|Presumed-male SK 48 at the Transvaal Museum|
Paranthropus robustus is a species of australopithecine from the Early and possibly Middle Pleistocene of the Cradle of Humankind, South Africa, about 2 to 1 or 0.6 million years ago. Discovered in 1938, it was among the first early hominins described and the first discovered robust australopithecine, and became the type species for the genus Paranthropus. However, it is argued that Paranthropus is an invalid grouping and synonymous with Australopithecus, so the species is also often classified as Australopithecus robustus.
Robust australopithecines are characterised by heavily built skulls capable of producing high stresses and bite forces, and some of the largest molars with the thickest enamel of any known ape. Males had more heavily built skulls than females. P. robustus may have had a genetic susceptibility for pitting enamel hypoplasia 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, with males substantially larger and more robust than females. Based on 3 specimens, males may have been 132 cm (4 ft 4 in) tall and females 110 cm (3 ft 7 in). Based on 4 specimens, males averaged 40 kg (88 lb) in weight and females 30 kg (66 lb). The brain volume of the specimen SK 1585 is estimated to have been 476 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 in the trees.
P. robustus seems to have consumed a high proportion of C4 savanna plants. In addition, P. robustus may have also consumed fruits, underground storage organs, and perhaps honey and termites. P. robustus may have used bones as tools to extract and process food. It is unclear if P. robustus lived in a harem society like gorillas 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 indicated by higher male mortality rates and assumed increased risk of predation to solitary individuals. P. robust contended with sabertooth cats, leopards, and hyenas 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 and the continual prolonging of dry cycles and subsequent retreat of such habitat.
The first remains, a partial skull including a part of the jawbone (TM 1517), were discovered in June 1938 at Kromdraai, South Africa, by local schoolboy Gert Terblanche. He gave the remains to South African conservationist Charles Sydney Barlow, who then relayed it to South African palaeontologist Robert Broom. A few weeks later, Broom recovered a distal right humerus (at the elbow joint), a proximal right ulna (also elbow), and 2 toe bones, which he assigned to the specimen. He also identified a distal toe phalanx bone which he believed belonged to a baboon, but has since been associated with the specimen. Broom noted the Kromdraai remains were especially robust compared to other hominins. 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. In 1936, Broom had described "Plesianthropus transvaalensis" (now synonymised with A. africanus) from the Sterkfontein Caves only 2 km (1.2 mi) west from Kromdraii. Broom considered all these species, which dated to the Pleistocene and were found in the same general vicinity (now called the "Cradle of Humankind"), evidence of a greater diversity of hominins in the Pliocene out of which they and modern humans descended from, and that several hominin taxa existed alongside human ancestors. In August 1938, Broom classified the robust Kromdraai remains into a new genus as Paranthropus robustus. "Paranthropus" derives from Ancient Greek παρα para beside or alongside; and άνθρωπος ánthropos man.
In 1948, at the nearby Swartkrans Cave, Broom and Robinson described "P. crassidens" based on a subadult jaw, SK 6, which was younger than the fossils from Kromdraai. They believed that the Swartkrans Paranthropus were reproductively isolated from Kromdraai Paranthropus and they eventually speciated. At this point in time, humans and ancestors were classified into the family Hominidae, and other apes into "Pongidae"; in 1950, Broom suggested separating early hominins into the subfamilies Australopithecinae (A. africanus and "P. transvaalensis"), "Paranthropinae" (P. robustus and "P. crassidens"), and "Archanthropinae" ("A. 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 1 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 Australopithecus in 1951, wanting to limit hominin genera to only that and Homo, 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 (orangutans), and so on, and in 2019 it was again argued to be a valid genus. 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 proponent against synonymisation.
In 1939, Broom hypothesised that Paranthropus 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 that Gigantopithecus was, respectively, the direct ancestor of the Asian H. erectus or closely related, much debate followed over whether or not Gigantopithecus was a hominin or if it was a non-human ape. In 1972, Robinson suggested including Gigantopithecus into "Paranthropinae", with the Miocene 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, contrasting this with a very small build for A. africanus (which he referred to as "H. africanus") and supposed cultural and hunting ability in the earliest human lineages which "paranthropines" lacked. With the popularisation of cladistics 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.
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 today), though it is unclear if this classification is completely sound.
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 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 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 (by those that believe Paranthropus is valid) that P. robustus was the ancestor of P. boisei, but the assignment of the 2.5 million year old East African skull KNM WT 17000 to the genus as P. aethiopicus in 1985 invalidated such a model. Instead, P. robustus and P. boisei evolved many robust features independently, and they were proposed to have both evolved from P. aethiopicus assuming Paranthropus is valid.
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 in Sterkfontein, Gondolin, Coopers, and Drimolen Caves.
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) 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.
At Sterkfontein, only the specimens StW 566 and StW 569 are firmly assigned to the species, 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, zebras, lions, 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.
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 fro Member 3 is KB 5223, but its classification is debated. The animals 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 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). Palaeomagnatism 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. At least 17 P. robustus individuals have been recovered from Kromdraai B. About 75% of other mammalian remains are monkeys, including leaf-eating colobine monkeys, the 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. The ear of the juvenile KB 6067 from Member 3 is consistent with that of P. robustus, but the dimensions of the cochlea 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.
Gondolin Cave has yielded 3 hominin specimens: a right third premolar assigned to early Homo (G14018), a partial left gracile australopithecine 1st or 2nd molar (GDA-1), and a robust australopithecine 2nd 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 and excavator L. Dihasu. GDA-2—measuring 18.8 mm × 18.1 mm (0.74 in × 0.71 in), an area of 340 mm2 (0.53 sq in)—is exceptionally large for P. robustus, which has a recorded maximum of 290 mm2 (0.45 sq in), falling within the range of P. boisei 278–378 mm2 (0.431–0.586 sq in). So, the discoverers assigned it to Paranthropus species indeterminate 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.
Coopers Cave was first reported to yield P. robustus remains in 2000 by South African palaeoanthropologists Christine Steininger and Lee Rogers Berger at the annual American Association of Physical Anthropologists. Specimens include a crushed partial right face (COB 101), 3 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 Coopers Cave deposits may date to 1.87–1.56 million years ago.
Drimolen Cave was first discovered to have yielded hominin remains by Keyser in 1992, who, in 8 years, oversaw the recovery of 79 P. robustus specimens. Among these is the most complete P. robustus skull, DNH-7, preserving an articulated jawbone with almost all the teeth. 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 Coopers 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 confirmed P. robustus specimen. It was also associated with the H. ergaster/H. erectus skull DNH 134.
The genus Paranthropus (otherwise known as "robust australopithecines") now also includes the East African P. boisei and P. aethiopicus. It is still debated if this is a valid natural grouping (monophyletic) or an invalid grouping of similar-looking hominins (paraphyletic). Because skeletal elements are so limited in these species, their affinities with each other and to other australopithecines is difficult to gauge with accuracy. The jaws are the main argument for monophyly, but such anatomy is strongly influenced by diet and environment, and could in all likelihood 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. Arguments for which species is more closely related to which are based on how one draws the hominin family tree, and the exact classification of Australopithecus species with each other is quite contentious.
Typical of Paranthropus, P. robustus exhibits post-canine megadontia with enormous cheek teeth but human-sized incisors and canines. The premolars are shaped like molars. P. robustus had a tall face with slight prognathism (the face was not completely flat). The skull had a well-defined sagittal crest on the midline 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 (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. However, 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).
Upon species description, 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. A year later, British primatologist Wilfrid Le Gros Clark commented that, since only a part of the temporal bone on 1 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 like Homo, echoing what Phillip V. Tobias said while studying P. boisei skulls in 1967. In 2000, American neuroanthropologist Dean Falk 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. The stated overall brain anatomy of P. robustus was more like that of non-human apes.
The posterior semicircular canals in the inner ear of SK 46 and SK 47 are unlike those of Australopithecus or Homo, suggesting different locomotory 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. 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, 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.
Broom had noted that the ankle bone and humerus of the holotype TM 1517 were about the same dimensions of 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 140–150 cm (4 ft 7 in–4 ft 11 in) and a weight of 68–91 kg (150–201 lb). 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 1.2–1.4 m (4–4.5 ft) tall and 18–27 kg (40–60 lb) 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 40.5 kg (89 lb). Similarly, in 1988, American anthropologist Henry McHenry reported much lighter weights as well as notable sexual dimorphism for Paranthropus. McHenry measured the area of the cross section of the femoral head and scaled the dimensions of a modern human and an ape to such a cross-sectional area and noted the size. He then took the average of these 2 estimates. Based on SK 82 and SK SK 97, he reported an average weight of 47.1 kg (104 lb) for P. robustus. 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 28 kg (62 lb) human, 22% in a 43 kg (95 lb) human, and the remaining 43% bigger than the former but less than a 54 kg (119 lb) human except for KNM‐ER 1464 (an ankle bone). At Member 3, all individuals were consistent with a 45 kg (99 lb) human. 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. 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 126 cm (4 ft 2 in), 137 cm (4 ft 6 in), and 110 cm (3 ft 7 in) respectively. Based on just these three, he reported an average height of 132 cm (4 ft 4 in) for P. robustus males and 110 cm (3 ft 7 in) for females.
In 2001, palaeoanthropologist Randall L. Susman and colleagues, using two recently discovered proximal femoral fragments from Swartkrans, estimated an average of 42 kg (93 lb) for males and 30 kg (66 lb) 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 humans and chimps, but less than orangutans and gorillas. 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 55 kg (121 lb). In 2012, American anthropologist Trenton Holliday, using the same equation as McHenry on 3 specimens, reported an average of 37 kg (82 lb) with a range of 30–43 kg (66–95 lb). In 2015, biological anthropologist Mark Grabowski and colleagues, using 9 specimens, estimated an average of 32.3 kg (71 lb) for males and 24 kg (53 lb) for females.
Few vertebrae are assigned to P. robustus. The only thoracolumbar series (thoracic and lumbar series) preserved belongs to the juvenile SKW 14002, and either represents L1–L4 or L2–L5. SK 3981 preserves a T12 (last thoracic vertebra), and a lower lumbar vertebra. The T12 is relatively elongated, and the articular surface (where it joins with another vertebra) is kidney-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 chimps than healthy humans. Early hominin vertebrae are similar to those of a pathological human, including the only other T12 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 1 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 other australopithecines and are within the range of humans, and the transverse processes indicate powerful iliolumbar ligaments. These could have bearing on the amount of time spent upright compared to other australopithecines.
The pelvis is similar to the pelvises of A. africanus and A. afarensis, but it has a wider iliac blade and smaller acetabulum 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 modern humans.
The distal (lower) humerus of P. robustus falls within the variation of both modern humans and chimps, as the distal humerus is quite similar between humans and chimps. The radius of P. robustus is the same as in Australopithecus species. The wrist joint has the same manoeuverability 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 is flexed like non-human apes. It is possible this reflects some arboreal activity 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, like 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 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 StW 311, which either belongs to P. robustus or early Homo, seems to have habitually been placed in highly flexed positions, 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. P. robustus and H. habilis may have achieved about the same grade of bipedality.
In 1954, Robinson suggested that the heavily built skull of P. robustus and resultantly exorbitant bite force was indicative of a specialist diet specifically adapted for processing a narrow band of foods. 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 furthered this model that Paranthropus ate only hard foods 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 the 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.
P. robustus is now thought to have been an omnivore. The microwear patterns in P. robustus suggest hard food was infrequently consumed, and 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 chimps (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 bears 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 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 consumed seeds based on tooth chipping patterns. A high cavity rate could indicate honey consumption. 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 instead less capable of removing grit from dug-up food rather than purposefully seeking out more abrasive foods.
Australopithecine bone technology was first proposed by Dart in the 1950s with the now-disproven "osteodontokeratic culture", which he attributed to A. africanus at Makapansgat dating to 3–2.6 million years ago. 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 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 2 from Kromdraai B. Bone tools are most abundant when P. robustus remains far outnumber Homo remains and stone tools, so they are often attributed to P. robustus. The 2 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; however, it is possible that the stone tools were reworked. 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 of long bones from medium- to large-sized mammals, but tools sourced from mandibles, ribs, and horn cores have also been found. They were not manufactured or purposefully shaped for a task. However, since the bones 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 300 °C (572 °F), 52 to 300–400 °C (572–752 °F), 45 to 400–500 °C (752–932 °F), 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 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. However, the bones were not burnt inside the cave, and it is alternatively possible that the bones were naturally burnt in cyclically occurring wildfires (dry savanna grass as well as possible guano 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, South Africa, made by South African archaeologist Peter Beaumont in 2011.
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.
A 2007 study pointed out that P. robustus appears to have had pronounced sexual dimorphism, with males notably larger than females. This is commonly correlated with a male-dominated polygamous 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.
However, 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 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, a 2016 study 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 savanna baboons. 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.
A 2017 study 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 in addition to supporting chewing muscles. Further, the size of the sagittal crest (and the gluteus muscles) in male western lowland gorillas has been correlated with reproductive success. They 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 engorged canines in agonistic display (the canines of Paranthropus are comparatively small). However, it is also possible that male gorillas and orangutans require larger temporalis muscles to achieve a wider gape to better display the canines.
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 2.5–4.5 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 lemurs. 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 bonobos, but unlike humans, which could also indicate an apelike growth trajectory.
While growing, the front part of the jaw in P. robustus is depository whereas the sides are resorptive. 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. Unlike apes and gracile australopithecines, but like humans, the premaxillary suture between the premaxilla and the maxilla (on the palate) formed early in development. At early stages, the P. robustus jawbone is somewhat similar to that of modern humans, but the breadth grows 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 broaden, and the ramus of the mandible elongates, diverging from the modern human trajectory. Because the ramus is so tall, it is suggested that P. robustus experienced more anterior face rotation than modern humans and apes. Growth is 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 decreases with age, though it decreases 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 6 mm (0.24 in). This is more similar to gorillas whose roots typically measure 7 mm (0.28 in) when emerging from the gums (a later stage of dental development) whereas other hominins typically are under 5–6 mm (0.20–0.24 in).
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.
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 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 children born. In 1972, after estimating a fetal size of 1,230–1,390 g (2.7–3.1 lb) based on an adult female weight of 50 kg (110 lb), anthropologist Walter Leutenegger estimated fetal head size at about 110–160 cc (6.7–9.8 cu in), similar to a chimp. In 1973, using this and an equation between fetal 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 neonate brain size of 175 cc and weight of 1.9 kg (4.2 lb), 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 neonate brain size, weaning age, and birth interval; and for humans all metrics except birth interval.
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 (which all inhabited the Cradle of Humankind at different points in time). In contrast, chimps 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 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 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 had to 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 encapsulating TM 1517, Schepers noted a large rock, which would have weighed 75 g (2.6 oz), 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. However, 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 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.
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 and Megantereon, 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 1,000 m (3,300 ft) in elevation at the time of deposition. This would mean that, like chimps, they often inhabited areas with an average diurnal temperature of 25 °C (77 °F), dropping to 10 or 5 °C (50 or 41 °F) at night.
Australopithecine bones may have accumulated in caves due to large carnivore activity, dragging in carcasses, which was first explored in detail in 1983 by Brain. The juvenile P. robustus skullcap SK 54 has two puncture marks consistent with the lower canines of the leopard specimen SK 349 from the same deposits. Brain hypothesised that Dinofelis and perhaps also hunting hyenas specialised on killing australopithecines. However, carbon isotope analysis indicates these species predominantly ate large grazers, and 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. Baboons in this region modern day 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 associated themselves with medium-to-large herbivores, most notably with 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.
P. robustus also cohabited the Cradle of Humankind with H. ergaster/H. erectus. In addition, these two species resided alongside A. 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.
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 coincides 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 due to the great geographical range. However, the geographical range of P. robustus in the fossil record is roughly 500 km2 (190 sq mi), whereas the critically endangered eastern gorilla (with the smallest range of any African ape) inhabits 70,000 km2 (27,000 sq mi), the critically endangered western gorilla 700,000 km2 (270,000 sq mi), and the endangered chimp 2,600,000 km2 (1,000,000 sq mi). 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).
- African archaeology
- Australopithecus africanus – Extinct hominid from South Africa
- Australopithecus sediba – Two-million-year-old hominin from the Cradle of Humankind
- Homo ergaster – Extinct species or subspecies of archaic human
- Homo habilis – Archaic human species from 2.1 to 1.5 mya
- Homo naledi – Small-brained South African archaic human
- Homo rudolfensis – Extinct hominin from the Early Pleistocene of East Africa
- Paranthropus aethiopicus – Extinct species of hominin of East Africa
- Paranthropus boisei – Extinct species of hominin of East Africa
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