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:

• Australopithecus aethiopicus lived from 2.7 to 2.3 million years ago. It may be an ancestor of robustus and boisei, though it shows a ambiguous mixture of traits. The brain volume is small (410 cc), and in most respects the skull is closest to afarensis.

• Australopithecus boisei (pictured above) lived from 2.3 to 1.2 million years ago. Males weighed 49 kilos and stood 1.4 meters and females weighed 34 kilos and stood 1.1 meters -- making bosei both the largest of the australopithids (it had the largest molars of any hominid) and the robust australopithid with the most extreme sexual dimorphism. Despite its large size, the average brain volume was about 530 cc.

• Australopithecus robustus lived from 2.0 to 1.0 million years ago. It was similar to bosei and also had an average brain size of about 530 cc. -- indeed, some experts consider boisei and robustus to be variants of the same species. However, despite its name robustus was physically smaller: males weighed about 40 kilos and stood 1.3 meters, females weighed 32 kilos and stood 1.1 meters. This last of the australopithids had a body similar to that of africanus, though with very dissimilar skull and teeth.