[1] 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.

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 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.

Palaeoanthropologists Mary and Louis Leakey had conducted excavations in Tanzania since the 1930s, though work was postponed with the start of World War II.

[11] 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.

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.

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.

[24] 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.

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.

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.

[15]: 120  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.

[34] The skull features 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).

In contrast, the cheek teeth of both sexes are enormous (postcanine megadontia), and the greater surface area would have permitted the processing of larger quantities of food at once.

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.

[41] The wide range of size variation in skull specimens seems to indicate a great degree of sexual dimorphism with males being notably bigger than females.

[19] The ambiguously attributed, presumed female femur KNM-ER 1500 is estimated to have been of an individual about 124 cm (4 ft 1 in) tall[43] which would be consistent with the argument of sexual dimorphism,[19] but if the specimen does indeed belong to P. boisei, it would show a limb anatomy quite similar to that of the contemporary H.

[15]: 116 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 could either indicate that P. boisei used a combination of terrestrial walking as well as suspensory behaviour, or was completely bipedal but retained an ape-like upper body condition from some ancestor species due to a lack of selective pressure to lose them.

[21] 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.

[33] 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".

[49] 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.

In East Africa, a few have been encountered at Olduvai Gorge Beds I–IV, occurring over roughly 1.7 to 0.8 million years ago, and are usually made of limb bones and possibly teeth of large mammals, most notably elephants.

[57] Australopithecines are generally considered to have had a faster, apelike growth rate than modern humans largely due to dental development trends.

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 and slower life history due to environmental factors, such as early weaning age exhibited in modern indriid lemurs.

[63] 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.

[65] Other likely Oldowan predators of great apes include the hunting hyena Chasmaporthetes nitidula, the sabertoothed cats Dinofelis and Megantereon,[66] and the crocodile Crocodylus anthropophagus.