Zinjanthropus is an extinct hominin discovered by Mary Leakey in 1959 at Olduvai Gorge, Tanzania. The name Zinjanthropus literally means “East African Man.” The specimen for which the species Zinjanthropus boisei was named (OH5, or Olduvai Hominind 5) is also known as “Nutcracker man” because of its extremely large postcanine teeth (molars and premolars). Zinjan-thropus boisei was later reclassified as Australopithecus boisei (A. boisei) because of its similarities to South African robust australopithecines. Some scientists have advocated reclassifying it to the genus Paranthropus, a sister taxon to Australopithecus.
The fossil preserves a nearly complete cranium of an adult male. Its brain size is about 530 cubic centimeters (cc), which is larger than the average chimpanzee (390 cc) but much smaller than the average human (1400 cc). The most impressive aspects of this skull are the characters associated with its massive jaws and postcanine teeth. These teeth are nearly four times the size of those of modern humans, although the hominins themselves were much smaller. Associated with these large teeth are large jaws to house them. The skull presents a bony crest on the top for attaching large chewing muscles. In addition, it possesses wide cheekbones that flare out to allow the passage of these muscles. As a result the face is somewhat dished-out in profile. These adaptations to heavy chewing, together with tooth wear and chipping, have been interpreted as evidence that A. boisei was a specialized feeder with a diet composed mostly of hard, low-quality foods.
Since the original discovery of OH5, many more fossils of this species have been discovered. They are found primarily in Ethiopia, Tanzania, and Kenya in East Africa. The OH5 fossil has been dated to approximately 1.8 million years, but the species to which it belongs (A. boisei) spans 2.3 (from Omo, Ethiopia) to 1.2 million years (Olduvai Gorge, Tanzania). About 1.2 million years ago the species disappears from the fossil record. Many scientists believe their extinction is linked to overspecialization to a particular environment. They hypothesize that when the environment changed the species could not survive. However, recent analyses have suggested that, in fact, A. boisei (like Homo habilis) was a generalized feeder and that other explanations need to be invoked to explain their extinction.
The OH5 specimen was originally considered by Louis Leakey to represent a human ancestor. Later, after the discovery in 1960 of a less robust hominin also from Olduvai (Homo habilis), it was reassigned to an evolutionary side branch in human prehistory. Earlier, some scientists believed the East African A. boisei to be male representatives of the South African A. robustus (also a large-toothed hominin). When both males and females of A. boisei were found in East Africa this hypothesis, known as the Single Species Hypothesis, was discarded. It is now accepted that more than one hominin species lived at the same time and there was great diversity in early hominin evolution. The exact evolutionary relationship between A. boisei and other early hominin species is still debated within paleoanthropology. However, most paleoanthropologists agree that A. boisei was an extinct hominin side branch that did not contribute to later human evolution.
- Tobias, P. (Ed.). (1967). The cranium and maxillary dentition of Australopithecus (Zinjanthropus) boisei. Cambridge: Cambridge University Press.
- Walker, A., Leakey, R. E. (1988). The evolution of Australopithecus boisei. In F. E. Grine (Ed.), Evolutionary history of the “robust” Australopithecine (pp. 247-258). New York: Aldine de Gruyter.
- Wood, B. A. (1988). Are ‘robust’ australopithecines a monophyletic group? In F. E. Grine (Ed.), Evolutionary history of the “robust” Australopithecine (pp. 269-284). New York: Aldine de Gruyter.
- Wood, B. A., & Strait, D. S. (2004). Patterns of resource use in early Homo and Paranthropus. Journal of Human Evolution, 46, 119-162.