Final answer:
The defining trait of hominins is bipedalism, which denotes the ability to walk on two legs. This trait distinguishes hominins from other primates and required substantial anatomical changes. Other notable traits include smaller jaws and teeth, shortened arms, upright posture, and dexterity in the hands.
Step-by-step explanation:
The defining trait of hominins is bipedalism. Hominins are part of the human family tree and are distinguished from other primates by their ability to walk on two legs. This shift to bipedal locomotion required significant anatomical adaptations, such as a broader pelvis, longer legs, arched feet, and changes in spine curvature. Early hominin species like Homo habilis exhibited an increased brain size, especially in the frontal lobes, and tool use, which is characteristic of the genus Homo.
Other traits associated with the evolution of hominins include smaller jaws and teeth, shortening of the arms relative to the legs, and a fully upright posture. Moreover, early Homo species like Homo erectus, although nearly as efficient at bipedal walking as modern humans, retained a relatively small brain and larger jaws and teeth compared to modern humans. Homo naledi and H. floresiensis show that even with a small brain size, traits like bipedalism and tool use can be present, challenging the idea that a larger brain is a requisite for intelligence.