Paranthropus boisei

Paranthropus boisei is a species of australopithecine from the Early Pleistocene of East Africa about 2.3 to 1.34 or 1 million years ago. The holotype specimen, OH 5, was discovered by palaeoanthropologist Mary Leakey in 1959, and described by her husband Louis a month later. It was originally placed into its own genus as "Zinjanthropus boisei", but is now relegated to Paranthropus along with other robust australopithecines. However, it is argued that Paranthropus is an invalid grouping and synonymous with Australopithecus, so the species is also often classified as Australopithecus boisei.

Paranthropus boisei
Temporal range: Early Pleistocene, 2.3–1.34/1 Ma
↓
Reconstruction of the holotype specimen OH 5
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Hominidae
Subfamily: Homininae
Tribe: Hominini
Genus: †Paranthropus
Species:
†P. boisei
Binomial name
†Paranthropus boisei
(Louis Leakey, 1959)
Synonyms

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. boisei is the most robust of this group. Brain size was about 450–550 cc (27–34 cu in), similar to other australopithecines. Some skulls are markedly smaller than others, which is taken as evidence of sexual dimorphism where females are much smaller than males, though body size is difficult to estimate given only one specimen, OH 80, definitely provides any bodily elements. The presumed male OH 80 may have been 156 cm (5 ft 1 in) tall and 50 kg (110 lb) in weight (assuming improbable humanlike proportions), and the presumed female KNM-ER 1500 124 cm (4 ft 1 in) tall (though its species designation is unclear). The arm and hand bones of OH 80 and KNM-ER 47000 suggest P. boisei was arboreal to a degree and was possibly capable of manufacturing tools.

P. boisei was originally believed to have been a specialist of hard foods, such as nuts, due to its heavily built skull, but it was more likely a generalist feeder of predominantly abrasive C4 plants, such as grasses or underground storage organs. Like gorillas, the apparently specialised adaptations of the skull may have only been used with less desirable fallback foods, allowing P. boisei to inhabit a wider range of habitats than gracile australopithecines. P. boisei may have been able to make Oldowan stone tools and butcher carcasses. P. boisei mainly inhabited wet, wooded environments, and coexisted with H. habilis, H. rudolfensis, H. ergaster, and H. erectus. These were likely preyed upon by the large carnivores of the time, including big cats, crocodiles, and hyenas.

Taxonomy

Research history
Map of Paranthropus finds (P. boisei in red)

The first remains—Olduvai Hominin (OH) 3, a baby canine and large molar tooth)—were unearthed in 1955 in Olduvai Gorge, Tanzania.[1] On July 17, 1959, palaeoanthropologist Mary Leakey discovered a skull, OH 5.[2] The remains were clearly australopithecine (not Homo), and at the time, the only australopithecine genera described were Australopithecus by Raymond Dart and Paranthropus (the South African P. robustus) by Robert Broom, and there were arguments that Paranthropus was synonymous with Australopithecus. Palaeoanthropologist Louis Leakey (Mary's husband) believed the skull had a mix of traits from both genera, briefly listing 20 differences, and so placed it into a new genus as "Zinjanthropus boisei" on August 15. The genus derives from the medieval term for East Africa, "Zanj", and the species name was in honour of Charles Watson Boise, the Leakeys' benefactor.[2] Soon after OH 5's discovery, Louis presented "Z. boisei" to the 4th Pan-African Congress on Prehistory in Kinshasa, Democratic Republic of the Congo.[1] In 1960, American anthropologist John Talbot Robinson pointed out that the supposed differences are due to OH 5 being slightly larger than P. robustus, and so recommended the species be reclassified as P. boisei. Louis rejected Robinson's argument.[3] Following this, it was debated if P. boisei was simply an East African variant of P. robustus until 1967 when South African palaeoanthropologist Phillip V. Tobias gave a far more detailed description of OH 5 in a monograph (edited by Louis). However, they still retained Zinjanthropus and recommended demoting it to subgenus level as Australopithecus (Zinjanthropus) boisei, considering Paranthropus to be synonymous with Australopithecus.[1]

By the time OH 5 was discovered, the Leakey's had spent 24 years excavating the area for early hominin remains, but had instead recovered mainly other animal remains as well as the Oldowan stone tool industry.[1] Because OH 5 was associated with the tools and processed animal bones, they presumed it to have been the toolmaker. Attribution of the tools was promptly switched to the bigger-brained H. habilis upon its description in 1964.[1] In 1965, the skull was dated to 1.75 million years ago based on potassium-argon dating of anortoclase crystals from an overlying tuff (volcanic ash) bed.[4]

Illustration of KGA10-525, one of the largest and most complete P. boisei specimens[5]

The first jawbone, Peninj 1, was discovered Lake Natron just north of Olduvai Gorge in 1964.[6]:107[7][8] Especially from 1966–1975, several more specimens revealing facial elements were reported from the Shungura Formation, Ethiopia; Koobi Fora and Chesowanja, Kenya; and Omo and Konso, Ethiopia.[6]:109 In 1997, the first specimen with associated skull and jawbone (and also one of the largest specimens), KGA10-525, was discovered in Konso.[5] In 1999, a jawbone was recovered from Malema, Malawi, extending the species' southernmost range over 2,000 km (1,200 mi) from Olduvai Gorge.[6]:109 The first definitive bodily elements of P. boisei associated with facial elements, OH 80 (isolated teeth with an arm and a leg), were discovered in 2013. Previously, body remains lacking unambiguous diagnostic skull elements had been dubiously assigned to the species, namely the partial skeleton KNM-ER 1500 associated with a small jawbone fragment. OH 80 was also associated with Oldowan stone tools.[9] In 2015, based on OH 80, American palaeoanthropologist Michael Lague recommended assigning the isolated humerus specimens KNM-ER 739, 1504, 6020, and 1591 from Koobi Fora to P. boisei.[10] In 2020, the first associated hand bones were reported, KNM-ER 47000 (which also includes a nearly complete arm), from Ileret, Kenya.[11]

Classification

The genus Paranthropus (otherwise known as "robust australopithecines") typically includes P. boisei, P. aethiopicus, and P. robustus. It is also debated if Paranthropus 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. boisei and P. robustus. 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.[6]:117–121

Because P. boisei and P. aethiopicus are both known from East Africa and P. aethiopicus is only confidently identified from the skull KNM WT 17000 and a few jaws and isolated teeth, it is debated if P. aethiopicus should be subsumed under P. boisei or if the differences stemming from archaicness justifies species distinction. The terms P. boisei sensu lato ("in the broad sense") and P. boisei sensu stricto ("in the strict sense") can be used to respectively include and exclude P. aethiopicus from P. boisei when discussing the lineage as a whole.[6]:106–107 P. boisei is the most robust of the robust australopithecines, whereas the South African P. robustus is smaller with comparatively more gracile features.[6]:120

African hominin timeline (in mya)
View references
HumanHomo nalediHomo rhodesiensisHomo ergasterAustralopithecus sedibaParanthropus robustusParanthropus boiseiHomo rudolfensisHomo habilisAustralopithecus garhiParanthropus aethiopicusLD 350-1Australopithecus bahrelghazaliKenyanthropusAustralopithecus deyiremedaAustralopithecus africanusAustralopithecus afarensisAustralopithecus anamensisArdipithecus ramidusArdipithecus kadabba
Peninj 1 showing the enlarged cheek teeth and reduced front teeth

P. aethiopicus is the earliest member of the genus, with the oldest remains, from the Ethiopian Omo Kibish Formation, dated to 2.6 million years ago (mya) at the end of the Pliocene.[12] It is possible that P. aethiopicus evolved even earlier, up to 3.3 mya, on the expansive Kenyan floodplains of the time.[13] The oldest P. boisei remains date to about 2.3 mya from Malema.[12] The youngest record of P. boisei comes Olduvai Gorge (OH 80) about 1.34 mya;[9] however, due a large gap in the hominin fossil record, P. boisei may have persisted until 1 mya.[6]:109 P. boisei changed remarkably little over its nearly 1 million year existence.[14]

Anatomy
P. boisei bust at the Hall of Human Origins, Washington DC, by John Gurche

The P. boisei skull is heavily built, and features a defined brow ridge, receding forehead, rounded bottom margins of the eye sockets, inflated and concave cheek bones, a thick palate, and a robust and deep jawbone. This is generally interpreted as having allowed P. boisei to resist high stresses while chewing,[15] though the thick palate could instead be a byproduct of facial lengthening.[16] The skull large rough patches (rugosities) on the cheek and jawbones, and males have pronounced sagittal (on the midline) and temporonuchal (on the back) crests, which indicate a massive masseter muscle (used in biting down) placed near the front of the head (increasing mechanical advantage). This is typically considered to be evidence of a high bite force. The enormous cheek teeth (postcanine megadontia) of both sexes would have increased the pressure felt by food.[15] The chewing surface of P. boisei molars averages 756 mm2 (1.172 sq in); in comparison, it is 334 mm2 (0.518 sq in) in modern human molars.[17] The molars are bunodont, featuring low and rounded cusps. The premolars resemble molars (are molarised), which may indicate P. boisei required an extended chewing surface for processing a lot of food at the same time. The enamel on the cheek teeth are among the thickest of any known ape, which would help resist high stresses while biting. The incisors and canines are reduced, which would hinder biting off chunks of large food pieces.[15]:128–132

In a sample of 10 P. boisei specimens, brain size varied from 444–545 cc (27.1–33.3 cu in) with an average of 487.5 cc (29.75 cu in).[18] However, the lower-end specimen, Omo L338‐y6, is a juvenile, and many skull specimens have a highly damaged or missing frontal bone which can alter brain volume estimates.[19] The brain volume of australopithecines generally ranged from 400–500 cc (24–31 cu in), and for contemporary Homo 500–900 cc (31–55 cu in).[20]

The wide range of size variation in skull specimens seems to indicate a great degree of sexual dimorphism with males notably bigger than females. However, it is difficult to predict with accuracy the true dimensions of living males and females due to the lack of definitive P. boisei skeletal remains, save for the presumed male OH 80. Based on an approximation of 400 mm (1.3 ft) for the femur before it was broken and using modern humanlike proportions (which is probably an unsafe assumption), OH 80 was about 156.3 cm (5 ft 2 in) tall in life.[9] For comparison, modern human men and women in the year 1900 averaged 163 cm (5 ft 4 in) and 152.7 cm (5 ft), respectively.[21] The femoral head, the best proxy for estimating body mass, is missing, but using the shaft, OH 80 weighed about 50 kg (110 lb) assuming humanlike proportions, and 61.7 kg (136 lb) using the proportions of a non-human ape.[9] The ambiguously attributed, presumed female femur KNM-ER 1500 is estimated to have been about 124 cm (4 ft 1 in) tall[22] which would be consistent with the argument of sexual dimorphism,[9] but if the specimen does indeed belong to P. boisei, it would show a limb anatomy quite similar to that of the contemporary H. habilis.[6]:116

OH 80 femoral shaft (left) and ulna (right)
Scale bar=1 cm (0.39 in)

Instead, the OH 80 femur, more like H. erectus femora, is quite thick, features a laterally flattened shaft, and indicates similarly arranged gluteal, pectineal, and intertrochanteric lines around the hip joint. Nonetheless, the intertrochanteric line is much more defined in OH 80, the gluteal tuberosity is more towards the midline of the femur, and the mid-shaft in side-view is straighter, which likely reflect some difference in load-bearing capabilities of the leg. Unlike P. robustus, the arm bones of OH 80 are heavily built, and the elbow joint shows similarities to that of modern gibbons and orangutans. This indicates suspensory behaviour in P. boisei.[9] The hand of KNM-ER 47000 shows Australopithecus-like anatomy lacking the third metacarpal styloid process (which allows the hand to lock into the wrist to exert more pressure), a weak thumb compared to modern humans, and curved phalanges (finger bones) which are typically interpreted as adaptations for climbing. Nonetheless, despite lacking a particularly forceful precision grip like Homo, the hand was still dextrous enough to handle and manufacture simple tools.[11]

Diet

In 1954, Robinson suggested that the heavily built skull of Paranthropus (at the time only including 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 it was unable to adapt to the volatile climate of the Pleistocene, unlike the much more adaptable Homo.[15] It was also once thought P. boisei cracked open nuts and similar hard foods with its powerful teeth, giving OH 5 the nickname "Nutcracker Man".[23]

However, in 1981, English anthropologist Alan Walker found that the microwearing patterns on the molars were inconsistent with a diet high in hard foods, and were effectively indistinguishable from the pattern seen in the molars of fruit-eating (frugivorous) mandrills, chimps, and orangutans.[24] The microwearing on P. boisei molars is different than that on P. robustus molars, and indicates that P. boisei, unlike P. robustus, very rarely ever ate hard foods. Carbon isotope analysis reports a diet of predominantly C4 plants, such as low quality and abrasive grasses and sedges.[25][26][27][27] Thick enamel is consistent with grinding abrasive foods.[15] The microwear patterns in P. robustus have been thoroughly examined, and suggest that the heavy build of the skull was only relevant when eating less desirable fallback foods. A similar scheme may have been in use by P. boisei.[15] 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.[28]

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.[29] 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.[28] In this model, P. boisei may have been a generalist feeder with a predilection for USOs,[30][27] and may have gone extinct due to an aridity trend and a resultant decline in USOs in tandem with increasing competition with baboons and Homo.[31] Like modern forest chimps and baboons, australopithecines likely foraged for food in the cooler morning and evening instead of in the heat of the day.[32]

OH 80 was found associated with a mass of Oldowan stone tools and animal bones bearing evidence of butchery. This could potentially indicate P. boisei was manufacturing the Oldowan tradition and ate meat to some degree.[9]

Palaeoecology
Crocodylus anthropophagus (restoration above) may have eaten early hominins.[33]

P. boisei seems to have resided predominantly in wet, wooded environments, such as wetlands along lakes and rivers, wooded or arid shrublands, and semiarid woodlands,[27] but its presence in the savanna-dominated Malawian Chiwondo Beds implies it could tolerate a range of habitats.[34] During the Pleistocene, there seems to have been coastal and montane forests in Eastern Africa. More expansive river valleys–namely the Omo River Valley–may have served as important refuges for forest-dwelling creatures. Being cut off from the forests of Central Africa by a savanna corridor, these East African forests would have promoted high rates of endemism, especially during times of climatic volatility.[35] 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.[32]

P. boisei coexisted with H. habilis, H. rudolfensis, and H. ergaster / H. erectus, but it is unclear how they interacted.[1] To explain why P. boisei was associated with Oldowan tools despite not being the knapper, Louis Leakey and colleagues, when describing H. habilis, suggested that one possibility was P. boisei was killed by H. habilis,[36] perhaps as food.[37] However, when describing P. boisei 5 years earlier, he said, "There is no reason whatever, in this case, to believe that the skull [OH 5] represents the victim of a cannibalistic feast by some hypothetical more advanced type of man."[2] The leg OH 35, which either belongs to P. boisei or H. habilis, shows evidence of leopard predation.[38] Other likely Olduvan predators of great apes include the hunting hyaena Chasmaporthetes nitidula, the sabertoothed cats Dinofelis and Megantereon,[39] and the crocodile Crocodylus anthropophagus.[33]

gollark: ALSOSTAYTEMENTTHISSENSEBEINGKNOWNASMAKEPOTENTILYTUXONE?
gollark: Agree completely I, this grammaring be the not the bad.
gollark: Are you saying that bad grammar is INHERITED now?
gollark: You are *vital* to the UNWAVERING PASTURELAND project.
gollark: Oh, there is, but I meant specifically Tux1.

See also

References
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