Final answer:
Raymond Dart insisted the Taung Child was on the human line due to its human-like dental features and anterior foramen magnum, indicative of bipedalism. Though its brain size was like that of a chimpanzee, these traits suggested an evolutionary connection to modern humans.
Step-by-step explanation:
Comparison of the Taung Child and Modern Humans
The Taung Child, classified as Australopithecus africanus, exhibited several key features that led Raymond Dart to argue it was on the human evolutionary line rather than that of apes.
Despite the small brain size of 422 cc, which is comparable to that of a chimpanzee, the Taung Child did not possess pronounced brow ridges and had circular orbits, minimal prognathism, small canines, and no diastema, all traits that align more closely with modern humans.
The most compelling evidence, however, was the anterior placement of the foramen magnum, indicative of bipedalism and an upright posture, which is distinct from the postures suggested by great ape anatomy.
Australopithecus africanus is believed to have lived between two and three million years ago. Comparatively, Australopithecus afarensis, known through specimens like Lucy, had a similar brain size but some morphological differences, such as longer arms suitable for arboreal activities.
Nonetheless, both species exhibit adaptations for bipedal locomotion, aligning them closer to humans than great apes in this aspect.
The significance of the Taung Child's discovery was in identifying features that marked a divergence from apes, even as the overall physiognomy suggested an evolutionary pathway that retained some ape-like characteristics.
Dart's insistence on the human lineage of this specimen underlines the importance of bipedalism as a defining characteristic of hominin evolution.