The correct answer is reduction in tooth size.
The anatomy of the foot and pelvis are the feature of Australopithecus afarensis that is used to definitively classify this species as a hominin.
Australopithecines are an adaptive radiation of early hominins, all of which were to some extent bipedal, had brains that were only slightly larger than those of apes, and had adaptations to a diet that included at least occasionally hard-to-chew items. They have been identified through research on perhaps ten species that lived in central, eastern, and southern Africa between 4.2 million and 1.0 million years ago. Understanding australopithecines is essential to comprehending not only the diversity of early hominins but also the origins of Homo. Our genus Homo probably originated from this radiation, albeit we are unsure from which species.
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