The discovery of the specimen OH 5 (“Zinj”) in 1959, by Mary Leakey, was a watershed in the history of paleoanthropology. The find vindicated Louis and Mary Leakey’s work at Olduvai (which had been relatively fruitless over the previous 30 years), which led to renewed research interest in the area, added an important stage in a relatively sparse hominid lineage at the time, and also was important in focusing attention on multidisciplinary research. The fairly complete cranium (sans mandible) was given the species name Zinjanthropus boisei by Louis Leakey, which eventually became known as Australopithecus boisei. However, Zinjanthropus lives on in the well known nickname of OH 5, “Zinj”.
Specimens attributed to A. boisei have been found mostly in Ethiopia, Tanzania, and Kenya in East Africa. The oldest has been found at Omo, Ethiopia, dating to approximately 2.3 myr (L. 74a-21), and the youngest has been found at Olduvai Gorge, dating to approximately 1.2 myr (OH 3 and OH 38).A. boisei seems to be the end point of a lineage that that was adapted to high masticatory stress needed to deal with hard low-quality foods. This species is sometimes called “hyper-robust” due to the relative and absolute size of their postcanines. This lineage may have died out due to overspecialization to a specific environment, and when the environment changed, evolution could not keep up. This seems to be the generally accepted idea regarding boisei, and there seems little hard evidence to contradict it.
The most striking feature of the A. boisei specimens is the degree of megadontia. This species has the absolute largest teeth found in any hominid group, with teeth similar in size to gorillas (who weigh as much as 10 times as much). They are often referred to as hyper-robust due to the massive postcanine megadontia. The features of boisei are best described in relation to the other “robusts” (including aethiopicus), since this best shows some of the features that exclude aethiopicus from the “robust” lineage in favor of africanus. Features that line up boisei as a descendent of africanus rather than aethiopicus include:
- The face is more vertically set, more orthognathic (variability in this trait).
- There is anterior teeth reduction.
- There is a continued increase in postcanine teeth size.
- There is a larger cranial capacity (500–550 cc).
- The sagittal crest is on the mid-brain case, not the posterior.
Since aethiopicus and africanus are contemporary, only one can be the ancestor to boisei, and africanus seems a more likely scenario. Perhaps more importantly, boisei shares unique traits with later species that undoubtedly link it with africanus (who is likely the ancestor of these later species). Features that link boisei withA. robustus (and possibly early Homo) include:
- Some structural brain differences reflected in endocasts, such as: A) greater frontal lobe breadth B) expanded parietal cortex C) increased cerebral height (high cerebral to cerebellar height) D) cerebellar lobes “tucked in” and not projecting laterally or posteriorly.
- Increased flexion of the cranial base.
- Shortening of the base and decrease in the angle of the petrous pyramids.
- More anterior foramen magnum position.
- Deeper mandibular fossa with well-delineated, projecting, articular eminence.
- Nearly horizontal orientation of nuchal plane.
- Expanded height of occipital plane of the occiput, with a concomitant low inion position.
- Decreased facial prognathism, especially subnasal.
- Shortened distance between the tooth row and the mandibular fossa.
- Reduced posterior component of temporalis muscle.
- Weakly developed or absence of pneumatized bone in the temporal squama.
The hyper-robusticity of boisei did not extend to their body size, as they are approximately only 10% larger than africanus (and a whopping 60% smaller than aethiopicus). A. boisei also shows a progression toward a more modern form relative to its ancestors, as sexual dimorphism is reduced (with males 1.3 time larger than females).
A. boisei was very important in clearing up a controversy that raged in the 1960s over the idea of the “Single Species Hypothesis” (championed by University of Michigan professor Milford Wolpoff). The single species hypothesis states that every environmental niche can only support one species, and that in hominids, “monkey-see monkey-do” holds true. Thus, if contemporary hominid groups came into contact, they would have the same behaviors, would attempt to fill the same niche, and conflict would ensue with one species the evolutionary victor, and one the loser (or would create an atmosphere where multiple hominid groups could not arise, due to the competition). It was argued that the known remains belonged to a single sexually dimorphic species, with the males attributed to robustus, and the females attributed to africanus. The discovery of boisei of both sexes in the same site, dated to the same time, showed that even if the South African material was a single sexually dimorphic species, boisei was a different species contemporary with it, bringing into doubt the validity of the single species hypothesis.
It is now generally accepted that (in the instance of the South African australopithecines) there are two separate species, and not a single sexually dimorphic one. While some claim that this was the “death” of the single species hypothesis, it really only showed that two different species could be contemporary, and modified single species concept (one highly variable population concept) is alive and well, and difficult to prove or disprove, which guarantees it will be around and debated for a long time to come.
Australopithecus boisei is an important species both in the history of paleoanthropological research and in constructing the phylogeny of the hominid lines. The features it shares or lacks with contemporary and earlier species makes relatively clear the relative phylogeny of the “robust” australopithecines. It also is present at a time when stone tools become much more common, and may have even made and used some. In the end, however, it seems that boisei became too specialized, and died with climatic and/or environmental shifts.
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