molars compared to earlier species. [13] [14] [15] H. sapiens shows significantly smaller molars, mandible, and a prominence of the chin. [15]
Postcanine megadontia is commonly associated with the repeated consumption of tough plant-like material, which can be referred to as "low-quality food stuffs". [16] [17] The substances were integral to the diet of extinct hominids, and their molars were subject to the constant occlusal attrition from the stress of vigorous mastication. [18] The development and evolution of this trait was characterized by a thick coating of enamel surrounding the molars and premolars, mitigating the detrimental effects of the tough diet. [19] As such, this postcanine dentition is capable of “crushing and grinding” the tough shoots and leaves common to the diet of an early hominid. [20] Australopithecus Paranthropus, for example, was perhaps the most noteworthy hominid to display this trait, an adaptation perhaps due to its varied and encompassing diet . [17] Note, postcanine megadontia is hypothesized to have no correlation to durophagy, but is rather a crucial development in hominids that allowed for preservation of occlusal quality. [16]
Increased postcanine size can be correlated with the evolution of other physiological traits [17] [21] [22] [23] Inverse trends of brain mass and molar size point to diet and food processing as a linking factor; encephalization is a crucial consideration in the development of tool usage and extraoral food processing that was observed in Homo species, but not in Australopithecines. [17] Post canine enlargement has also been significantly positively correlated with basal metabolic rate, independently of body size. [21] Larger primates tend to need larger teeth to process more food to meet the energy requirements of a larger body, [22] but the evolution of postcanine megadontia is more likely due to the quality of the diet. The tough, “low-quality food stuffs" consumed by robust Australopithecines, coupled with their lack of food processing technology, lead to an enlargement of the occlusal surface of the molars. [17] [21] [23]
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. [24] 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. [24] Many modern primates who lack such dietary features have been shown to occasionally rely on "fall-back foods" of these sorts, implying robust Australopithecines developed postcanine megadontia as they had to more heavily rely on such foods. [24]
While postcanine megadontia denotes the enlargement of the premolars and molars found in early hominid ancestors, it did not affect the structural organization of the cusps that make up those teeth, and thus, were used similarly to the premolars and molars that modern humans possess today. The premolars and molars of modern hominids and those affected by postcanine megadontia both have two and between four and five cusps respectively. [25] [26] The enlargement of the teeth affected by postcanine megadontia, without a difference in the arrangement or number of cusps that compose them, imparted an increased ability for grinding and crushing matter compared to modern day humans. While the form of the teeth themselves is not affected by postcanine megadontia, the ratio of molar teeth volume to total teeth volume is dramatically larger in specimens with postcanine megadontia compared to their modern human counterparts, an adaptation that shows signs of evolutionary convergence when compared to the form and function of the teeth found in many modern herbivores. [27]
Postcanine 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.” [28] 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. [28] For example, the large transverse dimensions of corpses of Homo floresiensis 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. [29] However, as Homo evolved, the amount of 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. [29] '
Megadontia works by increasing the ratio of minimum to maximum second moments of area or the efficiency of a shape to resist bending or malformation. [30] This resistance is tied to whichever specific axis about which the bending is being applied. [31] 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 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. [31]
Some important genes involved in the development and regulation of tooth formation include BMP4, FGF8, and homeobox genes such as MSX1, PAX9, PITX2, SHOX2, Barx1, and Shh to name a few. [32] [33] Research suggests that homeobox genes are mainly responsible for much of the variance in tooth morphogenesis observed in fossilized hominids. [32] It is theorized that tooth enlargement is due to several different gene mechanisms, none of them fully understood, and that selection acts on molars and pre-molars as a singular unit rather than on individual teeth. [32]
There are a few theories concerning the selection mechanisms. One theory is that selection acts on the variation in the molars and pre-molars presented by the homeobox genes in hominid species. [32] Another theory suggests that postcanine megadontia resulted from the spatial reassignment by homeobox genes that increased postcanine tooth size while simultaneously decreasing the size of the canines. [32]
For the transition from megadontia to normal-sized postcanine teeth and its inverse relationship to brain size, one hypothesis proposes that an inactivation of the MYH16 gene, which resulted in an increase in brain size, decreased temporal muscle mass. [30] The decrease in muscle allowed for the brain to grow, which might have allowed early hominids to develop tools. [30] A second hypothesis suggests that the SRGAP2 gene is responsible instead. [30] The inhibition of this gene allows for an increase in brain development. [30]
Compared to present day humans, early hominids such as Paranthropus aethiopicus and Australopithecus garhi [34] had significantly larger dental morphology in their molars and premolars and smaller incisors. The hominids possessing postcanine megadontia had thick molar enamel, premolars with molarized roots, and lower molars that had additional capsules. [35] Rather than inheriting their early hominid ancestors’ large sized 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 mandibles can rarely fit a third molar. [36]
The morphology of teeth among species offers insight into their diet and phylogeny. The constant chewing in primate’s diets created a selection in primate molar shape, notably possessing cusp tips to ingest seeds. [18] In order to achieve maximum chewing efficiency as the food toughness increases, folivorous primates tend to have larger postcanines than frugivorous primates. [37] In primates, positive allometry exists between the size of postcanine 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. [38] [39] Other species with herbivorous diets have adaptations in their postcanines in order to eat plant material, [40] but the term postcanine megadontia typically refers to the dental adaptation in the hominid group. [2]
Ardipithecus is a genus of an extinct hominine that lived during the Late Miocene and Early Pliocene epochs in the Afar Depression, Ethiopia. Originally described as one of the earliest ancestors of humans after they diverged from the chimpanzees, the relation of this genus to human ancestors and whether it is a hominin is now a matter of debate. Two fossil species are described in the literature: A. ramidus, which lived about 4.4 million years ago during the early Pliocene, and A. kadabba, dated to approximately 5.6 million years ago. Initial behavioral analysis indicated that Ardipithecus could be very similar to chimpanzees, however more recent analysis based on canine size and lack of canine sexual dimorphism indicates that Ardipithecus was characterised by reduced aggression, and that they more closely resemble bonobos.
Homininae, also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the tribe Hominini ―and 2) the tribe Gorillini (gorillas). Alternatively, the genus Pan is sometimes considered to belong to its own third tribe, Panini. Homininae comprises all hominids that arose after orangutans split from the line of great apes. The Homininae cladogram has three main branches, which lead to gorillas, and to humans and chimpanzees via the tribe Hominini and subtribes Hominina and Panina. There are two living species of Panina and two living species of gorillas, but only one extant human species. Traces of extinct Homo species, including Homo floresiensis have been found with dates as recent as 40,000 years ago. Organisms in this subfamily are described as hominine or hominines.
Homo habilis is an extinct species of archaic human from the Early Pleistocene of East and South Africa about 2.8 million years ago to 1.65 million years ago (mya). Upon species description in 1964, H. habilis was highly contested, with many researchers recommending it be synonymised with Australopithecus africanus, the only other early hominin known at the time, but H. habilis received more recognition as time went on and more relevant discoveries were made. By the 1980s, H. habilis was proposed to have been a human ancestor, directly evolving into Homo erectus which directly led to modern humans. This viewpoint is now debated. Several specimens with insecure species identification were assigned to H. habilis, leading to arguments for splitting, namely into "H. rudolfensis" and "H. gautengensis" of which only the former has received wide support.
Kenyanthropus is a genus of extinct hominin identified from the Lomekwi site by Lake Turkana, Kenya, dated to 3.3 to 3.2 million years ago during the Middle Pliocene. It contains one species, K. platyops, but may also include the 2 million year old Homo rudolfensis, or K. rudolfensis. Before its naming in 2001, Australopithecus afarensis was widely regarded as the only australopithecine to exist during the Middle Pliocene, but Kenyanthropus evinces a greater diversity than once acknowledged. Kenyanthropus is most recognisable by an unusually flat face and small teeth for such an early hominin, with values on the extremes or beyond the range of variation for australopithecines in regard to these features. Multiple australopithecine species may have coexisted by foraging for different food items, which may be reason why these apes anatomically differ in features related to chewing.
Paranthropus is a genus of extinct hominin which contains two widely accepted species: P. robustus and P. boisei. However, the validity of Paranthropus is contested, and it is sometimes considered to be synonymous with Australopithecus. They are also referred to as the robust australopithecines. They lived between approximately 2.9 and 1.2 million years ago (mya) from the end of the Pliocene to the Middle Pleistocene.
Australopithecus is a genus of early hominins that existed in Africa during the Pliocene and Early Pleistocene. The genera Homo, Paranthropus, and Kenyanthropus evolved from some Australopithecus species. Australopithecus is a member of the subtribe Australopithecina, which sometimes also includes Ardipithecus, though the term "australopithecine" is sometimes used to refer only to members of Australopithecus. Species include A. garhi, A. africanus, A. sediba, A. afarensis, A. anamensis, A. bahrelghazali and A. deyiremeda. Debate exists as to whether some Australopithecus species should be reclassified into new genera, or if Paranthropus and Kenyanthropus are synonymous with Australopithecus, in part because of the taxonomic inconsistency.
Homo rudolfensis is an extinct species of archaic human from the Early Pleistocene of East Africa about 2 million years ago (mya). Because H. rudolfensis coexisted with several other hominins, it is debated what specimens can be confidently assigned to this species beyond the lectotype skull KNM-ER 1470 and other partial skull aspects. No bodily remains are definitively assigned to H. rudolfensis. Consequently, both its generic classification and validity are debated without any wide consensus, with some recommending the species to actually belong to the genus Australopithecus as A. rudolfensis or Kenyanthropus as K. rudolfensis, or that it is synonymous with the contemporaneous and anatomically similar H. habilis.
The Olduvai Gorge or Oldupai Gorge in Tanzania is one of the most important paleoanthropological localities in the world; the many sites exposed by the gorge have proven invaluable in furthering understanding of early human evolution. A steep-sided ravine in the Great Rift Valley that stretches across East Africa, it is about 48 km long, and is located in the eastern Serengeti Plains within the Ngorongoro Conservation Area in the Olbalbal ward located in Ngorongoro District of Arusha Region, about 45 kilometres from Laetoli, another important archaeological locality of early human occupation. The British/Kenyan paleoanthropologist-archeologist team of Mary and Louis Leakey established excavation and research programs at Olduvai Gorge that achieved great advances in human knowledge and are world-renowned. The site is registered as one of the National Historic Sites of Tanzania.
Gigantopithecus is an extinct genus of ape that lived in southern China from 2 million to approximately 300-200,000 years ago during the Early to Middle Pleistocene, represented by one species, Gigantopithecus blacki. Potential identifications have also been made in Thailand, Vietnam, and Indonesia. The first remains of Gigantopithecus, two third molar teeth, were identified in a drugstore by anthropologist Ralph von Koenigswald in 1935, who subsequently described the ape. In 1956, the first mandible and more than 1,000 teeth were found in Liucheng, and numerous more remains have since been found in at least 16 sites. Only teeth and four mandibles are known currently, and other skeletal elements were likely consumed by porcupines before they could fossilise. Gigantopithecus was once argued to be a hominin, a member of the human line, but it is now thought to be closely allied with orangutans, classified in the subfamily Ponginae.
Australopithecus africanus is an extinct species of australopithecine which lived between about 3.3 and 2.1 million years ago in the Late Pliocene to Early Pleistocene of South Africa. The species has been recovered from Taung, Sterkfontein, Makapansgat, and Gladysvale. The first specimen, the Taung child, was described by anatomist Raymond Dart in 1924, and was the first early hominin found. However, its closer relations to humans than to other apes would not become widely accepted until the middle of the century because most had believed humans evolved outside of Africa. It is unclear how A. africanus relates to other hominins, being variously placed as ancestral to Homo and Paranthropus, to just Paranthropus, or to just P. robustus. The specimen "Little Foot" is the most completely preserved early hominin, with 90% of the skeleton intact, and the oldest South African australopith. However, it is controversially suggested that it and similar specimens be split off into "A. prometheus".
Paranthropus aethiopicus is an extinct species of robust australopithecine from the Late Pliocene to Early Pleistocene of East Africa about 2.7–2.3 million years ago. However, it is much debated whether or not Paranthropus is an invalid grouping and is synonymous with Australopithecus, so the species is also often classified as Australopithecus aethiopicus. Whatever the case, it is considered to have been the ancestor of the much more robust P. boisei. It is debated if P. aethiopicus should be subsumed under P. boisei, and the terms P. boisei sensu lato and P. boisei sensu stricto can be used to respectively include and exclude P. aethiopicus from P. boisei.
Australopithecus garhi is a species of australopithecine from the Bouri Formation in the Afar Region of Ethiopia 2.6–2.5 million years ago (mya) during the Early Pleistocene. The first remains were described in 1999 based on several skeletal elements uncovered in the three years preceding. A. garhi was originally considered to have been a direct ancestor to Homo and the human line, but is now thought to have been an offshoot. Like other australopithecines, A. garhi had a brain volume of 450 cc (27 cu in); a jaw which jutted out (prognathism); relatively large molars and premolars; adaptations for both walking on two legs (bipedalism) and grasping while climbing (arboreality); and it is possible that, though unclear if, males were larger than females. One individual, presumed female based on size, may have been 140 cm tall.
Paranthropus robustus is a species of robust australopithecine from the Early and possibly Middle Pleistocene of the Cradle of Humankind, South Africa, about 2.27 to 0.87 million years ago. It has been identified in Kromdraai, Swartkrans, Sterkfontein, Gondolin, Cooper's, and Drimolen Caves. Discovered in 1938, it was among the first early hominins described, and became the type species for the genus Paranthropus. However, it has been argued by some that Paranthropus is an invalid grouping and synonymous with Australopithecus, so the species is also often classified as Australopithecus robustus.
Paranthropus boisei is a species of australopithecine from the Early Pleistocene of East Africa about 2.5 to 1.15 million years ago. The holotype specimen, OH 5, was discovered by palaeoanthropologist Mary Leakey in 1959 at Olduvai Gorge, Tanzania and described by her husband Louis a month later. It was originally placed into its own genus as "Zinjanthropus boisei", but is now relegated to Paranthropus along with other robust australopithecines. However, it is also argued that Paranthropus is an invalid grouping and synonymous with Australopithecus, so the species is also often classified as Australopithecus boisei.
Australopithecina or Hominina is a subtribe in the tribe Hominini. The members of the subtribe are generally Australopithecus, and it typically includes the earlier Ardipithecus, Orrorin, Sahelanthropus, and (sometimes) Graecopithecus. All these closely related species are now sometimes collectively termed australopiths or homininians. They are the extinct, close relatives of modern humans and, together with the extant genus Homo, comprise the human clade. Members of the human clade, i.e. the Hominini after the split from the chimpanzees, are now called Hominina.
Australopithecus bahrelghazali is an extinct species of australopithecine discovered in 1995 at Koro Toro, Bahr el Gazel, Chad, existing around 3.5 million years ago in the Pliocene. It is the first and only australopithecine known from Central Africa, and demonstrates that this group was widely distributed across Africa as opposed to being restricted to East and southern Africa as previously thought. The validity of A. bahrelghazali has not been widely accepted, in favour of classifying the specimens as A. afarensis, a better known Pliocene australopithecine from East Africa, because of the anatomical similarity and the fact that A. bahrelghazali is known only from 3 partial jawbones and an isolated premolar. The specimens inhabited a lakeside grassland environment with sparse tree cover, possibly similar to the modern Okavango Delta, and similarly predominantly ate C4 savanna foods—such as grasses, sedges, storage organs, or rhizomes—and to a lesser degree also C3 forest foods—such as fruits, flowers, pods, or insects. However, the teeth seem ill-equipped to process C4 plants, so its true diet is unclear.
Australopithecus sediba is an extinct species of australopithecine recovered from Malapa Cave, Cradle of Humankind, South Africa. It is known from a partial juvenile skeleton, the holotype MH1, and a partial adult female skeleton, the paratype MH2. They date to about 1.98 million years ago in the Early Pleistocene, and coexisted with Paranthropus robustus and Homo ergaster / Homo erectus. Malapa is interpreted as having been a natural death trap, the base of a long vertical shaft which creatures could accidentally fall into. A. sediba was initially described as being a potential human ancestor, and perhaps the progenitor of Homo, but this is contested and it could also represent a late-surviving population or sister species of A. africanus which had earlier inhabited the area.
Homo luzonensis, also locally called "Ubag" after a mythical caveman, is an extinct, possibly pygmy, species of archaic human from the Late Pleistocene of Luzon, the Philippines. Their remains, teeth and phalanges, are known only from Callao Cave in the northern part of the island dating to before 50,000 years ago. They were initially identified as belonging to modern humans in 2010, but in 2019, after the discovery of more specimens, they were placed into a new species based on the presence of a wide range of traits similar to modern humans as well as to Australopithecus and early Homo. In 2023, a study revealed that the fossilized remains of the Callao Man has been found out to be years old and much older than previously known.
Changes to the dental morphology and jaw are major elements of hominid evolution. These changes were driven by the types and processing of food eaten. The evolution of the jaw is thought to have facilitated encephalization, speech, and the formation of the uniquely human chin.
Australopithecus deyiremeda is an extinct species of australopithecine from Woranso–Mille, Afar Region, Ethiopia, about 3.5 to 3.3 million years ago during the Pliocene. Because it is known only from three partial jawbones, it is unclear if these specimens indeed represent a unique species or belong to the much better-known A. afarensis. A. deyiremeda is distinguished by its forward-facing cheek bones and small cheek teeth compared to those of other early hominins. It is unclear if a partial foot specimen exhibiting a dextrous big toe can be assigned to A. deyiremeda. A. deyiremeda lived in a mosaic environment featuring both open grasslands and lake- or riverside forests, and anthropologist Fred Spoor suggests it may have been involved in the Kenyan Lomekwi stone-tool industry typically assigned to Kenyanthropus. A. deyiremeda coexisted with A. afarensis, and they may have exhibited niche partitioning to avoid competing with each other for the same resources, such as by relying on different fallback foods during leaner times.