This is an article I wrote in 2010 for a now defunct bioanthropology website.
The robust Australopithecus coexisted with other hominids for over a million years. Members of this family include Paranthropus Boisei, Paranthropus Robustus and Paranthropus Aethiopicus. P. Aethiopicus is the most ancient of the three. Age estimates of the “Black Skull” range from 2.4 to 2.7 million years and although P. Aethiopicus had a small cranial capacity (410 cc), the specimen exhibits traits that set the robust Australopithecus apart from their gracile relatives.
Traits such as a zygomatic arch and saggital crest (the zygomatic arch appears as two bony protrusions above the cheek, the saggital crest is the protrusion on the top of the head; in other words, like modern gorillas, robust australopithecines appear to have been adapted to heavy chewing and/or hard-feeding).
P. Aethiopicus may not be the immediate ancestor of Robustus or Boisei, but the evidence is still too scant to say either way. P. Aethiopicus, or one of its undiscovered offshoots, is likely the common ancestor of both P. Boisei and P. Robustus.
The robust australopithecines displayed sexually dimorphism. There is a well-established relationship between dimorphism and polygamy in primates. It has been suggested they made use of tools, but no evidence has yet been found.
Chimpanzees, proud stick-wielders, have cranial capacities similar to early hominids, so it is not at all ridiculous to suppose australopithecines, both gracile and robust, used tools.
P. Boisei’s brain is small compared to our own, but 510ccs is more than respectable for a primate of its size (Homo habilis, our immediate ancestor and a contemporary of P. Boisei, had an average cranial capacity of 750cc, which is about half that of modern human’s brain).The trend of encephalization observed in both homo and paranthropus is a tantalizing example of parallel evolution.
Since H. Sapien’s antecessors and cousin species are all extinct, it is easy to forget hominids are no exemption from the evolutionary phenomenon of branching. This conclusion is wrong. It becomes increasingly wrong as more and more dead-end hominid species are uncovered. It is now clear many types of nearly sentient apes competed with one another.
Mary Leakey’s discovery of the Zinj Skull was a momentous occasion for anthropologists and biologists because it forces us to remember we are part of the natural world. Branching is as much a part of the natural history of homo sapiens as it is of lizards or birds or protista, and, up until quite recently, humans coexisted with creatures much like themselves.