Oh 5

''Paranthropus boisei'' is a species of australopithecine from the Early Pleistocene of East Africa about 2.5 to 1.15 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 also argued that ''Paranthropus'' is an invalid grouping and synonymous with ''Australopithecus'', so the species is also often classified as ''Australopithecus boisei''.

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 , 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 tall and in weight, and the presumed female KNM-ER 1500 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.

''P. boisei'' was originally believed to have been a specialist species 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'' and ''H. ergaster''/''erectus''. These were likely preyed upon by the large carnivores of the time, including big cats, crocodiles and hyenas.

Research history

Discovery

Palaeoanthropologists Mary and Louis Leakey had conducted excavations in Tanzania since the 1930s, though work was postponed with the start of World War II. They returned in 1951, finding mostly ancient tools and fossils of extinct mammals for the next few years. In 1955, they unearthed a hominin baby canine and large molar tooth in Olduvai Gorge, catalogue ID Olduvai Hominin (OH) 3.

On the morning of July 17, 1959, Louis felt ill and stayed at camp while Mary went out to Bed I's Frida Leakey Gully. Sometime around 11:00 AM, she noticed what appeared to be a portion of a skull poking out of the ground, OH 5. The dig team created a pile of stones around the exposed portion to protect it from further weathering.Mary Leakey, ''Excavations'', 227. Active excavation began the following day; they had chosen to wait for photographer Des Bartlett to document the entire process. The partial cranium was fully unearthed August 6, though it had to be reconstructed from its fragments which were scattered in the scree. Louis published a short summary of the find and context the following week.

Louis determined OH 5 to be a subadult or adolescent based on dental development, and he and Mary nicknamed it "Dear Boy". After they reconstructed the skull and jaws, newspapers began referring to it as "Nutcracker Man" due to the large back teeth and jaws which gave it a resemblance to vintage nutcrackers. South African palaeoanthropologist Phillip Tobias, a colleague of the Leakeys, has also received attribution for this nickname. The cranium was taken to Kenya after its discovery and was there until January 1965 when it was placed on display in the Hall of Man at the National Museum of Tanzania in Dar es Salaam.Staniforth, 155.

Other specimens

Louis preliminarily supposed OH 5 was about half a million years old, but in 1965, American geologists Garniss Curtis and Jack Evernden dated OH 5 to 1.75 million years ago using potassium–argon dating of anortoclase crystals from an overlying tuff (volcanic ash) bed. Such an application of geochronology was unprecedented at the time.

The first identified jawbone, Peninj 1, was discovered Lake Natron just north of Olduvai Gorge in 1964. Especially from 1966 to 1975, several more specimens revealing facial elements were reported from the Shungura Formation, Ethiopia; Koobi Fora and Chesowanja, Kenya; and Omo and Konso, Ethiopia. Among the notable specimens found include the well preserved skull KNM-ER 406 from Koobi Fora in 1970. In 1997, the first specimen with both the skull and jawbone (and also one of the largest specimens), KGA10-525, was discovered in Konso. In 1999, a jawbone was recovered from Malema, Malawi, extending the species' southernmost range over from Olduvai Gorge. 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. 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''. In 2020, the first associated hand bones were reported, KNM-ER 47000 (which also includes a nearly complete arm), from Ileret, Kenya.

Naming

The remains were clearly australopithecine (not of the genus ''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''. Louis believed the skull had a mix of traits from both genera, briefly listing 20 differences, and so used OH 5 as the basis for the new genus and species "''Zinjanthropus boisei''" on August 15, 1959. The genus name derives from the medieval term for East Africa, "Zanj", and the specific name was in honour of Charles Watson Boise, the Leakeys' benefactor. He initially considered the name "''Titanohomo mirabilis''" ("wonderful Titan-like man").

Soon after, Louis presented "''Z.''" ''boisei'' to the 4th Pan-African Congress on Prehistory in Léopoldville, Belgian Congo (now Kinshasa, Democratic Republic of the Congo). Dart made his now famous joke, "... what would have happened if he ''A. africanus'' specimen Mrs. Ples had met Dear Boy one dark night." At the time of discovery, there was resistance to erecting completely new genera based on single specimens, and the Congress largely rejected "''Zinjanthropus''". In 1960, American anthropologist John Talbot Robinson pointed out that the supposed differences between "''Zinjanthropus''" and ''Paranthropus'' are due to OH 5 being slightly larger, and so recommended the species be reclassified as ''P. boisei''. Louis rejected Robinson's proposal. 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). Tobias and Louis still retained "''Zinjanthropus''", but recommended demoting it to subgenus level as ''Australopithecus ("Zinjanthropus") boisei'', considering ''Paranthropus'' to be synonymous with ''Australopithecus''. Synonymising ''Paranthropus'' with ''Australopithecus'' was first suggested by anthropologists Sherwood Washburn and Bruce D. Patterson in 1951, who recommended limiting hominin genera to only ''Australopithecus'' and ''Homo''.

Classification

The genus ''Paranthropus'' (otherwise known as "robust australopithecines") typically includes ''P. boisei'', ''P. aethiopicus'' and ''P. robustus''. It is 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''.

Before ''P. boisei'' was described (and ''P. robustus'' was the only member of ''Paranthropus''), Broom and Robinson continued arguing that ''P. robustus'' and ''A. africanus'' (the then only known australopithecines) were two distinct lineages. However, 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. In 1975, the ''P. boisei'' skull KNM-ER 406 was demonstrated to have been contemporaneous with the ''H. ergaster''/''erectus'' skull KNM ER 3733, which 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''.

Such arguments are based on how one draws the hominin family tree, and the exact classification of ''Australopithecus'' species with each other is quite contentious. For example, if the South African '' A. sediba'' (which evolved from ''A. africanus'') is considered the ancestor or closely related to the ancestor of ''Homo'', then this could allow for ''A. africanus'' to be placed more closely related to ''Homo'' than to ''Paranthropus''. This would leave the Ethiopian '' A. garhi'' as the ancestor of ''P. aethiopicus'' instead of ''A. africanus'' (assuming ''Paranthropus'' is monophyletic, and that ''P. aethiopicus'' evolved at a time in East Africa when only ''A. garhi'' existed there).

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.

''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

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