The Human Fossil Record, Part 6: The Dispersal of the Australopithecines, Cont’d
The South African Forms
As noted in the last post, the discovery of the Taung child in South Africa fueled interest that other hominins could be found as well. With the help of noted anatomist Robert Broom, and anthropologist John T. Robinson, Raymond Dart excavated several other sites in the South African cave system, Makapansgat, Swartkrans, Sterkfontein and Kromdraai, all of which yielded australopithecine remains of both an early gracile form and later robust form. The earlier form, dated to between 3.0 and 2.0 million years ago and to which the Taung find belonged, was called Australopithecus africanus (see Figure 6). Exact dating for these cave sites is hampered by the fact that the cave openings are vertical and it is not clear how the hominins got there. Unlike the east African forms, A. africanus was a very lightly built form, with no crests of any kind. The overall cranial capacity of the finds is between 430 and 520 cubic centimeters, slightly larger than that of A. afarensis and comparable to the east African forms. Like the east African forms, the teeth of this form were larger in the back and smaller in the front than A. afarensis suggesting to some an herbivorous diet.
Slightly more recent in time is A. robustus. This is a scaled-up version of A. africanus and continues the trends seen in that form (See Figure 7). Although having a generally similar facial structure as A. africanus, A. robustus also has crests like the eastern African forms, suggesting a similar dietary adaptation. There is also a continuation of the trend toward larger back teeth and smaller front teeth to accommodate this. While A. robustus and A. boisei are both considered “robust” australopithecines, the considerable differences in their facial architectures suggest to most researchers different evolutionary trajectories.
A recent discovery has been made by Lee Berger and colleagues (Berger et al. 2010) at the site of Malapa in South Africa, of yet another form of Australopithecus, which cannot be accommodated into the sequence of A. africanus and A. robustus, and is different enough from each of them to warrant its own species designation: A. sediba. This form is characterized by having a small cranium with no crests and a flat face, in contrast with all other known australopithecines (See Figure 8). Berger suggests that this form is descended from A. africanus and existed parallel to A. robustus. It has also been suggested that there are post-cranial elements in the hip and leg bones that align it with later Homo, although, at this point, the evidence for this position is scant.
The Odd Men Out: Kenyanthropus Platyops and Australopithecus Bahrelghazali
Up until the last few years, australopithecine evolution had been assumed to have taken place in the entire east African Rift Valley and South Africa. That changed in 1995 when researchers working on the banks of the Sahel River in Chad, discovered what they described as yet another australopithecine, A. bahrelghazali. Nicknamed “Abel”, this find was initially dated to between 3.0 and 3.5 million years ago, and consists only of a jaw fragment with six teeth. Since its discovery, the date of the find has been challenged by Beauvilain (Beauvilain 2008), who argues “Abel was collected at the foot of the shoreline of the last episode of Lake Megachad, which is only a few thousand years old.” While this does not imply that the find is this old, it does cast doubt on the ability to determine just how old it is. The morphology does appear to be australopithecine, however, extending the possible reach of this remarkable genus out to central Africa. Little has been said about it since then, however.
The other find of note was a discovery in northern Kenya, in Lomekwi of a mostly complete but crushed cranium and some facial bones of an individual that shows considerable facial flatness and is lacking in any of the specializations present in the robust australopithecines in terms of dietary adaptations (See Figure 9). It was hence called Kenyanthropus platyops (flat-faced man from Kenya). Dated to approximately 3.5 million years ago, little is known about this find. Its cranial capacity is between that of A. afarensis and A. africanus, at around 400 cubic centimeters (Leakey, et al. 2001). Due to the condition of the cranium, it is difficult to draw any inferences about where this find fits in the phylogeny of the early hominins, and some authors are relegating it to a regional variant of A. afarensis (White 2003). Michael Balter, in fact, recently asked the question Whatever happened to Kenyanthropus platyops?, and the fossil seems to have not generated much interest in recent years.
The Place of Australopithecus
It is tempting to look at these remains and think privately, “these are nothing but apes. What is the fuss?” Such has been the viewpoint of the Institute for Creation Research’s Duane Gish (Gish and Research 1985) and John Morris, who remarked “From the neck down, certain clues suggested to Johanson that Lucy walked a little more erect than today's chimps. This conclusion, based on his interpretation of the partial hip bone and a knee bone, has been hotly contested by many paleoanthropologists”(Morris 1994). This is incorrect. There was never any doubt in any of the researcher’s minds that from A. afarensis, the australopithecines walked upright, albeit with a gait not quite like that of modern humans. It is, further, instructive to compare the skulls of australopithecines and chimpanzees side by side (Figure 10). This shows several of the characteristics in the australopithecines that are derived relative to the ape condition. First, the brain case is decidedly larger, on the order of 100 cubic centimeters, and, second, the teeth do not extend beyond the tooth row as they do in the chimpanzee. While this comparison is certainly artificial in that the chimpanzee is a modern animal, the fact remains that the brains of australopithecines were already advanced beyond any known ape species either then or now. As importantly, however, is that the foramen magnum hole is always at the bottom of the skull in all australopithecine remains for which that area of the skull is present, indicating bipedality (movement on two rear legs), a condition never found in apes.
The Oldowan Tool Tradition
At the site of Gona, in Ethiopia, stone tools were unearthed dating to between 2.4 and 2.6 million years ago. More advanced than those used by A. afarensis, these consisted of crude choppers, bifaces and rudimentary blades. Other tools have been found at Olduvai (the type site) but none in direct association with any hominin remains. While it is thought that the users of these tools may have been australopithecines, there is simply no direct evidence to support this.
The Extinction of the Australopithecines
In both east and south Africa, the australopithecines hold sway until approximately 1.2 to 1.1 million years ago. Then they simply disappear. Remains are few enough that it is not possible to say for sure why or over what period of time this happened. As can be surmised from the evidence of the dentition and facial muscles, the australopithecines became very dietarily specialized, leading them down a restricted evolutionary path. Consequently, the most widely supported explanation for their extinction is that they were simply out-competed by the new kid on the block: Homo, which first makes its appearance around 2.3 million years ago. With the advent of Homo came a hominin with a vastly expanded braincase and direct association with stone tools.
We have now covered the ground between the earliest demonstrable hominins, in Ardipithecus and the most prolific ones, the australopithecines. Next, we will move to forms that begin to show the characteristics that we can recognize, from a physical perspective, as human.
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