necessary feature for active hunting on the open savannah. The tendon acts as a spring, storing energy when it is stretched and releasing it again as the foot pushes off for the next stride (Bramble & Lieberman, 2004). This can save up to 50% of the metabolic energy costs of rapid locomotion. Trends in general morphology of the skull Dental arcades : in an ape, the teeth are arranged in a rectangular dental arcade, where the left and right cheek teeth are in two parallel lines. Australopith dental arcades tend to be more rectangular than parabolic, but in Homo species the dental arcade is a full parabola, broader at the back than at the front. There is also a strong trend in tooth size, such that the cheek teeth (in particular) of modern humans are smaller than those of australopithecines. And even within H. sapiens there has been a marked decrease in tooth size over the last 30,000 years. Crests and ridges : both the great apes and early hominids have obvious crests and ridges on their skulls. The most obvious are the sagittal and nuchal crests and the brow ridges. Where present, the sagittal crests provide anchorage for large chewing muscles, and are thus most prominent in species where the diet comprises hard, tough material requiring a lot of chewing. (They are also larger in males than in females, an example of sexual dimorphism.) Brow ridge development in hominids may also be related to diet, as large brow ridges