A robust Australopithecine that lived about 2 million years ago. The first specimen was discovered by Mary Leaky in 1959 at Olduvai Gorge. Boisei was dubbed "Nutcracker Man" because of its great, big molars that looked to be mighty enough to crush nuts.
Like the other robust Australopithecines, Boisei had very large molars and thick jaws with tiny canines for the purpose of chomping fibers and roots. It had a fairly heavy build, and its brain size was about a third of modern man's. Boisei had a fairly pronounced sexual dimorphism. Boisei was not an ancestor of modern man, but rather evidently an offshot at some earlier point along man's lineage.

Also called Paranthropus boisei, this so-called "hyper-robust" hominid species lived between 2.3 to 1.0 million years ago. Fossils have been unearthed in the African Rift Valley, Olduvai, and Koobi Fora and West Turkana in Kenya. Many specimens have been found, with the majority being cranial and dental. The initial find (a fairly complete cranium without the mandible) was given the name Zinjanthropus boisei by Louis Leakey, but was later renamed Australopithecus boisei.

Cranial capacity: 400-500 cc
Weight: About 70 kg in males, females at 70% male body weight
Height: 1.2 – 1.4 m
Tool usage: Tools have been found alongside Boisei fossils, but are more likely attributable to Homo habilis fossils in the same area
Habitat: Semiarid savanna

Characteristics
Boisei was sexually dimorphic, but less so than its predecessors (with males being 1.3 times larger than females). This may indicate that the species was progressing towards a more modern appearance. The body size was generally similar to other Australopithecines, being about ten percent larger than Africanus. The species had a large cranium with very prominent sagittal and nuchal crests, and a large, flat face. The extreme flaring of the zygomatic arches further widened the face. There is a decreased facial prognathism as compared to previous hominids. The anteriorly positioned foramen magnum indicates bipedalism.

This species has the largest dentition of any hominid group (megadontia, with teeth similar in size to gorillas) and a parabolic dental arcade. Both the molars and premolars were extremely large, with thick enamel and a flat wear pattern. The canines and incisors were extremely small. This highly specialized dentition indicates a diet of tough vegetation and other hard too chew foods. The jaw and cheekbones were massive.

Significance
The discovery of Australopithecus boisei debunked the popular “Single Species Hypothesis” of evolution. This theory stated that any environmental niche will only be able to support one single species, and that among hominids, similar species would emulate each other. If that was true, contemporary hominids that came into contact would attempt to fill the same niche and, in doing so, one species would prevail over the other. This would mean that multiple hominid groups could not coexist.

Before the naming of Boisei as a separate species, the south African fossils were thought to be of one extremely sexually dimorphic species, with robust hominids being the males and gracile hominids being the females. When Boisei fossils of both sexes were found in one location, it proved that there could be two coexisting hominid species in one supporting environment.

Too Good?
Boisei is thought by some to have died out due to overspecialization. Adapted to an environment where a large jaw and post-canines were needed (due perhaps to low quality foods), this species may have then been at a disadvantage because of their hyper-robust dentition when the environment changed. There is little evidence contradicting this theory, and the dentition of this species does indicate an extreme level of adaptation to one particular environmental niche.

Sources:
http://chief.anth.uconn.edu/faculty/mcbrearty/old%20site/afrinhom.htm
http://www.humanevolution.f2s.com/boisei.html
http://www.talkorigins.org/faqs/homs/species.html#boisei

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