Final answer:
The cranial capacity of Australopithecus afarensis was about 400 cc, comparable to chimpanzees, indicating their cognitive level. They were adept at both bipedal movement and tree dwelling, with a pelvis structure similar to modern humans.
Step-by-step explanation:
The cranial capacity of Australopithecus afarensis is an important feature in understanding our ancestral past. This species, which lived approximately 3.9 to 2.9 million years ago, had a brain size of around 400 cubic centimeters (cc), similar to that of a common chimpanzee. This capacity reflects their cognitive abilities and is crucial in the timeline of human evolution demonstrating the beginning of encephalization, although not as advanced as in later species like Homo habilis and Homo erectus.
Australopithecus afarensis exhibits a combination of bipedal and arboreal traits, with a pelvis structure similar to modern humans, suggesting bipedal locomotion, alongside long, curved finger and toe bones, indicative of life in the trees. Their prognathism, or the forward projection of the face, is particularly noted among the gracile australopiths, a distinction from the robust species like Paranthropus. These cranial and skeletal features provide valuable insight into the behavior, lifestyle, and evolutionary stage of A. afarensis.
Other notable features include longer arms compared to modern humans, supporting the theory of their arboreal activity. The presence of smaller canines and molars than those of apes, yet larger than modern humans, and a relatively more anterior position of the foramen magnum indicate a dietary and bipedal adaptation. Their skeleton, including the famous fossil known as "Lucy", represents a significant milestone in anthropological discoveries, offering a clearer image of one of our earliest known ancestors.