"Taung Child", Australopithecus africanus
Discovered by Raymond Dart in 1924 at Taung in South Africa. The find consisted of a full face, teeth and jaws, and an endocranial cast of the brain. It is between 2 and 3 million years old, but it and most other South African fossils are found in cave deposits that are difficult to date. The teeth of this skull showed it to be from an infant about 5 or 6 years old (it is now believed that australopithecines matured faster than humans, and that the Taung child was about 3). The brain size was 410 cc, and would have been around 440 cc as an adult. The large rounded brain, canine teeth which were small and not apelike, and the position of the hole at the base of the skull convinced Dart that this was a bipedal human ancestor, which he named Australopithecus africanus (African southern ape). Although the discovery became famous, Dart's interpretation was rejected by the scientific community until the mid-1940's, following the discovery of other similar fossils.
Credit: photo and (edited) text courtesy of Talk Origins.