Paranthropus boisei was first discovered by Mary Leaky in 1959, and
was first termed Zinjanthropus boisei or Zinj. The oldest
Paranthropus boisei was found at Omo, Ethiopia and dates to approximately
2.3 million years ago, while the youngest was found at Olduvai Gorge, and
dates to approximately 1.2 million years ago. P. boisei is best
known for its enormous postcanines, and seems to be the end point of a lineage
that was adapted to high masticatory stress needed to deal with hard low-quality
foods. One theory is that P. boisei died out because of overspecialization
to a specific environment, which prohibited it from adapting to a rapidly
changing environment and climate. Compared to other robust species,
P. boisei has a larger cranial capacity (500-550 cc), a more vertically
set face, and a sagittal crest on the mid-brain case, as opposed to the posterior.
It is widely accepted that P. boisei’s ancestor is A. africanus
. The discovery of P. boisei was important because it disproved
Milford Wolpoff’s “Single Species Hypothesis" that was so popular in the
1960s. This hypothesis stated that every environmental niche could
only support one species. Following his logic, P. robustus
was thought to be made up of all males, while A. africanus was thought
to be made up of all females. Because P. boisei of both sexes
were found in the same site and were dated to the same time, the discovery
proved that even if the South African material was a single sexually dimorphic
species, P. boisei was a different species contemporary with it,
bringing into doubt the validity of the single species hypothesis.
Because of a lack of archaelogical evidence, there is very little known about Paranthropus aethiopicus. However, it is generally accepted that P. aethiopicus falls somewhere between the “robust” and “gracile” australopithecines. The first specimen of P. aethiopicus was found by a French expedition led by Camille Arambourg and Yves Coppens in southern Ethiopia in 1967. P. aethiopicus was named a new species partly because of its V-shaped jaw, which was different from other robust australopithecus known to have lived in the area. The most famous specimen of P. aethiopicus was discovered west of Lake Turkana and was dated to 2.5 million years old. It was known as “Black Skull” because mineral uptake during fossilization gave the specimen a blue-black color. The “Black Skull” specimen is similar to a male A. afarensis, but has a very small cranal capacity (410 cc) and a more developed masticatory apparatus. Like A. afarensis , A. aethiopicus has a flattened cranial base and large anterior tooth sockets. A. aethiopicus was shown to be the possible base of the boisei lineage; more primitive than robustus yet not ancestral to it. There is, however, much debate about this.
P. robustus was first discovered
by Dr. Robert Broom in South Africa in 1938. Generally, P. robustus
has been found in three
different locations: Swartkrans, Dreimulen, and Kromdraai. P.
robustus is believed to have lived from 2.0 – 1.0 million years ago.
The species has a significantly larger cranial capacity than A. africanus
, and is more similar to a modern brain. In addition, P. robustus
has better developed muscle markings, more prominent tori, and thicker buttressing
structures than A. africanus. P. robustus also had a
substantially bigger postcanine tooth size in comparison to A. africanus
, indicating that robustus was not an herbivore that subsisted on hard gritty
nuts and plants, but rather an omnivore. The most vigorous debate surrounding
P. robustus has been whether or not it is a different species than
P. boisei or simply a geographic species of a wide-ranging variable population.
Most experts seem to agree that P. robustus and P. boisei have
separate lineages that follow similar evolutionary trends.