Paranthropus robustus

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.

Paranthropus robustus
Temporal range: Pleistocene, 2–1/0.6 Ma
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Presumed-male SK 48 at the Transvaal Museum
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Hominidae
Subfamily: Homininae
Tribe: Hominini
Genus: †Paranthropus
Species:
†P. robustus
Binomial name
†Paranthropus robustus
Broom, 1938
Synonyms
  • Australopithecus robustus
  • P. crassidens

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. P. robustus may have had a genetic susceptibility for pitting enamel hypoplasia on the teeth. The species is though to have exhibited marked sexual dimorphism, with males substantially larger and more robust than females. The brain volume of the specimen SK 1585 is estimated to have been 476 cc. 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). 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.

In regard to plant foods, P. robustus likely had a similar diet to the contemporary Homo ergaster/Homo erectus. P. robustus is thought to 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.

Research history
Location of the Cradle of Humankind
Fossil-bearing caves (P. robustus is known from 1, 7, and 11–14)

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. 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.[1] "Paranthropus" derives from Ancient Greek παρα para beside or alongside; and άνθρωπος ánthropos man.[2] 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.[3]

TM 1517, the holotype specimen

In 1948, at the nearby Swartkrans Cave, Broom and South African palaeontologist John Talbot 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.[4] By the 21st century, "P. crassidens" had more or less fallen out of use in favour of P. robustus.[5] In 1951, anthropologists Sherwood Washburn and Bruce D. Patterson recommended limiting hominin genera to only Australopithecus and Homo.[6] It has since been debated whether or not Paranthropus is a junior synonym of Australopithecus.[5]

The genus Paranthropus (otherwise known as "robust australopithecines") now also includes the East African P. boisei and P. aethiopicus. It is unclear 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; and propose that P. robustus is closely related to or descended from A. africanus.[7]

 
 

A. africanus

 
 
 

P. aethiopicus

 
 
 

P. boisei

 
 

P. robustus

 
 
 
 
 
 

A. africanus

 
 

P. robustus

 
 
 
 

P. aethiopicus

 
 

P. boisei

 
 
Family tree showing Paranthropus as monophyletic (top) and paraphyletic (bottom)[7]:119

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.[5] In 2000, the most complete P. robustus skull, DNH-7, was discovered, preserving an articulated jawbone with almost all the teeth.[8] In 2020, the oldest P. robustus specimen, the braincase DNH 152, was dated to 2.04–1.95 million years ago.[9] P. robustus is recorded in Swartkrans until Member 3 dated to 1–0.6 mya (the Middle Pleistocene), though more likely the younger side of the estimate, which could mean this species outlived East African Paranthropus.[10]

Anatomy

Skull
Female DNH-7 (left) vs male SK 48 (right)

The P. robustus brain volume was first measured in 1972 by American physical anthropologist Ralph Holloway using the skullcap SK 1585, which is missing part of the frontal bone, and he reported a volume of about 530 cc.[11] In 2000, frontal bone anatomy was filled in using the P. boisei specimens KNM-ER 407, OH 5, and KNM-ER 732, and the brain volume of SK 1585 was recalculated at about 476 cc. Overall brain anatomy of P. robustus appears to have been more like that of non-human apes.[12]

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.[13] However, the well-defined sagittal crest and inflated cheeks are absent in the presumed-female skull DNH-7, so male P. robustus may have been much larger and heavily built than females (P. robustus was sexually dimorphic).[8]

The teeth of SKX 11

Dental development seems to have followed about the same timeframe as it does in modern humans and most other hominins, but, since P. robustus molars are markedly larger, rate of tooth eruption would have been accelerated.[14] The early age of the eruption and the fast rate of tooth root extension for the permanent first molar of the 3–4 year old specimen SK 63 is consistent with that of non-human apes.[15]

Size

Broom had noted that the ankle bone and humerus of the presumed male TM 1517 were about the same dimensions of that of a modern San woman, and so assumed humanlike proportions in P. robustus. The first attempt at estimating Paranthropus body size was made by Robinson in 1972. 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).[16]

In 1991, American anthropologist Henry McHenry reported much lighter weights as well as notable sexual dimorphism. He 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 individuals 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.[17] 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.[18] 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.[19]

In 2001, two more femoral fragments were discovered, which would give an average of about 42 kg (93 lb) for males and 30 kg (66 lb) for females. If these four individuals are representative of the entire species, 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. erectus in Swartkrans, but male H. erectus were much bigger at 55 kg (121 lb).[20]

Limbs

The humerus of P. robustus has a mosaic anatomy with some aspects in common with modern humans and others with various non-human great apes.[21] 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) is better suited to maintaining stability when the forearm is flexed like non-human apes. It is possible this reflects some arboreal activity in the trees as is postulated in other australopithecines.[22] The 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.[23]

Paranthropus were bipeds, and their hips, legs, and feet resemble A. afarensis and modern humans.[24][25] The pelvis is similar to A. afarensis, but the hip joints are smaller in P. robustus. The physical similarity implies a similar walking gait. This would mean that P. robustus was less efficient at walking than modern humans, but nonetheless much more able than non-human apes.[26] 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.[27]

Pathology

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.[28]

There have been 12 identified cases of dental cavities in P. robustus, indicating a rate similar to modern humans. 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 teeth which occurs when teeth become worn down and have 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.[29]

Palaeobiology

Diet and technology

In 1954, Robinson suggested that the heavily built skull of P. robustus 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.[30] However, 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.[30] 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.[31]

P. robustus is now thought to have been an omnivore, with a diet similar to early Homo.[30] P. robustus likely subsisted on mainly C4 savanna plants and C3 forest plants, which could indicate either seasonal shifts in diet or seasonal migration from forest to savanna.[32] The plant portion of its diet was nearly identical to that of H. ergaster.[33] In leaner times P. robustus may have fallen back on brittle food. P. robustus likely also consumed seeds[32][34] and possibly tubers or termites.[35] A high cavity rate could indicate honey consumption.[29] Juvenile P. robustus may have relied more on tubers than adults, given the elevated levels of Strontium compared to adults in teeth from Swartkrans Cave, which, in the area, was most likely sourced from tubers. Dentin exposure on juvenile teeth could indicate early weaning, or a more abrasive diet than adults which wore away the cementum and enamel coatings, or both. It is also possible juveniles were less capable of removing grit from dug-up food rather than purposefully seeking out more abrasive foods.[36]

Bone tools dating between 2.3 and 0.6 mya—found in abundance in Swartkrans,[35] Kromdraai, and Drimolen Caves—are often associated with P. robustus, and may have been used to cut or process vegetation,[23] process fruits (namely marula fruit), strip tree bark,[37] or dig up tubers or termites.[35][37][38] Though Homo is also known from these caves, their remains are comparatively scarce to Paranthropus, making Homo-attribution less likely.[38] The form of P. robustus incisors appear to be intermediate between H. erectus and modern humans, which could indicate less food processing done by the teeth due to preparation with simple tools.[36] The tools also cooccur with Homo-associated Oldawan and possibly Acheulian stone tool industries, but stone tools from Kromdraai could possibly be attributed to P. robustus as no Homo have been found there yet. The bone tools were typically sourced from the shaft of long bones from medium- to large-sized mammals, but tools made sourced from mandibles, ribs, and horn cores have also been found.[38] The bone tools were not manufactured or purposefully shaped for a task. However, since the bones display no weathering (and were not scavenged randomly), and there is a preference displayed for certain bones, raw materials were likely specifically hand picked.[38]

Burnt bones were also associated with the inhabitants of Swartkrans, which could indicate some of the earliest fire usage.[39] However, these bones were found in Member 3, where Paranthropus remains are rarer than H. erectus, and it is also possible the bones were burned in a wildfire and washed into the cave as it is known the bones were not burned onsite.[40][41]

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.[42][43] 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).[44]

However, 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 productive to better defend the troop from predators in the more exposed environment, much like savanna baboons.[43] A 2011 Strontium isotope study of P. robustus teeth from the dolomite Sterkfontein Valley found that, like other hominins, but unlike other great apes, P. robustus females were more likely to leave their place of birth (patrilocal). This also discounts the plausibility of a harem society, which would have resulted in a matrilocal society due to heightened male–male competition. Males did not seem to have ventured very far from the valley, which could either indicate small home ranges, or that they preferred dolomitic landscapes due to perhaps cave abundance or factors related to vegetation growth.[42] Nonetheless, because male P. robustus appear to have had a higher mortality rate than females, it is possible that males were more likely to be kicked out of a group, and these lone males had a higher risk of predation.[43]

Palaeoecology
SK 54 being dragged by the leopard SK 349

The Pleistocene Cradle of Humankind was mainly dominated by the springbok Antidorcas recki, but other antelope, giraffes, and elephants were also seemingly abundant megafauna. Other known primates are early Homo, the Hamadryas baboon, and the extinct colobine monkey Cercopithecoides williamsi. The carnivore assemblage comprises the sabertoothed cats Dinofelis and Megantereon, and the hyena Lycyaenops silberbergi. These animals indicate a mixed, open-to-closed landscape featuring perhaps montane grasslands and shrublands[45] Australopithecine bones may have accumulated in caves due to big cat activity, dragging in carcasses. 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. In 1983, South African palaeontologist Charles Kimberlin Brain hypothesised that Dinofelis and perhaps also hunting hyenas (specifically Euryboas and Hyaenictis) specialised on killing australopithecines. He 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.[46]

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.[47]

P. robustus also cohabited the Cradle of Humankind with H. ergaster/H. erectus.[20][40][9] In addition, these two species resided alongside A. sediba from 2.04–1.95 million years ago. Around 2.07 million years ago, just before the arrival of P. robustus and H. erectus, A. africanus went extinct from the area. 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.[9]

See also

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