Paranthropus Robustus

''Paranthropus robustus'' is a species of robust australopithecine from the Early and possibly Middle Pleistocene of the Cradle of Humankind, South Africa, about 2.27 to 0.87 (or, more conservatively, 2 to 1) million years ago. It has been identified in Kromdraai, Swartkrans, Sterkfontein, Gondolin, Cooper's, and Drimolen Caves. Discovered in 1938, it was among the first early hominins described, and became the type species for the genus '' Paranthropus''. However, it has been argued by some that ''Paranthropus'' is an invalid grouping and synonymous with ''Australopithecus'', so the species is also often classified as ''Australopithecus robustus''.

Robust australopithecines—as opposed to gracile australopithecines—are characterised by heavily built skulls capable of producing high stresses and bite forces, as well as inflated cheek teeth (molars and premolars). 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 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'' 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 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 as indicated by higher male mortality rates and assumed increased risk of predation to solitary individuals. ''P. robustus'' 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 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), 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 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 genus, as ''Paranthropus robustus''. "'' Paranthropus''" derives from the Ancient Greek 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. In 1936, Broom had described "''Plesianthropus transvaalensis''" (now synonymised with ''A. africanus'') from the Sterkfontein Caves only west from Kromdraai. All these species dated to the Pleistocene and were found in the same general vicinity (now called the " Cradle of Humankind"). Broom considered them evidence of a greater diversity of hominins in the Pliocene 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, Gondolin, 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 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 ''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.

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). 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 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 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 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.

''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 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 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 " gracile 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 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 cheek teeth but human-sized incisors 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 (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 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.

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 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 like ''Homo'', echoing what 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. 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, 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 vertebrae 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-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.

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 those of modern humans.

Limbs

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 chimpanzees. The radius 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 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 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. ''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 regressions 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 humans and chimpanzees, 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 . 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 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. 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 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 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 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.

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 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, in 2011, palaeoanthropologist Sandi Copeland and colleagues studied the strontium isotope 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 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. 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 (''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 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 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, 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 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 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, 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 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 (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. Unlike other 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 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

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