''Paranthropus robustus'' is a

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

of robust australopithecine from the

Early Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

and possibly

Middle Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek (d ...

Pleistocene of the

Cradle of Humankind The Cradle of Humankind is a paleoanthropological site and is located about northwest of Johannesburg, South Africa, in the Gauteng province. Declared a World Heritage Site by UNESCO in 1999, the site is home to the largest concentration of ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring count ...

, about 2.27 to 0.87 (or, more conservatively, 2 to 1) million years ago. It has been identified in Kromdraai, Swartkrans,

Sterkfontein Sterkfontein (Afrikaans for ''Strong Spring'') is a set of limestone caves of special interest to paleo-anthropologists located in Gauteng province, about northwest of Johannesburg, South Africa in the Muldersdrift area close to the town of K ...

Gondolin In Tolkien's legendarium, Gondolin was a secret city of Elves in the First Age of Middle-earth. The story of the Fall of Gondolin tells of the founding of the city; of the arrival there of Tuor, a prince of Men; of the betrayal of the city t ...

, Cooper's, and Drimolen Caves. Discovered in 1938, it was among the first early hominins described, and became the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen( ...

for the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

Paranthropus ''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 '' Au ...

''. However, it has been argued by some that ''Paranthropus'' is an invalid grouping and synonymous with ''

Australopithecus ''Australopithecus'' (, ; ) is a genus of early hominins that existed in Africa during the Late Pliocene and Early Pleistocene. The genus ''Homo'' (which includes modern humans) emerged within ''Australopithecus'', as sister to e.g. ''Austral ...

'', so the species is also often classified as ''Australopithecus robustus''. Robust australopithecines—as opposed to

gracile Gracility is slenderness, the condition of being gracile, which means slender. It derives from the Latin adjective ''gracilis'' (masculine or feminine), or ''gracile'' ( neuter), which in either form means slender, and when transferred for examp ...

australopithecines—are characterised by heavily built skulls capable of producing high stresses and bite forces, as well as inflated

cheek teeth Cheek teeth or post-canines comprise the molar and premolar teeth in mammals. Cheek teeth are multicuspidate (having many folds or tubercles). Mammals have multicuspidate molars (three in placentals, four in marsupials, in each jaw quadrant) and ...

( molars and premolars). Males had more heavily built skulls than females. ''P. robustus'' may have had a genetic susceptibility for

pitting enamel hypoplasia Enamel hypoplasia can take a variety of forms, but all types are associated with a reduction of enamel formation due to disruption in ameloblast production. One of the most common types, Pitting Enamel Hypoplasia (PEH), ranges from small circular pi ...

on the teeth, and seems to have had a dental cavity rate similar to non-agricultural modern humans. The species is thought to have exhibited marked

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...

, with males substantially larger and more robust than females. Based on 3 specimens, males may have been tall and females . Based on 4 specimens, males averaged in weight and females . The brain volume of the specimen SK 1585 is estimated to have been 476 cc, and of DNH 155 about 450 cc (for comparison, the brain volume of contemporary ''

Homo ''Homo'' () is the genus that emerged in the (otherwise extinct) genus '' Australopithecus'' that encompasses the extant species ''Homo sapiens'' (modern humans), plus several extinct species classified as either ancestral to or closely relat ...

'' varied from 500 to 900 cc). ''P. robustus'' limb anatomy is similar to that of other australopithecines, which may indicate a less efficient walking ability than modern humans, and perhaps some degree of arboreality (movement in the trees). ''P. robustus'' seems to have consumed a high proportion of C₄ savanna plants. In addition, it may have also eaten fruits, underground storage organs (such as roots and tubers), and perhaps

honey Honey is a sweet and viscous substance made by several bees, the best-known of which are honey bees. Honey is made and stored to nourish bee colonies. Bees produce honey by gathering and then refining the sugary secretions of plants (primar ...

and

termite Termites are small insects that live in colonies and have distinct castes ( eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blat ...

s. ''P. robustus'' may have used bones as tools to extract and process food. It is unclear if ''P. robustus'' lived in a

harem Harem ( Persian: حرمسرا ''haramsarā'', ar, حَرِيمٌ ''ḥarīm'', "a sacred inviolable place; harem; female members of the family") refers to domestic spaces that are reserved for the women of the house in a Muslim family. A har ...

society like

gorilla Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four ...

s or a multi-male society like baboons. ''P. robustus'' society may have been patrilocal, with adult females more likely to leave the group than males, but males may have been more likely to be evicted as indicated by higher male mortality rates and assumed increased risk of predation to solitary individuals. ''P. robustus'' contended with sabertooth cats,

leopard The leopard (''Panthera pardus'') is one of the five extant species in the genus '' Panthera'', a member of the cat family, Felidae. It occurs in a wide range in sub-Saharan Africa, in some parts of Western and Central Asia, Southern Russia ...

s, and

hyena Hyenas, or hyaenas (from Ancient Greek , ), are feliform carnivoran mammals of the family Hyaenidae . With only four extant species (each in its own genus), it is the fifth-smallest family in the Carnivora and one of the smallest in the clas ...

s on the mixed, open-to-closed landscape, and ''P. robustus'' bones probably accumulated in caves due to big cat predation. It is typically found in what were mixed open and wooded environments, and may have gone extinct in the Mid-Pleistocene Transition characterised by the continual prolonging of dry cycles and subsequent retreat of such habitat.

Taxonomy

Research history

Discovery

The first remains, a partial skull including a part of the jawbone (

TM 1517 TM 1517 is a fossilized skull and lower mandible of the species ''Paranthropus robustus''. It was discovered at Kromdraai, South Africa in 1938 by Robert Broom. Its characteristics include bony ear tubes positioned below the plane of the cheek bo ...

), were discovered in June 1938 at the Kromdraai cave site, South Africa, by local schoolboy Gert Terblanche. He gave the remains to South African conservationist Charles Sydney Barlow, who then relayed them to South African palaeontologist Robert Broom. Broom began investigating the site, and, a few weeks later, recovered a right distal

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

(the lower part of the upper arm bone), a proximal right ulna (upper part of a lower arm bone) and a distal phalanx bone of the big toe, all of which he assigned to TM 1517. He also identified a distal toe phalanx which he believed belonged to a baboon, but has since been associated with TM 1517. Broom noted the Kromdraai remains were especially robust compared to other hominins. In August 1938, Broom classified the robust Kromdraai remains into a new

, as ''Paranthropus robustus''. "''

''" derives from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

παρα ''para'', beside or alongside; and άνθρωπος ''ánthropos'', man. At this point in time, Australian anthropologist Raymond Dart had made the very first claim (quite controversially at the time) of an early ape-like human ancestor in 1924 from South Africa, '' Australopithecus africanus'', based on the

Taung child The Taung Child (or Taung Baby) is the fossilised skull of a young '' Australopithecus africanus''. It was discovered in 1924 by quarrymen working for the Northern Lime Company in Taung, South Africa. Raymond Dart described it as a new specie ...

. In 1936, Broom had described "''Plesianthropus transvaalensis''" (now synonymised with ''A. africanus'') from the

Caves only west from Kromdraai. All these species dated to the

Pleistocene The Pleistocene ( , often referred to as the '' Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed ...

and were found in the same general vicinity (now called the "

"). Broom considered them evidence of a greater diversity of hominins in the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Swartkrans Cave in 1948. ''P. robustus'' was only definitively identified at Kromdraai and Swartkrans until around the turn of the century when the species was reported elsewhere in the Cradle of Humankind at Sterkfontein,

, Cooper's, and Drimolen Caves. The species has not been found outside this small area.

"''P. crassidens''"

In 1948, at the nearby Swartkrans Cave, Broom described "''P. crassidens''" (distinct from ''P. robustus'') based on a subadult jaw, SK 6, because Swartkrans and Kromdraai clearly dated to different time intervals based on the diverging animal assemblages in these caves. At this point in time, humans and allies were classified into the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Hominidae, and non-human great apes into " Pongidae"; in 1950, Broom suggested separating early hominins into the subfamilies Australopithecinae (''Au. africanus'' and "''Pl. transvaalensis''"), "Paranthropinae" (''Pa. robustus'' and "''Pa. crassidens''"), and "Archanthropinae" ("''Au. prometheus''"). This scheme was widely criticised for being too liberal in demarcating species. Further, the remains were not firmly dated, and it was debated if there were indeed multiple hominin lineages or if there was only a single one leading to humans. Most prominently, Broom and South African palaeontologist John Talbot Robinson continued arguing for the validity of ''Paranthropus''. Anthropologists Sherwood Washburn and Bruce D. Patterson were the first to recommend synonymising ''Paranthropus'' with ''

'' in 1951, wanting to limit hominin genera to only that and ''

'', and it has since been debated whether or not ''Paranthropus'' is a junior synonym of ''Australopithecus''. In the spirit of tightening splitting criteria for hominin taxa, in 1954, Robinson suggested demoting "''P. crassidens''" to subspecies level as "''P. r. crassidens''", and also moved the Indonesian '' Meganthropus'' into the genus as "''P. palaeojavanicus''". ''Meganthropus'' has since been variously reclassified as a synonym of the Asian '' Homo erectus'', "'' Pithecanthropus dubius''", ''Pongo'' (

orangutan Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the gen ...

s), and so on, and in 2019 it was again argued to be a valid genus. In 1949, also in Swartkrans Cave, Broom and Robinson found a mandible which they preliminary described as "intermediate between one of the ape-men and true man," classifying it as a new genus and species "''Telanthropus capensis''". Most immediate reactions favoured synonymising "''T. capensis''" with "''P. crassidens''", whose remains were already abundantly found in the cave. In 1957, though, Italian biologist Alberto Simonetta moved it to the genus "''Pithecanthropus''", and Robinson (without a specific reason why) decided to synonymise it with ''H. erectus'' (African ''H. erectus'' are sometimes called ''

H. ergaster ''Homo ergaster'' is an extinct species or subspecies of archaic humans who lived in Africa in the Early Pleistocene. Whether ''H. ergaster'' constitutes a species of its own or should be subsumed into ''H. erectus'' is an ongoing and unres ...

'' today). In 1965, South African palaeoanthropologist Phillip V. Tobias questioned whether this classification is completely sound or not. By the 21st century, "''P. crassidens''" had more or less fallen out of use in favour of ''P. robustus''. American palaeoanthropologist Frederick E. Grine is the primary opponent of synonymisation of the two species.

''Gigantopithecus''

In 1939, Broom hypothesised that ''P. robustus'' was closely related to the similarly large-toothed ape '' Gigantopithecus'' from Asia (extinct apes were primarily known from Asia at the time) believing ''Gigantopithecus'' to have been a hominin. Primarily influenced by the mid-century opinions of Jewish German anthropologist Franz Weidenreich and German-Dutch palaeontologist

Ralph von Koenigswald Gustav Heinrich Ralph (often cited as G. H. R.) von Koenigswald (13 November 1902 – 10 July 1982) was a German-Dutch paleontologist and geologist who conducted research on hominins, including ''Homo erectus''. His discoverie ...

that ''Gigantopithecus'' was, respectively, the direct ancestor of the Asian ''H. erectus'' or closely related, much debate followed over whether ''Gigantopithecus'' was a hominin or a non-human ape. In 1972, Robinson suggested including ''Gigantopithecus'' in "Paranthropinae", with the

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recent" ...

Pakistani "''G. bilaspurensis''" (now '' Indopithecus'') as the ancestor of ''Paranthropus'' and the Chinese ''G. blacki''. He also believed that they both had a massive build. In contrast, he reported a very small build for ''A. africanus'' (which he referred to as "''Homo''" ''africanus'') and speculated it had some cultural and hunting abilities, being a member of the human lineage, which "paranthropines" lacked. With the popularisation of

cladistic Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived ch ...

s by the late 1970s to 1980s, and better resolution on how Miocene apes relate to later apes, ''Gigantopithecus'' was entirely removed from Homininae, and is now placed in the subfamily Ponginae with orangutans.

''P. boisei''

In 1959, another and much more robust australopithecine was discovered in East Africa, '' P. boisei'', and in 1975, the ''P. boisei'' skull KNM-ER 406 was demonstrated to have been contemporaneous with the ''H. ergaster/H. erectus'' skull KNM ER 3733 (which is considered a human ancestor). This is generally taken to show that ''Paranthropus'' was a

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to ''Homo'', both developing from some ''Australopithecus'' species, which at the time only included ''A. africanus''. In 1979, a year after describing '' A. afarensis'' from East Africa, anthropologists Donald Johanson and

Tim D. White Tim D. White (born August 24, 1950) is an American paleoanthropologist and Professor of Integrative Biology at the University of California, Berkeley. He is best known for leading the team which discovered Ardi, the type specimen of ''Ardipithecu ...

suggested that ''A. afarensis'' was instead the last common ancestor between ''Homo'' and ''Paranthropus'', and ''A. africanus'' was the earliest member of the ''Paranthropus'' lineage or at least was ancestral to ''P. robustus'', because ''A. africanus'' inhabited South Africa before ''P. robustus'', and ''A. afarensis'' was at the time the oldest known hominin species at roughly 3.5 million years old. Now, the earliest-known South African australopithecine (" Little Foot") dates to 3.67 million years ago, contemporaneous with ''A. afarensis''. The matter is still debated. It was long assumed that if ''Paranthropus'' is a valid genus then ''P. robustus'' was the ancestor of ''P. boisei'', but in 1985, anthropologists Alan Walker and Richard Leakey found that the 2.5-million-year-old East African skull KNM WT 17000—which they assigned to a new species '' A. aethiopicus''—was ancestral to ''A. boisei'' (they considered ''Paranthropus'' synonymous with ''Australopithecus''), thus establishing the ''boisei'' lineage as beginning long before ''robustus'' had existed.

Classification

The genus ''Paranthropus'' (otherwise known as "robust australopithecines", in contrast to the "

australopithecines") now also includes the East African ''P. boisei'' and ''P. aethiopicus''. It is still debated if this is a valid natural grouping (

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic ...

) or an invalid grouping of similar-looking hominins (

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

). Because skeletal elements are so limited in these species, their

affinities In post-classical history, an affinity was a collective name for the group ( retinue) of (usually) men whom a lord gathered around himself in his service; it has been described by one modern historian as "the servants, retainers, and other foll ...

with each other and with other australopithecines are difficult to gauge with accuracy. The jaws are the main argument for monophyly, but jaw anatomy is strongly influenced by diet and environment, and could have evolved independently in ''P. robustus'' and ''P. boisei''. Proponents of monophyly consider ''P. aethiopicus'' to be ancestral to the other two species, or closely related to the ancestor. Proponents of paraphyly allocate these three species to the genus ''Australopithecus'' as ''A. boisei'', ''A. aethiopicus'', and ''A. robustus''. In 2020, palaeoanthropologist Jesse M. Martin and colleagues' phylogenetic analyses reported the monophyly of ''Paranthropus'', but also that ''P. robustus'' had branched off before ''P. aethiopicus'' (that ''P. aethiopicus'' was ancestral to only ''P. boisei''). The exact classification of ''Australopithecus'' species with each other is quite contentious.

Anatomy

Head

Skull

Typical of ''Paranthropus'', ''P. robustus'' exhibits post-canine megadontia with enormous

but human-sized

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, w ...

s and canines. The premolars are shaped like molars. The enamel thickness on the cheek teeth is relatively on par with that of modern humans, though australopithecine cheek tooth enamel thickens especially at the tips of the cusps, whereas in humans it thickens at the base of the cusps. ''P. robustus'' has a tall face with slight prognathism (the jaw jutted out somewhat). The skulls of males have a well-defined sagittal crest on the midline of the skullcap and inflated cheek bones, which likely supported massive temporal muscles important in biting. The cheeks project so far from the face that, when in top-view, the nose appears to sit at the bottom of a concavity (a dished face). This displaced the eye sockets forward somewhat, causing a weak brow ridge and receding forehead. The inflated cheeks also would have pushed the

masseter muscle In human anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it ...

(important in biting down) forward and pushed the tooth rows back, which would have created a higher bite force on the premolars. The ramus of the jawbone, which connects the lower jaw to the upper jaw, is tall, which would have increased lever arm (and thereby, torque) of the masseter and medial pterygoid muscles (both important in biting down), further increasing bite force. The well-defined sagittal crest and inflated cheeks are absent in the presumed-female skull DNH-7, so Keyser suggested that male ''P. robustus'' may have been more heavily built than females (''P. robustus'' was sexually dimorphic). The Drimolen material, being more basal, is comparatively more gracile and consequently probably had a smaller bite force than the younger Swartkrans and Kromdraii ''P. robustus''. The brows of the former also are rounded off rather than squared, and the sagittal crest of the presumed-male DNH 155 is more posteriorly (towards the back of the head) positioned. The posterior semicircular canals in the inner ear of

SK 46 SK 46 is the fossilized partial cranium and palate of the species ''Paranthropus robustus''. It was discovered in Swartkrans, South Africa by local quarrymen and Robert Broom in 1949. It is estimated to be 1.5-1.8 million years old. Its character ...

and SK 47 are unlike those of the apelike ''Australopithecus'' or ''Homo'', suggesting different locomotory and head movement patterns, since inner ear anatomy affects the vestibular system (sense of balance). The posterior semicircular canals of modern humans are thought to aid in stabilisation while running, which could mean ''P. robustus'' was not an endurance runner. Paranthropus robustus female

Brain

Upon describing the species, Broom estimated the fragmentary braincase of TM 1517 as 600 cc, and he, along with South African anthropologist Gerrit Willem Hendrik Schepers, revised this to 575–680 cc in 1946. For comparison, the brain volume of contemporary ''Homo'' varied from 500 to 900 cc. A year later, British primatologist Wilfrid Le Gros Clark commented that, since only a part of the

temporal bone The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebral cortex. The temporal bones are overlaid by the sides of the head known as the temples, and house the structures of the ears ...

on one side is known, brain volume cannot be accurately measured for this specimen. In 2001, Polish anthropologist Katarzyna Kaszycka said that Broom quite often artificially inflated brain size in early hominins, and the true value was probably much lower. In 1972, American physical anthropologist Ralph Holloway measured the skullcap SK 1585, which is missing part of the frontal bone, and reported a volume of about 530 cc. He also noted that, compared to other australopithecines, ''Paranthropus'' seems to have had an expanded

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cere ...

like ''Homo'', echoing what Tobias said while studying ''P. boisei'' skulls in 1967. In 2000, American neuroanthropologist

Dean Falk Dean Falk (born June 25, 1944) is an American academic neuroanthropologist who specializes in the evolution of the brain and cognition in higher primates. She is the Hale G. Smith Professor of Anthropology and a Distinguished Research Professor ...

and colleagues filled in frontal bone anatomy of SK 1585 using the ''P. boisei'' specimens KNM-ER 407, OH 5, and KNM-ER 732, and recalculated the brain volume to about 476 cc. They stated overall brain anatomy of ''P. robustus'' was more like that of non-human apes. In 2020, the nearly complete skull DNH 155 was discovered and was measured to have had a brain volume of 450 cc.

Blood vessels

In 1983, while studying SK 1585 (''P. robustus'') and KNM-ER 407 (''P. boisei'', which he referred to as ''robustus''), French anthropologist Roger Saban stated that the parietal branch of the middle meningeal artery originated from the posterior branch in ''P. robustus'' and ''P. boisei'' instead of the anterior branch as in earlier hominins, and considered this a derived characteristic due to increased brain capacity. It has since been demonstrated that, at least for ''P. boisei'', the parietal branch could originate from either the anterior or posterior branches, sometimes both in a single specimen on opposite sides of the skull. Regarding the dural venous sinuses, in 1983, Falk and anthropologist Glenn Conroy suggested that, unlike ''A. africanus'' or modern humans, all ''Paranthropus'' (and ''A. afarensis'') had expanded occipital and marginal (around the foramen magnum) sinuses, completely supplanting the transverse and sigmoid sinuses. They suggested the setup would have increased blood flow to the internal vertebral venous plexuses or

internal jugular vein The internal jugular vein is a paired jugular vein that collects blood from the brain and the superficial parts of the face and neck. This vein runs in the carotid sheath with the common carotid artery and vagus nerve. It begins in the post ...

, and was thus related to the reorganisation of the blood vessels supplying the head as an immediate response to bipedalism, which relaxed as bipedalism became more developed. In 1988, Falk and Tobias demonstrated that early hominins (at least ''A. africanus'' and ''P. boisei'') could have both an occipital/marginal and transverse/sigmoid systems concurrently or on opposite halves of the skull.

Torso

Few

vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characterist ...

e are assigned to ''P. robustus''. The only thoracolumbar series ( thoracic and lumbar series) preserved belongs to the juvenile SKW 14002, and either represents the 1st to the 4th lumbar vertebrae, or the 2nd to the 5th. SK 3981 preserves a 12th thoracic vertebra (the last in the series), and a lower lumbar vertebra. The 12th thoracic vertebra is relatively elongated, and the articular surface (where it joins with another vertebra) is

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; bloo ...

-shaped. The T12 is more compressed in height than that of other australopithecines and modern apes. Modern humans which suffer from spinal disc herniation often have vertebrae that are more similar to those of chimpanzees than healthy humans. Early hominin vertebrae are similar to those of a pathological human, including the only other 12th thoracic vertebra known for ''P. robustus'', the juvenile SK 853. Conversely, SK 3981 is more similar to those of healthy humans, which could be explained as: SK 3981 is abnormal, the vertebrae took on a more humanlike condition with maturity, or one of these specimens is assigned to the wrong species. The shape of the lumbar vertebrae is much more similar to that of Turkana Boy (''H. ergaster''/''H. erectus'') and humans than other australopithecines. The pedicles (which jut out diagonally from the vertebra) of the lower lumbar vertebra are much more robust than in other australopithecines and are within the range of humans, and the transverse processes (which jut out to the sides of the vertebra) indicate powerful iliolumbar ligaments. These could have bearing on the amount of time spent upright compared to other australopithecines. DNH 43 ventral

The pelvis is similar to the pelvises of ''A. africanus'' and ''A. afarensis'', but it has a wider iliac blade and smaller

acetabulum The acetabulum (), also called the cotyloid cavity, is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint. Structure There are three bones of the ''os coxae'' (hip bone) tha ...

and hip joint. Like modern humans, the ilium of ''P. robustus'' features development of the surface and thickening of the posterior superior iliac spine, which are important in stabilising the sacrum, and indicates lumbar lordosis (curvature of the lumbar vertebrae) and thus bipedalism. The anatomy of the sacrum and the first lumbar vertebra (at least the vertebral arch), preserved in DNH 43, are similar to those of other australopithecines. The pelvis seems to indicate a more-or-less humanlike hip joint consistent with bipedalism, though differences in overall pelvic anatomy may indicate ''P. robustus'' used different muscles to generate force and perhaps had a different mechanism to direct force up the spine. This is similar to the condition seen in ''A. africanus''. This could potentially indicate the lower limbs had a wider range of motion than those of modern humans.

Limbs

The distal (lower)

of ''P. robustus'' falls within the variation of both modern humans and chimps, as the distal humerus is quite similar between humans and chimpanzees. The

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

of ''P. robustus'' is comparable in form to ''Australopithecus'' species. The wrist joint had the same maneuverability as that of modern humans rather than the greater flexion achieved by non-human apes, but the head of radius (the elbow) seems to have been quite capable of maintaining stability when the forearm was flexed like non-human apes. It is possible this reflects some arboreal activity (movement in the trees) as is controversially postulated in other australopithecines. SKX 3602 exhibits robust radial styloid processes near the hand which indicate strong brachioradialis muscles and extensor retinaculae. Like humans, the finger bones are uncurved and have weaker muscle attachment than non-human apes, though the proximal phalanges are smaller than in humans. The intermediate phalanges are stout and straight like humans, but have stouter bases and better developed flexor impressions. The distal phalanges seem to be essentially humanlike. These could indicate a decreased climbing capacity compared to non-human apes and ''P. boisei''. The ''P. robustus'' hand is consistent with a humanlike precision grip which would have made possible the production or usage of tools requiring greater motor functions than non-human primate tools. The femur, as in ''P. boisei'' and ''H. habilis'', is flattened anteroposteriorly (on the front and back side). This may indicate a walking gait more similar to early hominins than to modern humans (less efficient gait). Four femora assigned to ''P. robustus''—SK 19, SK 82, SK 97, and SK 3121—exhibit an apparently high

anisotropic Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's phys ...

trabecular bone (at the hip joint) structure, which could indicate reduced mobility of the hip joint compared to non-human apes, and the ability to produce forces consistent with humanlike bipedalism. The

femoral head The femoral head (femur head or head of the femur) is the highest part of the thigh bone ( femur). It is supported by the femoral neck. Structure The head is globular and forms rather more than a hemisphere, is directed upward, medialward, and a ...

StW 311, which either belongs to ''P. robustus'' or early ''Homo'', seems to have habitually been placed in highly flexed positions based on the wearing patterns, which would be consistent with frequent climbing activity. It is unclear if frequent squatting could be a valid alternative interpretation. The textural complexity of the kneecap SKX 1084, which reflects cartilage thickness and thus usage of the knee joint and bipedality, is midway between modern humans and chimps. The big toe bone of ''P. robustus'' is not dextrous, which indicates a humanlike foot posture and range of motion, but the more distal ankle joint would have inhibited the modern human toe-off

gait cycle A (bipedal) gait cycle is the time period or sequence of events or movements during locomotion in which one foot contacts the ground to when that same foot again contacts the ground, and involves propulsion of the centre of gravity In physics, ...

. ''P. robustus'' and ''H. habilis'' may have achieved about the same grade of bipedality.

Size

Broom had noted that the ankle bone and humerus of the holotype TM 1517 were about the same dimensions as that of a modern San woman, and so assumed humanlike proportions in ''P. robustus''. In 1972, Robinson estimated ''Paranthropus'' as having been massive. He calculated the humerus-to-femur ratio of ''P. robustus'' by using the presumed female humerus of STS 7 and comparing it with the presumed male femur of STS 14. He also had to estimate the length of the humerus using the femur assuming a similar degree of sexual dimorphism between ''P. robustus'' and humans. Comparing the ratio to humans, he concluded that ''P. robustus'' was a heavily built species with a height of and a weight of . Consequently, Robinson had described its locomotory habits as, "a compromise between erectness and facility for quadrupedal climbing." In contrast, he estimated ''A. africanus'' (which he called "''H.''" ''africanus'') to have been tall and in weight, and to have also been completely bipedal. This was soon challenged in 1974 by American palaeontologist Stephen Jay Gould and English palaeoanthropologist David Pilbeam, who guessed from the available skeletal elements a much lighter weight of about . Similarly, in 1988, American anthropologist Henry McHenry reported much lighter weights as well as notable sexual dimorphism for ''Paranthropus''. McHenry plotted body size vs. the cross sectional area of the femoral head for a sample of just humans and a sample with all great apes including humans, and calculated

linear regression In statistics, linear regression is a linear approach for modelling the relationship between a scalar response and one or more explanatory variables (also known as dependent and independent variables). The case of one explanatory variable is ...

s for each one. Based on the average of these two regressions, he reported an average weight of for ''P. robustus'' using the specimens SK 82 and SK 97. In 1991, McHenry expanded his sample size, and also estimated the living size of Swartkrans specimens by scaling down the dimensions of an average modern human to meet a preserved leg or foot element (he considered the arm measurements too variable among hominins to give accurate estimates). At Members 1 and 2, about 35% of the ''P. robustus'' leg or foot specimens were the same size as those in a human, 22% in a human, and the remaining 43% bigger than the former but less than a human except for KNM‐ER 1464 (an ankle bone). At Member 3, all individuals were consistent with a human. Smaller adults thus seem to have been more common. McHenry also estimated the living height of 3 ''P. robustus'' specimens (male SK 82, male SK 97, and female or subadult SK 3155), by scaling down an average human to meet the estimated size of the preserved femur, as , , and , respectively. Based on just these three, he reported an average height of for ''P. robustus'' males and for females. In 2001, palaeoanthropologist Randall L. Susman and colleagues, using two recently discovered proximal femoral fragments from Swartkrans, estimated an average of for males and for females. If these four proximal femur specimens—SK 82, SK 97, SKW 19, and SK 3121—are representative of the entire species, they said that this degree of sexual dimorphism is greater than what is exhibited in

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

s and chimpanzees, but less than

s and

s. Female ''P. robustus'' were about the same estimated weight as female ''H. ergaster/H. erectus'' in Swartkrans, but they estimated male ''H. ergaster/H. erectus'' as much bigger at . In 2012, American anthropologist Trenton Holliday, using the same equation as McHenry on 3 specimens, reported an average of with a range of . In 2015, biological anthropologist Mark Grabowski and colleagues, using 9 specimens, estimated an average of for males and for females.

Palaeobiology

Diet

In 1954, Robinson suggested that the heavily built skull of ''P. robustus'' and resultantly exorbitant bite force was indicative of a

specialist Specialist may refer to: Occupations * Specialist (rank), a military rank ** Specialist (Singapore) * Specialist (arena football) * Specialist degree, in academia * Specialty (medicine) * Designated market maker, in the American stock market * ...

diet adapted for frequently cracking hard foods such as nuts. Because of this, the predominant model of ''Paranthropus'' extinction for the latter half of the 20th century was that they were unable to adapt to the volatile climate of the Pleistocene, unlike the much more adaptable ''Homo''. Subsequent researchers reinforced this model studying the musculature of the face, dental wearing patterns, and primate ecology. In 1981, English anthropologist Alan Walker, while studying the ''P. boisei'' skulls KNM-ER 406 and 729, pointed out that bite force is a measure of not only the total pressure exerted but also the surface area of the tooth over which the pressure is being exerted, and ''Paranthropus'' teeth are 4–5 times the size of modern human teeth. Because the chewing muscles are arranged the same way, Walker postulated that the heavy build was instead an adaptation to chew a large quantity of food at the same time. He also found that microwearing on 20 ''P. boisei'' molar specimens were indistinguishable from patterning recorded in mandrills, chimps, and orangutans. Despite subsequent arguments that ''Paranthropus'' were not specialist feeders, the predominant consensus in favour of Robinson's initial model did not change for the remainder of the 20th century. In 2004, in their review of ''Paranthropus'' dietary literature, anthropologists Bernard Wood and David Strait concluded that ''Paranthropus'' were most definitely generalist feeders, and that ''P. robustus'' was an

omnivore An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nut ...

. They found that the microwear patterns in ''P. robustus'' suggest hard food was infrequently consumed, and therefore the heavy build of the skull was only relevant when eating less desirable fallback foods. Such a strategy is similar to that used by modern gorillas, which can sustain themselves entirely on lower quality fallback foods year-round, as opposed to lighter built chimpanzees (and presumably gracile australopithecines) which require steady access to high quality foods. In 1980, anthropologists Tom Hatley and John Kappelman suggested that early hominins ( convergently with

bear Bears are carnivoran mammals of the family Ursidae. They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats throughout the N ...

s and pigs) adapted to eating abrasive and calorie-rich underground storage organs (USOs), such as roots and tubers. Since then, hominin exploitation of USOs has gained more support. In 2005, biological anthropologists

Greg Laden Gregory Thomas Laden is an American biological anthropologist and science blogger. Education Born in 1958, Laden received his B.A. from the University of the State of New York's Regents College program in 1984, and his M.A. and Ph.D. from Harv ...

and Richard Wrangham proposed that ''Paranthropus'' relied on USOs as a fallback or possibly primary food source, and noted that there may be a correlation between high USO abundance and hominin occupation. A 2006 carbon isotope analysis suggested that ''P. robustus'' subsisted on mainly C₄ savanna plants or C₃ forest plants depending on the season, which could indicate either seasonal shifts in diet or seasonal migration from forest to savanna. ''H. ergaster/H. erectus'' appears to have consumed about the same proportion of C₃ to C₄ based foods as ''P. robustus''. ''P. robustus'' likely also commonly cracked hard foods such as seeds or nuts, as it had a moderate tooth-chipping rate (about 12% in a sample of 239 individuals, as opposed to little to none for ''P. boisei''). A high cavity rate could indicate

consumption. Juvenile ''P. robustus'' may have relied more on tubers than adults, given the elevated levels of

strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ...

compared to adults in teeth from Swartkrans Cave, which, in the area, was most likely sourced from tubers.

Dentin Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by e ...

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 instead less capable of removing grit from dug-up food rather than purposefully seeking out more abrasive foods.

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 are highly speculative. In 2007, anthropologist Charles Lockwood and colleagues pointed out that ''P. robustus'' appears to have had pronounced

, with males notably larger than females. This is commonly correlated with a male-dominated

polygamous Crimes Polygamy (from Late Greek (') "state of marriage to many spouses") is the practice of marrying multiple spouses. When a man is married to more than one wife at the same time, sociologists call this polygyny. When a woman is married ...

society, such as the harem society of modern forest-dwelling silverback gorillas where one male has exclusive breeding rights to a group of females. Estimated male-female size disparity in ''P. robustus'' is comparable to gorillas (based on facial dimensions), and younger males were less robust than older males (delayed maturity is also exhibited in gorillas). Because the majority of sexed ''P. robustus'' specimens are male (or at least presumed male), males seem to have had a higher mortality rate than females. In a harem society, males are more likely to be evicted from the group given higher male–male competition over females, and lone males may have been put at a higher risk of predation. By this hypothesis, a female moving out of her birth group may have spent little time alone and transferred immediately to another established group. 20070819 Dierenpark Emmen (20)

However, in 2011, palaeoanthropologist Sandi Copeland and colleagues studied the

strontium isotope The alkaline earth metal strontium (38Sr) has four stable, naturally occurring isotopes: 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%). Its standard atomic weight is 87.62(1). Only 87Sr is radiogenic; it is produced by decay from t ...

ratio of ''P. robustus'' teeth from the dolomite Sterkfontein Valley, and found that like other hominins, but unlike other great apes, ''P. robustus'' females were more likely to leave their place of birth ( patrilocal). This 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. Similarly, in 2016, Polish anthropologist Katarzyna Kaszycka rebutted that, 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. If ''P. robustus'' preferred a savanna habitat, a multi-male society would have been more conducive in defending the troop from predators in the more exposed environment, much like baboons which live in the savanna. Even in a multi-male society, it is still possible that males were more likely to be evicted, explaining male-skewed mortality with the same mechanism. In 2017, anthropologist Katharine Balolia and colleagues postulated that, because male non-human great apes have a larger sagittal crest than females (particularly gorillas and orangutans), the crest may be influenced by

sexual selection Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (in ...

in addition to supporting chewing muscles. Further, the size of the sagittal crest (and the gluteus muscles) in male

western lowland gorilla The western lowland gorilla (''Gorilla gorilla gorilla'') is one of two Critically Endangered subspecies of the western gorilla (''Gorilla gorilla'') that lives in montane, primary and secondary forest and lowland swampland in central Afr ...

s has been correlated with reproductive success. Balolia ''et al.'' extended their interpretation of the crest to the males of ''Paranthropus'' species, with the crest and resultantly larger head (at least in ''P. boisei'') being used for some kind of display. This contrasts with other primates which flash the typically enlarged canines in

agonistic display Agonistic behaviour is any social behaviour related to fighting. The term has broader meaning than aggressive behaviour because it includes threats, displays, retreats, placation, and conciliation. The term "agonistic behaviour" was first implemen ...

(''Paranthropus'' likely did not do this as the canines are comparatively small), though it is also possible that the crest is only so prominent in male gorillas and orangutans because they require larger temporalis muscles to achieve a wider gape to better display the canines.

Technology

Cave sites in the Cradle of Humankind often have stone and bone tools, with the former attributed to early ''Homo'' and the latter generally to ''P. robustus'', as bone tools are most abundant when ''P. robustus'' remains far outnumber ''Homo'' remains. Australopithecine bone technology was first proposed by Dart in the 1950s with what he termed the " osteodontokeratic culture", which he attributed to ''A. africanus'' at Makapansgat dating to 3–2.6 million years ago. These bones are no longer considered to have been tools, and the existence of this culture is not supported. The first probable bone tool was reported by Robinson in 1959 at Sterkfontein Member 5. Excavations led by South African palaeontologist Charles Kimberlin Brain at Swartkrans in the late 1980s and early 1990s recovered 84 similar bone tools, and excavations led by Keyser at Drimolen recovered 23. These tools were all found alongside

Acheulean Acheulean (; also Acheulian and Mode II), from the French ''acheuléen'' after the type site of Saint-Acheul, is an archaeological industry of stone tool manufacture characterized by the distinctive oval and pear-shaped " hand axes" associat ...

stone tools, except for those from Swartkrans Member 1 which bore Oldowan stone tools. Thus, there are 108 bone tool specimens from the region in total, and possibly an additional two from Kromdraai B. The two stone tools (either "Developed Oldowan" or "Early Acheulean") from Kromdraai B could possibly be attributed to ''P. robustus'', as ''Homo'' has not been confidently identified in this layer, though it is possible that the stone tools were reworked (moved into the layer after the inhabitants had died). Bone tools may have been used to cut or process vegetation, process fruits (namely marula fruit), strip tree bark, or dig up tubers or termites. The form of ''P. robustus'' incisors appears to be intermediate between ''H. erectus'' and modern humans, which could possibly mean it did not have to regularly bite off mouthfuls of a large food item due to preparation with simple tools. The bone tools were typically sourced from the

shaft Shaft may refer to: Rotating machine elements * Shaft (mechanical engineering), a rotating machine element used to transmit power * Line shaft, a power transmission system * Drive shaft, a shaft for transferring torque * Axle, a shaft around whi ...

of long bones from medium- to large-sized mammals, but tools sourced from

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 teeth in place. The mandible sits beneath the maxilla. It is the only movable bon ...

s, ribs, and horn cores have also been found. They were not manufactured or purposefully shaped for a task, but since they display no weathering, and there is a preference displayed for certain bones, raw materials were likely specifically hand picked. This contrasts with East African bone tools which appear to have been modified and directly cut into specific shapes before using. In 1988, Brain and South African archaeologist A. Sillent analysed the 59,488 bone fragments from Swartkrans Member 3, and found that 270 had been burnt, mainly belonging to medium-sized antelope, but also zebra, warthog, baboon, and ''P. robustus''. They were found across the entire depth of Member 3, so fire was a regular event throughout its deposition. Based on colour and structural changes, they found that 46 were heated to below , 52 to , 45 to , and 127 above this. They concluded that these bones were, "the earliest direct evidence of fire use in the fossil record," and compared the temperatures with those achieved by experimental campfires burning

white stinkwood ''Celtis africana'', the white stinkwood, is a deciduous tree in the family Cannabaceae. Its habit ranges from a tall tree in forest to a medium-sized tree in bushveld and open country, and a shrub on rocky soil. It occurs in Yemen and over la ...

which commonly grows near the cave. Though some bones had cut marks consistent with butchery, they said it was also possible hominins were making fire to scare away predators or for warmth instead of cooking. Because both ''P. robustus'' and ''H. ergaster''/''H. erectus'' were found in the cave, they were unsure which species to attribute the fire to. As an alternative to hominin activity, because the bones were not burnt inside the cave, it is possible that they were naturally burnt in cyclically occurring

wildfire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...

s (dry savanna grass as well as possible

guano Guano (Spanish from qu, wanu) is the accumulated excrement of Seabird, seabirds or bats. As a manure, guano is a highly effective fertilizer due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant ...

or plant accumulation in the cave may have left it susceptible to such a scenario), and then washed into what would become Member 3. The now-earliest claim of fire usage is 1.7 million years ago at

Wonderwerk Cave Wonderwerk Cave is an archaeological site, formed originally as an ancient solution cavity in dolomite rocks of the Kuruman Hills, situated between Danielskuil and Kuruman in the Northern Cape Province, South Africa. It is a National Heritage ...

, South Africa, made by South African archaeologist Peter Beaumont in 2011, which he attributed to ''H. ergaster''/''H. erectus''.

Development

Australopithecines are generally considered to have had a faster, apelike growth rate than modern humans largely due to dental development trends. Broadly speaking, the emergence of the first permanent molar in early hominins has been variously estimated anywhere from 2.5 to 4.5 years, which all contrast markedly with the modern human average of 5.8 years. The 1st permanent molar of SK 63, which may have died at 3.4–3.7 years of age, possibly erupted at 2.9–3.2 years. In modern apes (including humans), dental development trajectory is strongly correlated with life history and overall growth rate, but it is possible that early hominins simply had a faster dental trajectory but a slower life history due to environmental factors, such as early weaning age as is exemplified in modern

indriid The Indriidae (sometimes incorrectly spelled Indridae) are a family of strepsirrhine primates. They are medium- to large-sized lemurs, with only four teeth in the toothcomb instead of the usual six. Indriids, like all lemurs, live exclusively on t ...

lemur Lemurs ( ) (from Latin ''lemures'' – ghosts or spirits) are wet-nosed primates of the superfamily Lemuroidea (), divided into 8 families and consisting of 15 genera and around 100 existing species. They are endemic to the island of Madaga ...

s. In TM 1517, fusion of the elements of the distal humerus (at the elbow joint) occurred before the fusion of the elements in the distal big toe phalanx, much like in chimps and

bonobo The bonobo (; ''Pan paniscus''), also historically called the pygmy chimpanzee and less often the dwarf chimpanzee or gracile chimpanzee, is an endangered great ape and one of the two species making up the genus '' Pan,'' the other being the co ...

s, but unlike humans, which could also indicate an apelike growth trajectory. While growing, the front part of the jaw in ''P. robustus'' is depository (so it grows) whereas the sides are resorptive (so they recede). For comparison, chimp jaws are generally depository reflecting prognathism, and modern humans resorptive reflecting a flat face. In ''Paranthropus'', this may have functioned to thicken the

palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly s ...

. Unlike other apes and gracile australopithecines, but like humans, the premaxillary suture between the premaxilla and 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 ...

(on the palate) formed early in development. At early stages, the ''P. robustus'' jawbone was somewhat similar to that of modern humans, but the breadth grew in ''P. robustus'', as to be expected from its incredible robustness in adulthood. By the time the first permanent molar erupts, the body of the mandible and the front jaw broadened, and the ramus of the mandible elongated, diverging from the modern human trajectory. Because the ramus was so tall, it is suggested that ''P. robustus'' experienced more anterior face rotation than modern humans and apes. Growth was most marked between the eruptions of the first and second permanent molars, most notably in terms of the distance from the back of the mouth to the front of the mouth, probably to make room for the massive postcanine teeth. Like humans, jaw robustness decreased with age, though it decreased slower in ''P. robustus''. Regardless if ''P. robustus'' followed a human or non-human ape dental development timeframe, the premolars and molars would have had an accelerated growth rate to achieve their massive size. In contrast, the presence of perikymata on the incisors and canines (growth lines which typically are worn away after eruption) could indicate these teeth had a reduced growth rate. The tooth roots of ''P. robustus'' molars may have grown at a faster rate than gracile australopithecines; the root length of SK 62's 1st molar, which was reaching emergence from the dental alveolus, is about . In contrast, those of other hominins reach after the tooth has emerged not only from the gums (a later stage of dental development). SK 62's growth trajectory is more similar to that of gorillas, whose roots typically measure when emerging from the gums. Females may have reached skeletal maturity by the time the third molar erupted, but males appear to have continued growing after reaching dental maturity, during which time they become markedly more robust than females ( sexual bimaturism). Similarly, male gorillas complete dental development about the same time as females, but continue growing for up to 5 or 6 years; and male mandrills complete dental development before females, but continue growing for several years more. It is debated whether or not ''P. robustus'' had a defined growth spurt in terms of overall height during adolescence, an event unique to humans among modern apes.

Life history

In 1968, American anthropologist Alan Mann, using dental maturity, stratified ''P. robustus'' specimens from Swartkrans into different ages, and found an average of 17.2 years at death (they did not necessarily die from old age), and the oldest specimen was 30–35 years old. He also reported an average of 22.2 years for ''A. africanus''. Using these, he argued these hominins had a humanlike prolonged childhood. In response, in 1971, biologist Kelton McKinley repeated Mann's process with more specimens, and (including ''P. boisei'') reported an average of 18 years. McKinley agreed with Mann that ''P. robustus'' may have had a prolonged childhood. McKinley also speculated that sexual maturity was reached at approximately 11 years because it is about halfway between the averages for chimps (9 years) and humans (13). Based on this, he concluded babies were birthed at intervals of 3 to 4 years using a statistical test to maximise the number of children born. In 1972, after estimating a foetal size of based on an adult female weight of , anthropologist Walter Leutenegger estimated foetal head size at about , similar to a chimp. In 1973, using this and an equation between foetal head size and gestation (assuming foetal growth rate of 0.6 for all mammals), biologist John Frazer estimated a gestation of 300 days for ''P. robustus''. In response, Leutenegger pointed out that apes have highly variable foetal growth rates, and "estimates on gestation periods based on this rate and birth weight are useless." In 1985, British biologists Paul H. Harvey and Tim Clutton-Brock came up with equations relating body size to life history events for primates, which McHenry applied to australopithecines in 1994. For ''P. robustus'', he reported newborn brain size of 175 cc and weight of , gestation 7.6 months, weaning after 30.1 months of age, maturation age 9.7 years, breeding age 11.4 years, birth interval 45 months, and lifespan 43.3 years. These roughly aligned with other australopithecines and chimps. However, for chimps, he got strongly inaccurate results when compared to actual data for newborn brain size, weaning age, and birth interval, and for humans all metrics except birth interval.

Pathology

Based on a sample of 402 teeth, ''P. robustus'' seems to have had a low incidence rate of about 12–16% for tertiary dentin, which forms to repair tooth damage caused by excessive wearing or dental cavities. This is similar to what was found for ''A. africanus'' and ''

H. naledi '' Homo naledi'' is an extinct species of archaic human discovered in 2013 in the Rising Star Cave, Cradle of Humankind, South Africa dating to the Middle Pleistocene 335,000–236,000 years ago. The initial discovery comprises 1,550 specime ...

'' (all three inhabited the Cradle of Humankind at different points in time). In contrast, chimpanzees have an incidence rate of 47%, and gorillas as much as 90%, probably due to a diet with a much higher content of tough plants. ''P. robustus'' seems to have had notably high rates of

(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, for the combined teeth of ''A. africanus'', '' A. sediba'', early ''Homo'', and ''H. naledi''. The condition of these holes covering the entire tooth is consistent with the modern human ailment amelogenesis imperfecta. 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 region concerned with thickening enamel also increased the risk of developing PEH. As many as four ''P. robustus'' individuals have been identified as having had dental cavities, indicating a rate similar to non-agricultural modern humans (1–5%). This is odd as ''P. robustus'' is thought to have had a diet high in gritty foods, and gritty foods should decrease cavity incidence rate, so ''P. robustus'' may have often consumed high-sugar cavity-causing foods. PEH may have also increased susceptibility to cavities. A molar from Drimolen 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 also presented either alveolar resportion, which is commonly associated with gum disease; or super-eruption of the tooth which occurs when it becomes worn down and has to erupt a bit more in order to maintain a proper bite, exposing the root in the process. 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, possibly due to a change in diet or mouth microbiome, or the loss of the adjacent molar. In a sample of 15 ''P. robustus'' specimens, all of them exhibited mild to moderate alveolar bone loss resulting from periodontal disease (the wearing away of the bone which supports the teeth due to gum disease). In contrast, in a sample of 10 ''A. africanus'' specimens, three exhibited no pathologies of the alveolar bone. Measuring the distance between the alveolar bone and the cementoenamel junction, ''P. robustus'' possibly suffered from a higher rate of tooth-attachment loss, unless ''P. robustus'' had a higher cervical height (the slightly narrowed area where the crown meets the root) in which case these two species had the same rate of tooth-attachment loss. If the former is correct, then the difference may be due to different dietary habits, chewing strategies, more pathogenic mouth microflora in ''P. robustus'', or some immunological difference which made ''P. robustus'' somewhat more susceptible to gum disease. While removing the

matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** '' The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...

encapsulating TM 1517, Schepers noted a large rock, which would have weighed , which had driven itself into the braincase through the parietal bone. He considered this evidence that another individual had killed TM 1517 by launching the rock as a projectile in either defense or attack, but the most parsimonious explanation is that the rock was deposited during the fossilisation process after TM 1517 had died. In 1961, science writer

Robert Ardrey Robert Ardrey (October 16, 1908 – January 14, 1980) was an American playwright, screenwriter and science writer perhaps best known for '' The Territorial Imperative'' (1966). After a Broadway and Hollywood career, he returned to his academic ...

noted two small holes about 2.5 cm (an inch) apart on the child skullcap SK 54, and believed this individual had been killed by being struck twice on the head in an assault; in 1970, Brain reinterpreted this as evidence of a leopard attack.

Palaeoecology

The Pleistocene Cradle of Humankind was mainly dominated by the springbok ''Antidorcas recki'', but other antelope, giraffes, and elephants were also seemingly abundant megafauna. The carnivore assemblage comprises the sabertoothed cats '' Dinofelis spp.'' and '' Megantereon spp.'', and the hyena '' Lycyaenops silberbergi''. Overall, the animal assemblage of the region broadly indicates a mixed, open-to-closed landscape featuring perhaps montane grasslands and shrublands. Australopithecines and early ''Homo'' likely preferred cooler conditions than later ''Homo'', as there are no australopithecine sites that were below in elevation at the time of deposition. This would mean that, like chimps, they often inhabited areas with an average diurnal temperature of , dropping to at night. ''P. robustus'' also cohabited the Cradle of Humankind with ''H. ergaster''/''H. erectus''. In addition, these two species resided alongside ''Australopithecus sediba'' which is known from about 2 million years ago at Malapa. The most recent ''A. africanus'' specimen, Sts 5, dates to about 2.07 million years ago, around the arrival of ''P. robustus'' and ''H. erectus''. It has been debated whether or not ''P. robustus'' would have had symbiotic, neutral, or antagonist relations with contemporary ''Australopithecus'' and ''Homo''. It is possible that South Africa was a refugium for ''Australopithecus'' until about 2 million years ago with the beginning of major climatic variability and volatility, and potentially competition with ''Homo'' and ''Paranthropus''.

Fossil-bearing deposits

;Swartkrans At Swartkrans, ''P. robustus'' has been identified from Members 1–3. ''Homo'' is also found in these deposits, but species identification in Members 1 and 2 is debated between ''H. ergaster/H. erectus'', '' H. habilis'', '' H. rudolfensis'', or multiple species. In total, over 300 ''P. robustus'' specimens representing over 130 individuals, predominantly isolated teeth, have been recovered from Swartkrans. Member 1 and Member 3 have several mammal species in common, making dating by animal remains (

biostratigraphy Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...

) yield overlapping time intervals. Like the East African Olduvai Bed I (2.03–1.75 million years ago) and Lower Bed II (1.75–1.70 million years ago), Member 1 preserved the antelope '' Parmularius angusticornis'', the wildebeest, and the Cape buffalo. The presence of the Hamadryas baboon and '' Dinopithecus'' could mean Members 1–3 were deposited 1.9–1.65 million years ago, though the presence of warthogs suggests some sections of the deposits could date to after 1.5 million years ago. Uranium–lead dating reports intervals of 3.21–0.45 million years ago for Member 1 (a very large error range), 1.65–1.07 million years ago for Member 2, and 1.04–0.62 million years ago for Member 3, though more likely the younger side of the estimate; this could mean ''P. robustus'' outlived ''P. boisei''. Cosmogenic nuclide geochronology has reported much more constrained dates of 2.2–1.8 million years ago for Member 1, and 0.96 million years ago for Member 3. No suitable section of Member 2 could be identified to date. ;Sterkfontein The cranium of an Australopithecus africanus (3967774317)

The cranium of an Australopithecus africanus (3967774317)

At Sterkfontein, only the specimens StW 566 and StW 569 are firmly assigned to ''P. robustus'', coming from the "Oldowan infill" dating to 2–1.7 million years ago in a section of Member 5. Earlier members yielded ''A. africanus''. In 1988, palaeoanthropologist Ronald J. Clarke suggested StW 505 from the earlier Member 4 was an ancestor to ''P. robustus''. The specimen is still generally assigned to ''A. africanus'', though the Sterkfontein hominins are known to have an exceedingly wide range of variation, and it is debated whether or not the materials represent multiple species instead of just ''A. africanus''. The appearance of the baboon '' Theropithecus oswaldi'',

zebra Zebras (, ) (subgenus ''Hippotigris'') are African equines with distinctive black-and-white striped coats. There are three living species: the Grévy's zebra (''Equus grevyi''), plains zebra (''E. quagga''), and the mountain zebra (''E. ...

lion The lion (''Panthera leo'') is a large Felidae, cat of the genus ''Panthera'' native to Africa and India. It has a muscular, broad-chested body; short, rounded head; round ears; and a hairy tuft at the end of its tail. It is sexually dimorphi ...

s, ostriches, springhares, and several grazing antelope in Member 5 indicates the predominance of open grasslands, but sediment analysis indicates the cave opening was moist during deposition, which could point to a well-watered wooded grassland. ;Kromdraai At Kromdraai, ''P. robustus'' has been unearthed at Kromdraai B, and almost all ''P. robustus'' fossils discovered in the cave have been recovered from Member 3 (out of 5 members). A total of 31 specimens representing at least 17 individuals have been recovered. The only potential ''Homo'' specimen from Member 3 is KB 5223, but its classification is debated. The ear bones of the juvenile KB 6067 from Member 3 is consistent with that of ''P. robustus'', but the dimensions of the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...

and oval window better align with the more ancient StW 53 from Sterkfontein Member 4 with undetermined species designation. KB 6067, therefore, may possibly be basal to (more ancient than) other ''P. robustus'' specimens, at least those for which ear morphology is known. Palaeomagnetism suggests Member 3 may date to 1.78–1.6 million years ago, Member 2 to before 1.78 million years ago, and Member 1 to 2.11–1.95 million years ago. The animal remains of Kromdraai A suggest deposition occurred anywhere between 1.89 and 1.63 million years ago, and the presence of Oldowan or Achulean tools indicates early ''Homo'' activity. The biostratigraphic dating of Kromdraai B is less clear as there are no animal species which are known to have existed in a narrow time interval, and many non-hominin specimens have not been assigned to a species (left at genus level). About 75% of mammalian remains other than ''P. robustus'' are monkeys, including leaf-eating colobine monkeys, possibly the earliest record of the Hamadryas baboon, '' Gorgopithecus'', and '' Papio angusticeps'' in South Africa. The absence of the baboons ''T. oswaldi'' and ''Dinopithecus'' could potentially mean Member 3 is older than Sterkfontein Member 5 and Swartkrans Member 1; which, if correct, would invalidate the results from palaeomagnetism, and make these specimens among the oldest representatives of the species. ;Gondolin Cave Sterkfontein Caves 69

Gondolin Cave has yielded 3 hominin specimens: a right third premolar assigned to early ''Homo'' (G14018), a partial left gracile australopithecine first or second molar (GDA-1), and a robust australopithecine second molar (GDA-2). The first hominin specimen (G14018) was found by German palaeontologist Elisabeth Vrba in 1979, and the other two specimens were recovered in 1997 by, respectively, South African palaeoanthropologist

Andre Keyser André Werner Keyser (8 March 1938, Pretoria – 15 August 2010, Pretoria), was a South African palaeontologist and geologist noted for his discovery of the Drimolen hominid site and of numerous hominid remains. In 1994 he discovered a fe ...

and excavator L. Dihasu. GDA-2—measuring , an area of —is exceptionally large for ''P. robustus'', which has a recorded maximum of . This falls within the range of ''P. boisei'' , so the discoverers assigned it to an indeterminate species of ''Paranthropus'' rather than ''P. robustus''. GDA-2 was found alongside the pig '' Metridiochoerus andrewsi'', which means the tooth must be 1.9–1.5 million years old. Using this and palaeomagnetism, it may date to roughly 1.8 million years ago. ;Cooper's Cave Cooper's Cave was first reported to yield ''P. robustus'' remains in 2000 by South African palaeoanthropologists Christine Steininger and Lee Rogers Berger. Specimens include a crushed partial right face (COB 101), three isolated teeth, a juvenile jawbone, and several skull fragments. The animal remains in the hominin-bearing deposit are similar to those of Swartkrans and Kromdraai A, so the Cooper's Cave deposits may date to 1.87–1.56 million years ago. ;Drimolen Cave Drimolen Cave was first discovered to have yielded hominin remains by Keyser in 1992, who, in eight years, oversaw the recovery of 79 ''P. robustus'' specimens. Among these are the most complete ''P. robustus'' skulls: the presumed female DNH-7 (which also preserved articulated jawbone with almost all the teeth), and presumed male DNH 155. It was also associated with the ''H. ergaster''/''H. erectus'' skull DNH 134. The Drimolen material preserves several basal characteristics relative to the Swartkrans and Kromdraai remains (meaning it may be older). The site is thought to be roughly 2–1.5 million years old based on animal remains which have also been recovered from Swartkrans Member 1. The animal assemblage is broadly similar to that of Cooper's Cave, meaning they probably are about the same age. In 2020, DNH 152 was palaeomagnetically dated to 2.04–1.95 million years ago, making it the oldest identified ''P. robustus'' specimen.

Predation

Australopithecine bones may have accumulated in caves due to large carnivores dragging in carcasses, which was first explored in detail by Brain in his 1981 book '' The Hunters or the Hunted?: An Introduction to African Cave Taphonomy''. The juvenile ''P. robustus'' skullcap SK 54 has two puncture marks consistent with the lower canines of the

specimen SK 349 from the same deposits. Brain hypothesised that ''Dinofelis'' and perhaps also

hunting hyena ''Chasmaporthetes'', also known as hunting or running hyena, is an extinct genus of hyenas distributed in Eurasia, North America, and Africa during the Pliocene-Pleistocene epochs, living from 4.9 million to 780,000 years ago, existing for about ...

s specialised on killing australopithecines, but carbon isotope analysis indicates these species predominantly ate large grazers, while the leopard, the sabertoothed ''Megantereon'', and the spotted hyena were more likely to have regularly consumed ''P. robustus''. Brain was unsure if these predators actively sought them out and brought them back to the cave den to eat, or inhabited deeper recesses of caves and ambushed them when they entered. Modern-day baboons in this region often shelter in sinkholes especially on cold winter nights, though Brain proposed that australopithecines seasonally migrated out of the Highveld and into the warmer Bushveld, only taking up cave shelters in spring and autumn. As an antipredator behaviour, baboons often associate themselves with medium-to-large herbivores, most notably impalas, and it is possible that ''P. robustus'' as well as other early hominins which lived in open environments did so also, given they are typically associated with an abundance of medium-to-large bovid and horse remains.

Extinction

Though ''P. robustus'' was a rather hardy species with a tolerance for environmental variability, it seems to have preferred wooded environments, and similarly most ''P. robustus'' remains date to a wet period in South Africa 2–1.75 million years ago conducive to such biomes. The extinction of ''P. robustus'' coincided with the Mid-Pleistocene Transition, and the doubling of glacial cycle duration. During glacial events, with more ice locked up at the poles, the tropical rain belt contracted towards the equator, subsequently causing the retreat of wetland and woodland environments. Before the transition, ''P. robustus'' populations possibly contracted to certain wooded refuge zones over 21,000-year cycles, becoming regionally extinct in certain areas until the wet cycle whereupon it would repopulate those zones. The continual prolonging of dry cycles may have caused its extinction, with the last occurrence in the fossil record 1–0.6 million years ago (though more likely 0.9 million years ago). ''Homo'' possibly was able to survive by inhabiting a much larger geographical range, more likely to find a suitable refuge area during unfavourable climate swings. However, the geographical range of ''P. robustus'' in the fossil record is roughly , whereas the critically endangered eastern gorilla (with the smallest range of any African ape) inhabits , the critically endangered western gorilla , and the

endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and inv ...

chimpanzee . Therefore, fossil distribution very unlikely represents the true range of the species; consequently, ''P. robustus'' possibly went extinct much more recently somewhere other than the Cradle of Humankind ( Signor–Lipps effect).

References

External links

Meet ''Australopithecus robustus''
—

John D. Hawks John Hawks is an associate professor of anthropology at the University of Wisconsin–Madison. He also maintains a paleoanthropology blog. Contrary to the common view that cultural evolution has made human biological evolution insignificant, ...

' website
''Paranthropus robustus''
- The Smithsonian's Human Origins Program
Human Timeline (Interactive)
– Smithsonian {{Taxonbar, from=Q310529 Pleistocene mammals of Africa Paranthropus Pleistocene primates Pliocene primates Pleistocene species extinctions Fossil taxa described in 1938 Prehistoric South Africa