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 at Olduvai Gorge, Tanzania 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

thumb|left|Map of [[Paranthropus finds (P. boisei in red)]]

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 a.m., 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. 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". 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.

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.

thumb|Various P. boisei specimens

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 1999, a jawbone was recovered from Malema<!--not Malema, Mozambique-->, Malawi, extending the species' southernmost range over from Olduvai Gorge. 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.&nbsp;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.&nbsp;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.

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). 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.&nbsp;boisei. Louis rejected Robinson's proposal. Following this, it was debated if P.&nbsp;boisei was simply an East African variant of P.&nbsp;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.

Classification

The genus Paranthropus (otherwise known as "robust australopithecines") typically includes P.&nbsp;boisei, P.&nbsp;aethiopicus and P.&nbsp;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.&nbsp;boisei and P.&nbsp;robustus. Proponents of monophyly consider P.&nbsp;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.&nbsp;boisei, A.&nbsp;aethiopicus and A.&nbsp;robustus. Now, the earliest known South African australopithecine ("Little Foot") dates to 3.67 million years ago, contemporaneous with A.&nbsp;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.&nbsp;africanus) is considered the ancestor or closely related to the ancestor of Homo, then this could allow for A.&nbsp;africanus to be placed more closely related to Homo than to Paranthropus. This would leave the Ethiopian A. garhi as the ancestor of P.&nbsp;aethiopicus instead of A.&nbsp;africanus (assuming Paranthropus is monophyletic, and that P.&nbsp;aethiopicus evolved at a time in East Africa when only A.&nbsp;garhi existed there).