Final answer:
The reduction of sexual dimorphism in A. afarensis and A. africanus, particularly regarding canine size, is an evolutionary trend reflecting changes in these hominid species' social structures, mating practices, and physical adaptations for bipedalism and arboreal activity.
Step-by-step explanation:
The question concerns the degree of sexual dimorphism, specifically concerning canine size, in two early hominid species: Australopithecus afarensis and Australopithecus africanus. These species, part of the evolutionary lineage leading to modern humans, are noted for a trend towards reduced differences in canine size between males and females, a characteristic linked to social behavior and mating practices. Australopithecus afarensis, dating from 3.9 to 2.9 million years ago, showcased reduced canine and molar size relative to apes, but larger than modern humans, along with several morphological features indicative of bipedal locomotion as well as arboreal activity. On the other hand, Australopithecus africanus, living between two and three million years ago, demonstrated an even larger brain capacity and still carried traits for bipedalism and some arboreal habits. These findings suggest a complex relationship between anatomy, behavior, and the environment in the evolution of human ancestors.