Exemplar: STS 71 [Sterkfontein] - 2.5 million y.a.About 3 million years ago, Australopithecus afarensis gave rise to two distinct evolutionary lines: one leading into the first humans, and the other into the robust australopithecines. Though its place is still unclear, AUSTRALOPITHECUS AFRICANUS is most likely the key transitional species toward the emergence of the human line.
A male africanus stood about 1.4 meters tall and weighed about 41 kilos, females were about 1.1 meters and 30 kilos. The species had a brain volume of 420cc to 500cc, somewhat larger than but still close to afarensis. This makes africanus noticably smaller than afarensis, if (as currently believed) the skull shown at left is from a male.
In primates, sexual dimorphism -- difference in the physical characteristics of males and females -- is expressed primarily in physical size and in the genitals. In general australopithid females weighed about 25% less than males, a size ratio that remained fairly stable throughout the different species of this line. Humans, in contrast, show less sexual dimorphism in stature: females are around 10%-15% smaller than males. Instead, the dimorphism emerges in a variety of reproductive characteristics -- genitals, breasts, parenting behavior.
It's really in the skull that africanus is noticeably different from afarensis: in the more vertical slope of the face, the narrower cheekbones and reduced browridges, and the more rounded shape of the cranium. Brain endocasts show significant increases in the frontal and parietal lobes in comparison to chimpanzee brains; the africanus brain clearly prefigures that of humans. The teeth are also more similar to human teeth than to those of modern apes; the canine teeth are smaller than those in afarensis, and the shape of the jaw is now fully parabolic, again like humans. These features suggest to me that africanus is better viewed as an ancestor of the earliest humans and not of the robust australopithids.
These later australopithids (such as Australopithecus bosei, right) are called "robust" because they are physically larger and because their skulls, jaws and jaw muscle attachments are more heavily built -- adapted to a diet of coarse, tough plants processed by laborious chewing. A saggital crest or ridge of bone formed along the top of the skull as an anchor for massive jaw muscles, the face widened to let the jaw muscles pass under the cheekbones, and the cheekbones merge into a waist of bone that circles the sides of the skull most of the way to the back of the head. The brow ridges thicken and fuse with the cheekbones to strengthen the face against the pressures of chewing. The rounded shape of the jaw is closer to afarensis than to any of the early humans.
Three species of australopithids form a distinct clade with fewer humanlike traits:
In the evolutionary chart I have placed africanus as an ancestor to the earliest humans, primarily because of its humanlike teeth and brain anatomy and the absence of the skull features that characterize the robust australopithids. But its place in the evolutionary story is still far from clear.