Final answer:
To distinguish hominins from other apes, scientists examine traits such as larger brain size, reduced prognathism, humanlike postcranial features, and adaptations for bipedal walking. Studies of fossils and anatomical features support the identification of these traits in ancient hominin species.
Step-by-step explanation:
When determining hominins from other apes, scientists look for specific criteria, many of which have been defined through the pioneering work of anthropologists like Mary Leakey, Louis Leakey, and John Napier. Hominins are characterized by a number of features that differentiate them from other hominoids, such as a larger brain size—over 600 cc for the genus Homo—a round, globular skull, reduced prognathism (less protrusion of the jaw), smaller jaws and mandibles, and humanlike postcranial features. Additionally, feet fully adapted for walking upright, a change in the position of the thumb allowing for better grip, and evidence of tool use are all indicative of a species belonging to the hominin clade.
Fossil evidence, including footprints dating back 3.6 million years found in Laetoli, Tanzania, supports bipedal locomotion in early hominins like Australopithecus. Over time, the hominin lineage, which includes genera such as Homo, Australopithecus, Paranthropus, and Ardipithecus, showed progressive changes towards modern human anatomy, including reduced sexual dimorphism and adaptations for bipedal movement, like shifts in pelvic shape.