Robust australopithecines

''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 2.6 and 1.2 million years ago (mya) from the end of the Pliocene.

''Paranthropus'' is characterised by robust skulls, with a prominent gorilla-like sagittal crest along the midline—which suggest strong chewing muscles—and broad, herbivorous teeth used for grinding. However, they likely preferred soft food over tough and hard food. ''Paranthropus'' species were generalist feeders, but ''P. robustus'' was likely an omnivore, whereas ''P. boisei'' was likely herbivorous and mainly ate bulbotubers. They were bipeds. Despite their robust heads, they had comparatively small bodies. Average weight and height are estimated to be at for ''P. robustus'' males, at for ''P. boisei'' males, at for ''P. robustus'' females, and at for ''P. boisei'' females.

They were possibly polygamous and patrilocal, but there are no modern analogues for australopithecine societies. They are associated with bone tools and contested as the earliest evidence of fire usage. They typically inhabited woodlands, and coexisted with some early human species, namely '' A. africanus'', ''H. habilis'' and ''H. erectus''. They were preyed upon by the large carnivores of the time, specifically crocodiles, leopards, sabertoothed cats and hyenas.

Taxonomy

Species

''P. robustus''

The genus ''Paranthropus'' was first erected by Scottish-South African palaeontologist Robert Broom in 1938, with the type species '' P. robustus''. "''Paranthropus''" derives from Ancient Greek παρα ''para'' beside or alongside; and άνθρωπος ''ánthropos'' man. The type specimen, a male braincase, TM 1517, was discovered by schoolboy Gert Terblanche at the Kromdraai fossil site, about southwest of Pretoria, South Africa. By 1988, at least six individuals were unearthed in around the same area, now known as the Cradle of Humankind.

In 1948, at Swartkrans Cave, in about the same vicinity as Kromdraai, Broom and South African palaeontologist John Talbot Robinson described ''P. crassidens'' based on a subadult jaw, SK 6. He believed later ''Paranthropus'' were morphologically distinct from earlier ''Paranthropus'' in the cave—that is, the Swartkrans ''Paranthropus'' were reproductively isolated from Kromdraai ''Paranthropus'' and the former eventually speciated. By 1988, several specimens from Swartkrans had been placed into ''P. crassidens''. However, this has since been synonymised with ''P. robustus'' as the two populations do not seem to be very distinct.

''P. boisei''

In 1959, ''P. boisei'' was discovered by Mary Leakey at Olduvai Gorge, Tanzania (specimen OH 5). Her husband Louis named it ''Zinjanthropus boisei'' because he believed it differed greatly from ''Paranthropus'' and ''Australopithecus''. The name derives from " Zinj", an ancient Arabic word for the coast of East Africa, and "boisei", referring to their financial benefactor Charles Watson Boise. However, this genus was rejected at Mr. Leakey's presentation before the 4th Pan-African Congress on Prehistory, as it was based on a single specimen. The discovery of the Peninj Mandible made the Leakey's reclassify their species as ''Australopithecus (Zinjanthropus) boisei'' in 1964, but in 1967, South African palaeoanthropologist Phillip V. Tobias subsumed it into ''Australopithecus'' as ''A. boisei''. However, as more specimens were found, the combination ''Paranthropus boisei'' became more popular.

It is debated whether the wide range of variation in jaw size indicates simply sexual dimorphism or a grounds for identifying a new species. It could be explained as groundmass filling in cracks naturally formed after death, inflating the perceived size of the bone. ''P. boisei'' also has a notably wide range of variation in skull anatomy, but these features likely have no taxonomic bearing.

''P. aethiopicus''

In 1968, French palaeontologists Camille Arambourg and Yves Coppens described "''Paraustralopithecus aethiopicus''" based on a toothless mandible from the Shungura Formation, Ethiopia (Omo 18). In 1976, American anthropologist Francis Clark Howell and Breton anthropologist Yves Coppens reclassified it as ''A. africanus''. In 1986, after the discovery of the skull KNM WT 17000 by English anthropologist Alan Walker and Richard Leakey classified it into ''Paranthropus'' as '' P. aethiopicus''. There is debate whether this is synonymous with ''P. boisei'', the main argument for separation being the skull seems less adapted for chewing tough vegetation.

In 1989, palaeoartist and zoologist Walter Ferguson reclassified KNM WT 17000 into a new species, ''walkeri'', because he considered the skull's species designation questionable as it comprised the skull whereas the holotype of ''P. aethiopicus'' comprised only the mandible. Ferguson's classification is almost universally ignored, and is considered to be synonymous with ''P. aethiopicus''.

Others

In 1963, while in the Congo, French ethnographer Charles Cordier assigned the name "P. congensis" to a super-strong, monstrous ape-man cryptid called "Kikomba", "Apamándi", "Abanaánji", "Zuluzúgu", or "Tshingómbe" by various native tribes which he heard stories about.

In 2015, Ethiopian palaeoanthropologist Yohannes Haile-Selassie and colleagues described the 3.5–3.2 Ma '' A. deyiremeda'' based on three jawbones from the Afar Region, Ethiopia. They noted that, though it shares many similarities with ''Paranthropus'', it may not have been closely related because it lacked enlarged molars which characterize the genus. Nonetheless, in 2018, independent researcher Johan Nygren recommended moving it to ''Paranthropus'' based on dental and presumed dietary similarity.

Validity

In 1951, American anthropologists Sherwood Washburn and Bruce D. Patterson were the first to suggest that ''Paranthropus'' should be considered a junior synonym of ''Australopithecus'' as the former was only known from fragmentary remains at the time, and dental differences were too minute to serve as justification. In face of calls for subsumation, Leakey and Robinson continued defending its validity. Various other authors were still unsure until more complete remains were found. ''Paranthropus'' is sometimes classified as a subgenus of ''Australopithecus''.

There is currently no clear consensus on the validity of ''Paranthropus''. The argument rests upon whether the genus is monophyletic—is composed of a common ancestor and all of its descendants—and the argument against monophyly (that the genus is paraphyletic) says that ''P. robustus'' and ''P. boisei'' evolved similar gorilla-like heads independently of each other by coincidence (convergent evolution), as chewing adaptations in hominins evolve very rapidly and multiple times at various points in the family tree (homoplasy). In 1999, a chimp-like ulna forearm bone was assigned to ''P. boisei'', the first discovered ulna of the species, which was markedly different from ''P. robustus'' ulnae, which could suggest paraphyly.

Evolution

''P. aethiopicus'' is the earliest member of the genus, with the oldest remains, from the Ethiopian Omo Kibish Formation, dated to 2.6 mya at the end of the Pliocene, Malawi. ''P. boisei'' changed remarkably little over its nearly one-million-year existence. ''Paranthropus'' had spread into South Africa by 2 mya with the earliest ''P. robustus'' remains.

It is sometimes suggested that ''Paranthropus'' and ''Homo'' are sister taxa, both evolving from ''Australopithecus''. This may have occurred during a drying trend 2.8–2.5 mya in the Great Rift Valley, which caused the retreat of woodland environments in favor of open savanna, with forests growing only along rivers and lakes. ''Homo'' evolved in the former, and ''Paranthropus'' in the latter riparian environment. However, the classifications of ''Australopithecus'' species is problematic.

Evolutionary tree according to a 2019 study:

Description

Skull

''Paranthropus'' had a massively built, tall and flat skull, with a prominent gorilla-like sagittal crest along the midline which anchored massive temporalis muscles used in chewing. Like other australopithecines, ''Paranthropus'' exhibited sexual dimorphism, with males notably larger than females. They had large molars with a relatively thick tooth enamel coating (post-canine megadontia), and comparatively small incisors (similar in size to modern humans), possibly adaptations to processing abrasive foods. The teeth of ''P. aethiopicus'' developed faster than those of ''P. boisei''.

''Paranthropus'' had adaptations to the skull to resist large bite loads while feeding, namely the expansive squamosal sutures. The notably thick palate was once thought to have been an adaptation to resist a high bite force, but is better explained as a byproduct of facial lengthening and nasal anatomy.

In ''P. boisei'', the jaw hinge was adapted to grinding food side-to-side (rather than up-and-down in modern humans), which is better at processing the starchy abrasive foods that likely made up the bulk of its diet. ''P. robustus'' may have chewed in a front-to-back direction instead, and had less exaggerated (less derived) anatomical features than ''P. boisei'' as it perhaps did not require them with this kind of chewing strategy. This may have also allowed ''P. robustus'' to better process tougher foods.

The braincase volume averaged about , comparable to gracile australopithecines, but smaller than ''Homo''. Modern human brain volume averages for men and for women.

Limbs and locomotion

Unlike ''P. robustus'', the forearms of ''P. boisei'' were heavily built, which might suggest habitual suspensory behaviour as in orangutans and gibbons. A ''P. boisei'' shoulder blade indicates long infraspinatus muscles, which is also associated with suspensory behavior. A ''P. aethiopicus'' ulna, on the other hand, shows more similarities to ''Homo'' than ''P. boisei''.

''Paranthropus'' were bipeds, and their hips, legs and feet resemble ''A. afarensis'' and modern humans. The pelvis is similar to ''A. afarensis'', but the hip joints are smaller in ''P. robustus''. The physical similarity implies a similar walking gait. Their modern-humanlike big toe indicates a modern-humanlike foot posture and range of motion, but the more distal ankle joint would have inhibited the modern human toe-off gait cycle. By 1.8 mya, ''Paranthropus'' and ''H. habilis'' may have achieved about the same grade of bipedality.

Height and weight

In comparison to the large, robust head, the body was rather small. Average weight for ''P. robustus'' may have been for males and for females; and for ''P. boisei'' for males and for females. At Swartkrans Cave Members 1 and 2, about 35% of the ''P. robustus'' individuals are estimated to have weighed , 22% about , and the remaining 43% bigger than the former but less than . At Member 3, all individuals were about . Female weight was about the same in contemporaneous ''H. erectus'', but male ''H. erectus'' were on average heavier than ''P. robustus'' males. ''P. robustus'' sites are oddly dominated by small adults, which could be explained as heightened predation or mortality of the larger males of a group. The largest-known ''Paranthropus'' individual was estimated at .

According to a 1991 study, based on femur length and using the dimensions of modern humans, male and female ''P. robustus'' are estimated to have stood on average , respectively, and ''P. boisei'' . However, the latter estimates are problematic as there were no positively identified male ''P. boisei'' femurs at the time. In 2013, a 1.34 Ma male ''P. boisei'' partial skeleton was estimated to be at least and .

Pathology

''Paranthropus'' seems to have had notably high rates of pitting enamel hypoplasia (PEH), where tooth enamel formation is spotty instead of mostly uniform. In ''P. robustus'', about 47% of baby teeth and 14% of adult teeth were affected, in comparison to about 6.7% and 4.3%, respectively, in any other tested hominin species. The condition of these holes covering the entire tooth is consistent with the modern human ailment amelogenesis imperfecta. However, since circular holes in enamel coverage are uniform in size, only present on the molar teeth, and have the same severity across individuals, the PEH may have been a genetic condition. It is possible that the coding-DNA concerned with thickening enamel also left them more vulnerable to PEH.

There have been 10 identified cases of cavities in ''P. robustus'', indicating a rate similar to modern humans. A molar from Drimolen, South Africa, showed a cavity on the tooth root, a rare occurrence in fossil great apes. In order for cavity-creating bacteria to reach this area, the individual would have had to have also presented either alveolar resportion, which is commonly associated with gum disease; or super-eruption of teeth which occurs when teeth become worn down and have to erupt a bit more in order to maintain a proper bite, and this exposed the root. The latter is most likely, and the exposed root seems to have caused hypercementosis to anchor the tooth in place. The cavity seems to have been healing, which may have been caused by a change in diet or mouth microbiome, or the loss of the adjacent molar.

Palaeobiology

Diet

It was once thought ''P. boisei'' cracked open nuts with its powerful teeth, giving OH 5 the nickname "Nutcracker Man". However, like gorillas, ''Paranthropus'' likely preferred soft foods, but would consume tough or hard food during leaner times, and the powerful jaws were used only in the latter situation. In ''P. boisei'', thick enamel was more likely used to resist abrasive gritty particles rather than to minimize chipping while eating hard foods. In fact, there is a distinct lack of tooth fractures which would have resulted from such activity.

''Paranthropus'' were generalist feeders, but diet seems to have ranged dramatically with location. The South African ''P. robustus'' appears to have been an omnivore, with a diet similar to contemporaneous ''Homo'' and nearly identical to the later '' H. ergaster'', and subsisted on mainly C4 savanna plants and C3 forest plants, which could indicate either seasonal shifts in diet or seasonal migration from forest to savanna. In leaner times it may have fallen back on brittle food. It likely also consumed seeds and possibly tubers or termites. A high cavity rate could indicate honey consumption.

The East African ''P. boisei'', on the other hand, seems to have been largely herbivorous and fed on C4 plants. Its powerful jaws allowed it to consume a wide variety of different plants, though it may have largely preferred nutrient-rich bulbotubers as these are known to thrive in the well-watered woodlands it is thought to have inhabited. Feeding on these, ''P. boisei'' may have been able to meet its daily caloric requirements of approximately 9,700 kJ after about 6 hours of foraging.

Juvenile ''P. robustus'' may have relied more on tubers than adults, given the elevated levels of strontium compared to adults in teeth from Swartkrans Cave, which, in the area, was most likely sourced from tubers. Dentin exposure on juvenile teeth could indicate early weaning, or a more abrasive diet than adults which wore away the cementum and enamel coatings, or both. It is also possible juveniles were less capable of removing grit from dug-up food rather than purposefully seeking out more abrasive foods.

Technology

Bone tools dating between 2.3 and 0.6 mya have been found in abundance in Swartkrans, Kromdraai and Drimolen caves, and are often associated with ''P. robustus''. Though ''Homo'' is also known from these caves, their remains are comparatively scarce to ''Paranthropus'', making ''Homo''-attribution unlikely. The tools also cooccur with ''Homo''-associated Oldawan and possibly Acheulian stone tool industries. The bone tools were typically sourced from the shaft of long bones from medium- to large-sized mammals, but tools made sourced from mandibles, ribs and horn cores have also been found. Bone tools have also been found at Oldawan Gorge and directly associated with ''P. boisei'', the youngest dating to 1.34 mya, though a great proportion of other bone tools from here have ambiguous attribution. Stone tools from Kromdraai could possibly be attributed to ''P. robustus'', as no ''Homo'' have been found there yet.

The bone tools were not manufactured or purposefully shaped for a task. However, since the bones display no weathering (and were not scavenged randomly), and there is a preference displayed for certain bones, raw materials were likely specifically hand-picked. This could indicate a similar cognitive ability to contemporary Stone Age ''Homo''.

Bone tools may have been used to cut or process vegetation, or dig up tubers or termites, The form of ''P. robustus'' incisors appear to be intermediate between ''H. erectus'' and modern humans, which could indicate less food processing done by the teeth due to preparation with simple tools.

Burnt bones were also associated with the inhabitants of Swartkrans, which could indicate some of the earliest fire usage. However, these bones were found in Member 3, where ''Paranthropus'' remains are rarer than ''H. erectus'', and it is also possible the bones were burned in a wildfire and washed into the cave as it is known the bones were not burned onsite.

Social structure

Given the marked anatomical and physical differences with modern great apes, there may be no modern analogue for australopithecine societies, so comparisons drawn with modern primates will not be entirely accurate.

''Paranthropus'' had pronounced sexual dimorphism, with males notably larger than females, which is commonly correlated with a male-dominated polygamous society. ''P. robustus'' may have had a harem society similar to modern forest-dwelling silverback gorillas, where one male has exclusive breeding rights to a group of females, as male-female size disparity is comparable to gorillas (based on facial dimensions), and younger males were less robust than older males (delayed maturity is also exhibited in gorillas).

However, if ''P. robustus'' preferred a savanna habitat, a multi-male society would have been more productive to better defend the troop from predators in the more exposed environment, much like savanna baboons. Further, among primates, delayed maturity is also exhibited in the rhesus monkey which has a multi-male society, and may not be an accurate indicator of social structure.

A 2011 strontium isotope study of ''P. robustus'' teeth from the dolomite Sterkfontein Valley found that, like other hominins, but unlike other great apes, ''P. robustus'' females were more likely to leave their place of birth (patrilocal). This also discounts the plausibility of a harem society, which would have resulted in a matrilocal society due to heightened male–male competition. Males did not seem to have ventured very far from the valley, which could either indicate small home ranges, or that they preferred dolomitic landscapes due to perhaps cave abundance or factors related to vegetation growth.

Life history

Dental development seems to have followed about the same timeframe as it does in modern humans and most other hominins, but, since ''Paranthropus'' molars are markedly larger, rate of tooth eruption would have been accelerated. Their life history may have mirrored that of gorillas as they have the same brain volume, which (depending on the subspecies) reach physical maturity from 12–18 years and have birthing intervals of 40–70 months.

Palaeoecology

Habitat

It is generally thought that ''Paranthropus'' preferred to inhabit wooded, riverine landscapes. The teeth of ''Paranthropus'', ''H. habilis'' and ''H. erectus'' are all known from various overlapping beds in East Africa, such as at Olduvai Gorge and the Turkana Basin. ''P. robustus'' and ''H. erectus'' also appear to have coexisted.

''P. boisei'', known from the Great Rift Valley, may have typically inhabited wetlands along lakes and rivers, wooded or arid shrublands, and semiarid woodlands, though their presence in the savanna-dominated Malawian Chiwondo Beds implies they could tolerate a range of habitats. During the Pleistocene, there seems to have been coastal and montane forests in Eastern Africa. More expansive river valleys—namely the Omo River Valley—may have served as important refuges for forest-dwelling creatures. Being cut off from the forests of Central Africa by a savanna corridor, these East African forests would have promoted high rates of endemism, especially during times of climatic volatility.

The Cradle of Humankind, the only area ''P. robustus'' is known from, was mainly dominated by the springbok ''Antidorcas recki'', but other antelope, giraffes and elephants were also seemingly abundant megafauna. Other known primates are early ''Homo'', the hamadryas baboon, and the extinct colobine monkey '' Cercopithecoides williamsi''.

Predators

The left foot of a ''P. boisei'' specimen (though perhaps actually belonging to ''H. habilis'') from Olduvai Gorge seems to have been bitten off by a crocodile, possibly '' Crocodylus anthropophagus'', and another's leg shows evidence of leopard predation. Other likely Olduvan predators of great apes include the hunting hyena ''Chasmaporthetes nitidula'', and the sabertoothed cats '' Dinofelis'' and ''Megantereon''. The carnivore assemblage at the Cradle of Humankind comprises the two sabertooths, and the hyena '' Lycyaenops silberbergi''.

Male ''P. robustus'' appear to have had a higher mortality rate than females. It is possible that males were more likely to be kicked out of a group, and these lone males had a higher risk of predation.

Extinction

It was once thought that ''Paranthropus'' had become a specialist feeder, and were inferior to the more adaptable tool-producing ''Homo'', leading to their extinction, but this has been called into question. However, smaller brain size may have been a factor in their extinction along with gracile australopithecines. ''P. boisei'' may have died out due to an arid trend starting 1.45 mya, causing the retreat of woodlands, and more competition with savanna baboons and ''Homo'' for alternative food resources.

South African ''Paranthropus'' appear to have outlasted their East African counterparts. The youngest record of ''P. boisei'' comes from Konso, Ethiopia about 1.4 mya; however, there are no East African sites dated between 1.4 and 1 mya, so it may have persisted until 1 mya. ''P. robustus'', on the other hand, was recorded in Swartkrans until Member 3 dated to 1–0.6 mya (the Middle Pleistocene), though more likely the younger side of the estimate.

See also

* ''Australopithecus''
* ''Ardipithecus''
* ''Graecopithecus''
* ''Orrorin''
* ''Sahelanthropus''

References

Further reading

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External links

Reconstructions of ''P. boisei''
by John Gurche
*

Human Timeline (Interactive)
– Smithsonian, National Museum of Natural History (August 2016).

{{Taxonbar, from=Q111463

Prehistoric primate genera
Pliocene primates
Pleistocene primates
Pleistocene extinctions
Cenozoic mammals of Africa
Pleistocene genus extinctions
Fossil taxa described in 1938