Final answer:
The earliest characteristic to evolve in hominin evolution was bipedalism, allowing upright walking, as evident in species such as Homo naledi and Homo erectus. This, along with other features like brain size and tool use, contribute to the criteria for classifying species under the genus Homo.
Step-by-step explanation:
Earliest Characteristic of Hominin Evolution
The first characteristic that evolved early in hominin evolution was bipedalism. This adaptation enabled our ancestors to walk upright on two legs, which is considered a defining trait of the hominin lineage. Homo naledi, like other early Homo species, displayed this trait with humanlike feet equipped with arches and ankles that are indicative of bipedal locomotion. Furthermore, by about 2.8 million years ago, species such as Homo erectus were nearly as efficient at upright walking as modern humans, possessing a broadened pelvis, elongated legs, and arched feet that facilitated bipedalism.
It's important to note that while bipedalism was a significant early characteristic, various traits have been used to classify a species under the genus Homo. In addition to bipedalism, the criteria include a larger brain size, reduced jaw protrusion, the use of tools, and particular skeletal structures like a round skull and fully adapted feet for walking. These traits collectively contribute to the mosaic of characteristics that define human evolution.