Why was a. africanus unlike any primate fossil skull that dart had ever seen?

 Australopithecus africanus, the fossil species to which the skull belonged, was unlike any primate fossil skull that Raymond Dart had ever seen because it exhibited a mix of both ape-like and human-like features. Dart, an Australian anatomist and anthropologist, discovered the first specimen of A. africanus, the Taung Child, in South Africa in 1924.

This skull was unique for several reasons:

1. It had a small brain size similar to that of a chimpanzee yet the position of the foramen magnum (the hole in the skull through which the spinal cord passes) was underneath the skull. This indicated an upright, bipedal posture, which is a key trait of humans rather than apes.

2. The teeth and jaws were more human-like. Although the skull had larger canines than modern humans, the teeth were smaller than those of apes and the dental arcade was more parabolic in shape, resembling that of humans.

3. The face was less protruding than that of apes, a characteristic of hominins, which include humans and their ancestors.

These features meant that A. africanus was a transitional form in human evolution, combining both primitive and more advanced traits. Prior to Dart's find, it was commonly believed that brain size increase was the first step in human evolution. However, the Taung Child indicated that bipedal locomotion preceded the expansion of brain size.