Final answer:
Hominids differ from other hominoids by having a larger brain size, hands capable of tool-making, and humanlike feet adapted for walking. Criteria for classifying a species under the genus Homo include brain size, skull shape, and evidence of tool use, among others.
Step-by-step explanation:
The distinguishing features of hominids from other hominoids, beyond skeletal differences, involve several key characteristics. These include a larger brain size, changes in the bone structure allowing for dexterity and tool use, humanlike feet adapted for bipedal locomotion, reduced prognathism, and smaller jaws and teeth. For instance, Homo habilis, one of the earliest species classified under the genus Homo, had a brain capacity of approximately 661-700 cc, which is larger compared to that of australopithecines. Additionally, the hands of H. habilis were capable of precision necessary for making tools due to the repositioning of the thumb, which facilitated an improved grip. The feet of early hominins like Homo naledi were nearly indistinguishable from those of modern humans, with arches and ankles designed for upright walking.
Furthermore, as proposed by Mary Leakey and others, specific criteria for classifying a species as Homo include a brain size over 600 cc, a more round and globular skull, evidence of tool use, and postcranial features similar to those of humans. However, the presence of mosaic traits in species like Homo naledi, which display a combination of primitive and modern characteristics, can sometimes complicate classification.
By 2.8 million years ago, species like Homo erectus were able to move bipedally nearly as efficiently as modern humans. While their pelvic, leg, and foot structure facilitated this mode of locomotion, they still exhibited significant differences from modern humans, such as larger jaws and teeth, a sloping forehead, and a small brain relative to their body size.