The region just north of Johannesburg, South Africa, is a formation of ancient limestone in which groundwater has formed numerous caves and sinkholes. Some of these caves are used by animals for cool shade, water, and minerals; some are used by leopards, or in ancient times, sabretooths. By accident and predation, the skeletons of animals fall or are dragged into these caves, including our relatives the hominins. After around 2 million years ago, the most common kind of hominin in these caves was a species we call Australopithecus robustus.
The word “robust” refers to size and strength. A. robustus was not very large in body size, but it had exceptionally large molar and premolar teeth, and a very large and thick mandible, or jawbone. The main muscles of the jaw, the temporalis muscles, were so large that they ran up the complete height of the skull to meet at the midline. The high ridge of bone where these muscles attached to the top of the skull is called the sagittal crest.
A. robustus is one of the best-represented species of early hominins. The first specimen to be found was TM 1517, a partial skeleton with cranial remains from Kromdraai, presently in the Cradle of Humankind World Heritage Site. The largest sample of A. robustus fossils come from Swartkrans, less than 3 km from Kromdraai. The iconic skull, SK 48, provides a good illustration of the anatomy of the cranium of A. robustus with its sagittal crest, large, thick cheekbones, and relatively large molar teeth.
The most obvious features that A. robustus shares with living people are related to locomotion. Human bipedality, or upright walking, caused many changes to the skeleton. A simple comparison of the distal end of the femur, the end nearest the knee, is enough to tell that A. robustus was bipedal like humans. Quadrupedal animals, who go on all fours, very rarely support their weight on one leg and do not have to balance their centers of mass over a single point. Their legs are typically oriented straight from the hip joint to the ground. Humans, in contrast, have to support their weight on one leg every time they take a step. To accomplish this, their legs must angle from the hip joint under the body’s center of mass. The human knee angles very obviously at the distal femur, so that when the condyles of the femur rest flat on the tibia (or a table), the shaft of the bone angles markedly from vertical.
This angle is called the valgus angle, and is one of the easiest-to-see traces of bipedality in fossil hominins.